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Old 10-26-2011, 07:12 AM   #1
dbetz
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Default Fuel Economy Tips

Was wondering if anyone has any good tips on improving fuel economy, besides the normal maintenance that is regularly discussed here? Like where to get gas, driving tips, fuel additives to use/avoid, etc.

In particular, when gas started hitting (Gasp!) $2 a gallon a few years back, I started looking into what I could do to stretch mileage. At the time I was (and still have) driving a 2002 Ford F-150, with the 5.7L V8. Getting about 12-13 MPG wasn't good enough for me, and I started exploring. A friend of mine recommended a K&N air filter - improved engine running but no noticeable MPG gain. Went to replace plugs and wires and found out that they designed that vehicle to NOT be user-friendly.

Later, when gas got really expensive, I inherited my Saturn from my mother-in-law (She upgraded to a LW200 wagon), but continued to explore my options. Through chatting with friends, I found that (Gasp!) not all gas is the same. I naively assumed that the big names carried similar gas, and that the local gas stations just marketed the bigger guys gas at a discount. After hearing many stories about water in the gas tank, etc., I started to look deeper into the issue. Needless to say, I was tremendously surprised at what I found out.

For about a year and a half, with the wagon (1999 SW2, Auto trans), documented my gas mileage and varied where I filled up. I tried to even out driving habits through multiple visits. (Yes, I am an engineer . . .).

Shell (6 Tanks) 29.5MPG
Marathon (1 Tank) 30.3MPG
Mobil (3 Tanks) 29.3MPG
Speedway (26 Tanks) 28.4MPG
BP (9 Tanks) 28.1MPG
Meijer (9 Tanks) 27.0MPG

Speedway is the closest to me and made a good reference fuel, which I went back to after switching each brand, to make sure something didn't change in the mean time. As you can see, MPG varied as much as 10%. Also the Meijer gas had negative affects that often carried into the next brand (Speedway, usually), reducing the MPG for at least one tank. I observed a 10%-15% change in my F-150 from getting off Meijer gas, as well. (from 12.5MPG to 14+MPG)

I also found quickly that plus fuel got me slightly worse mileage than regular.


The next thing I varied was driving habits. I couldn't figure out why, during a week of heavy snow here, I got WAY better gas mileage in my F-150, doing 35-45 on the hwy, in 4WD, than when I was at full speed in 2WD.

Of course, wind resistance increases with the square of the velocity, and with the wind cheating characteristics of an apartment building, it made sense that there would be a break-point somewhere below 90 on the F-150.(And, no, I never really believed the gov't claim that 55MPH is the optimal speed)

So I experimented, and found that at 75MPH, I was getting 13-14MPG, but if I went 65MPH on the expressway, I could get 16.5+MPG. And since the EPA Hwy rating is 17MPG, I figured getting 16.5+MPG with 140,000 miles on it was pretty good. (at 70MPH was a split between the two)

The wagon, being more streamlined, has a different curve. I found that I got worse gas mileage at 65MPH versus 75MPH. But I haven't explored it in too much detail.

I don't know if any of you highway runners have any thoughts?

What about other observations?

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Old 10-26-2011, 07:24 AM   #2
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Default Re: Fuel Economy Tips

The differences you noted in places you fuel are well within the norm of variance from one fill to the next even at the same station. I see that much and I nearly always buy the same fuel at the same station and the same pump. And because of how much I drive I buy fuel every 3rd day/night.
As for better at 75 than 65 there is no physical way. Again this would fall in to the normal variation from tank to tank. Over long period averages driving mostly highway (I have a 125 mile RT commute) last year I had a 650 mile commute (long story). The current commute is 90% highway. If I run my SL2 at 55-60 MPG runs just over 40. At 65 to 70 37 to 39 and over 70 it drops pretty quick to 32-35. Now if an SL2 had better gearing this would not be the case. Ignore advice for outlandish mods and such. Just keep the car maintained CORRECTLY and run it.

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Old 10-26-2011, 08:08 AM   #3
cheapybob
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Default Re: Fuel Economy Tips

If you want good fuel economy, it would be optimal to drive an SOHC with a 5 spd.

Drive light on the accelerator pedal and try to time traffic to maintain steady speeds with minimal throttle. MPG's on the SC2 at 10% throttle is about 50 MPG and at 15% its down to 40 MPG and at 20% its down around 30 MPG.

That said, and given what you have, about the best you can do is to maintain what you have for the best it can do.

You might also look at my hot air intake. I never tried it on a DOHC or automatic, but I'd guess it will work.

I run injector cleaner through about once a year, but otherwise have found no benefit MPG wise from fuel additives. I usually buy Shell or Speedway or BP fuel from high volume stations, and don't notice any difference between them, but haven't gone to the trouble of tracking the MPG from each.

IMO, the majority of variation in my MPG's are due to temperature, traffic and lights, length of trip, and highway vs suburb driving, not which brand of gas I was running, because it varies daily, and I only fill weekly.

PS: the cheap way to improve gearing for MPG's is to run a taller tire

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Old 10-26-2011, 08:11 AM   #4
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Default Re: Fuel Economy Tips

Okay Mr. Engineer, getting good gas mileage isnt that hard.

Forget aerodynamics.
Forget where you get your fuel(unless you want pure gas, then find a station).

Maintain your vehicle. Plugs, wires, thermostat, Brass ECTS, clean air filter, tires filled to 40 psi, lose any unnecessary weight. Replace/accomplish all of that, and you'll be seeing better mileage

...
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Old 10-26-2011, 08:26 AM   #5
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Default Re: Fuel Economy Tips

I found my best gains were from keeping my speed on flat ground in the 68-70mph range. I let the speed slowly bleed down when going uphill and then pick it back up when I am going down hill.

The obvious maintained issues are a no-brainer. The one thing I harp on with a lot of saturn owners is to check your coils. If they are much above 100k miles then replacements are normally in order.

Also, keep the extra weight in the car to a min. Clean out the truck (keep the spare, jack, etc) but loose anything that you don't really need to carry.

I'm getting 40-41mpg on a regular basis during my 130 mile/day commute.

If you can find gas without methanol in it you will see an instant 3 to 5 mpg increase.

Good luck
Highmile
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Old 10-26-2011, 08:48 AM   #6
dbetz
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Default Re: Fuel Economy Tips

Wow. . .

Said I was an engineer . . . which verifies the love for detail I was showing . . . not that I was some genius convinced I could game the system.

Never said it was that hard. Just wanted to know what was outside of what you have to repeat to everyone every day here. And I am sorry, but bad gas can do a whole lot of mess to an engine. Maybe a car's engine is more corrective than a small engine, but with most lawn-mower problems, bad gas is the majority problem. Knowing where to avoid "bad gas" and find "good gas" could help. Maybe others don't know this. I didn't.

Certain brands use a lot of ethanol. So I have been told, the ethanol can do quite a bit of bad on "older" cars. And with every brand working their own alchemy lab or additives and cleansers, it would only make sense that certain engines would respond better to certain mixes than others. In all the cars I drive, I have found exactly that. I only started to document it recently so I could verify I wasn't seeing only what I wanted to see. So what are they?

Dunkin' Donuts and Starbucks and Waffle House all sell "coffee", but no one will agree that they all are the "same." And if there are that big of variations in coffee, it only tracks that gas might also have variants. My Saturn likes the Starbucks from Marathon and hates Meijer Folger's. If all gas is the same, then why do I notice a difference? A difference determined over, for the most part, 2700 miles of driving for each brand.

This site debates ad infinitum the pros and cons of just about every part on the car, replaceable or otherwise. Especially oil. Why not discuss brands of gas and driving habits?

According to www.fueleconomy.gov, my 1999 SW2, auto trans should get 26MPG combined, 32 MPG hwy. So I don't think I am doing too bad getting nearly 30 combined MPG, before fixes. But just because I have "fixed" my car doesn't mean that I don't have more to learn.

As for aerodynamics, they do affect mileage etc. Especially when related to speed. Every car will have a certain "sweet spot" where the horsepower is greatest. This power will compete with all those things that drag, which is also unique to each car. Where the break even point is, where friction finally overwhelms gains in power, is, further, unique to every car.

Ever wonder why they dropped the "instant" fuel economy data on the electronic dash systems? Now you just get average. Because people were using the "instant" option to figure out that they got better economy outside of "government recommended" speeds. At 85MPH you could gain 3-4MPG over running at "posted" speeds. And so they were. Get there faster AND save gas. Bonus!

That being said, I figured I would share what I have seen, just like you guys. Maybe I would have something to contribute. Maybe no one cares.

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Old 10-26-2011, 08:59 AM   #7
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Default Re: Fuel Economy Tips

Quote:
Originally Posted by dbetz View Post
A friend of mine recommended a K&N air filter - improved engine running but no noticeable MPG gain.
That's because modern vehicles operate in one of two ECU modes- open loop and closed loop. Most of the time, the ECU is in closed loop- which means that the ECU is monitoring and listening to all of its sensors keeping the exhaust mixture right around stiochiometric. When the ECU switches over to open loop, the ECU just goes to a pre-programmed fuel table and basically dumps in fuel- the fuel mixture goes pretty rich at this point. The reason K&N filters (and similar) make more power is because they help to lean out the fuel mixture in open loop. A K&N filter will only make more power in open loop. I suppose that if you spent a lot of time in open loop, having a K&N filter could also save you some mpgs because you would accelerate faster and not be in open loop as long. The rest of the time you could have whatever filter you want. I believe K&Ns claim to giving you more MPGs comes from when you replace a normal filter that is virtually clogged with a K&N- but you'd see the same results if you replaced that clogged filter with a clean filter of the same type.

As far as highway vs city goes, I recently tried 2 tank fulls of daily commuting by taking a route that is 95% free flowing highway (speed limit 65- which meant I was doing about 70). While doing this, I recorded my second worse overall tank at 34.4 mpgs. My theory is that while I wasn't speeding up after stop lights, I was constantly applying throttle to maintain speed. While driving in the city, I can benefit from DFCO (decelleration fuel cut off- or engine braking) as I approach lights.

But I also replaced my worn out and extremely hard tires with some new tires that are softer. The weird thing is that I also got an alignment- it was totally out of whack. Since I got the tires and alignment, my mpgs seem to be a little down from what they were with the hard tires and bad alignment. Could also be the introduction of the winter blend gas bringing it down at the same time. Hard to tell.

Either way, I'm not too worried since my other option sits in the garage most days- an Explorer that gets 16.x mpgs. So my method of improving my mpgs by about 20 mpgs, is simply to grab the key to my Saturn instead of my Explorer.

And I'm also an engineer- although not an automotive one.

...
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Old 10-26-2011, 09:04 AM   #8
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Default Re: Fuel Economy Tips

Quote:
Originally Posted by dbetz View Post
Ever wonder why they dropped the "instant" fuel economy data on the electronic dash systems?
The new Explorers have the instantaneous read out. There's a guy on the Explorer forum who has spend over $1000 on a new intake and exhuast system trying to get 30 mpg highway using the instantaneous read out. Guess who spent over $1000 on shiny things with no improvement to his instantaneous highway mpg.......

...
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Old 10-26-2011, 09:12 AM   #9
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Default Re: Fuel Economy Tips

Quote:
Originally Posted by dbetz View Post
As for aerodynamics, they do affect mileage etc. Especially when related to speed. Every car will have a certain "sweet spot" where the horsepower is greatest. This power will compete with all those things that drag, which is also unique to each car. Where the break even point is, where friction finally overwhelms gains in power, is, further, unique to every car.
Some things you could do in this regard are to lower the vehicle, build a front air dam to keep air from going under it, remove your side mirrors and run some kind of low profile mirror, tape up any gaps in the body work (around the hood, front bumper fascia, doors, windows), close out the rear wheel wells so they are flush to the body, make underbody belly pans so the air flows smoothly under the car, make a boat tail so the air leaving the rear of the car does so smoothly and doesn't create any turbulence...... Lots of options.

...
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Old 10-26-2011, 09:41 AM   #10
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Default Re: Fuel Economy Tips

Well use reputable stations and you wont run into the problem of bad gas. And what im saying is that paying attention to what gas is "better" isnt gonna get you very far.

and forget about EPA estimates. I dont know where they get their numbers, but they almost always undercut the best mileage you can expect to see.

30mpg in a Saturn isnt something to be proud of either.

Between 50mph and 65 is where youll find the best mileage

...
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Old 10-26-2011, 09:54 AM   #11
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Default Re: Fuel Economy Tips

Quote:
Originally Posted by Tom92SCm View Post
Some things you could do in this regard are to lower the vehicle, build a front air dam to keep air from going under it, remove your side mirrors and run some kind of low profile mirror, tape up any gaps in the body work (around the hood, front bumper fascia, doors, windows), close out the rear wheel wells so they are flush to the body, make underbody belly pans so the air flows smoothly under the car, make a boat tail so the air leaving the rear of the car does so smoothly and doesn't create any turbulence...... Lots of options.
Wasn't trying to bill the idea of streamlining the car. An easy route would be to smack the highway retaining wall nice and flush to narrow the car like NASCAR drivers joke about.

Was more trying to build argument for a smaller profile car having a maximized MPG at a higher speed than that for a pickup truck. Thus giving an explanation for why my truck ran better at 65MPH and the wagon at 75MPH. The truck has more HP, but a significantly greater wind resistance to push through. The wagon would get to higher speeds, which, if I understand correctly, would mean the engine would be at higher RPM, meaning pushing toward higher torque/HP (and efficiency?), before the air resistance starts overcoming the gains in power. Sure, it is probably not THAT simple, but the principle is there.

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Old 10-26-2011, 09:59 AM   #12
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Default Re: Fuel Economy Tips

Quote:
Originally Posted by Highmile View Post
The obvious maintained issues are a no-brainer. The one thing I harp on with a lot of saturn owners is to check your coils. If they are much above 100k miles then replacements are normally in order.

If you can find gas without methanol in it you will see an instant 3 to 5 mpg increase.

Good luck
Highmile
'95 SL1 467k and counting
Thanks! I will check the coils out, as I have 148k on the car. Either way it will be fun to learn how to deal with them. Been referencing the threads here on how.

Didn't know that ethanol cut 3-5MPG. I mostly heard about damage from water and to fuel lines and various buildup issues. I'll keep an eye on who uses and how much.

David

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Old 10-26-2011, 10:01 AM   #13
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Default Re: Fuel Economy Tips

Use gas buddy; this is the link I use: http://www.lansinggasprices.com/ of course, you want to switch to your own area. It's not unusual for me to save 10~20 cents a gallon by checking just before I need to fuel up. By gassing up before the weekend and the "usual" 20~30 cent jump, I've saved as much as 50 cents a gallon.

+1 to what cheapybob said. also, I've read that below 40mph, air resistance is almost negligible on fuel economy
Quote:
Originally Posted by cheapybob View Post
You might also look at my hot air intake. I never tried it on a DOHC or automatic, but I'd guess it will work.tire
Yes, Carlos (username cfg83) has a hot air intake on his SW2 (also has the SOHC 5th gear, but IIRC he documented the mpg changes separately). Those were the two biggest gains for him.

Quote:
Originally Posted by underthehood View Post
As for better at 75 than 65 there is no physical way.
Sure there is... 10 mph tail wind while running at 75mph, 10 mph headwinid when driving 65 mph As you said, there are so many variables that a single run or tank of gas does not tell you much, and as mentioned gas stations are starting the winter blend switchover... between that and temperature variations, and all the other factors, you'll want to do a lot of testing.

I don't think water in the gas is a significant factor at all, anymore, since there is ethanol in most fuels

...
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And now (7/2010), a Craigslist 1997 SC2, white, 5 spd

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Old 10-26-2011, 10:03 AM   #14
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Default Re: Fuel Economy Tips

To optimize economy, you want the engine to be at a lower RPM. And how does that make sense, "a smaller profile car having a maximized MPG at a higher speed"?

The slower you go, the lower the engine speed will be, the less wind resistance there will be and the better mileage you will get(within reason, i.e. <50mph. Thats for any vehicle.

...
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Old 10-26-2011, 10:05 AM   #15
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Default Re: Fuel Economy Tips

Quote:
Originally Posted by Tom92SCm View Post
But I also replaced my worn out and extremely hard tires with some new tires that are softer. The weird thing is that I also got an alignment- it was totally out of whack. Since I got the tires and alignment, my mpgs seem to be a little down from what they were with the hard tires and bad alignment. Could also be the introduction of the winter blend gas bringing it down at the same time. Hard to tell.
.
Thanks for the info on the K&N. Now I know a bit more and why!

As for the tires, I was always under the impression that softer would lead to more traction, but also greater rolling resistance. Kind of like max or over inflating them leading to less traction, but greater road efficiency. Whether or not it would account for the full drop off I cannot say.

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Old 10-26-2011, 10:47 AM   #16
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Default Re: Fuel Economy Tips

Quote:
Originally Posted by AlexofNazareth View Post
To optimize economy, you want the engine to be at a lower RPM. And how does that make sense, "a smaller profile car having a maximized MPG at a higher speed"?

The slower you go, the lower the engine speed will be, the less wind resistance there will be and the better mileage you will get(within reason, i.e. <50mph. Thats for any vehicle.
OK, maybe I have always had a flawed understanding of engines. But when looking at a chart of HP and torque related to RPM for an engine, the graph is not a straight line. There is a point in the power band (hopefully I am using the term properly, if not I apologize), where torque and HP are maximized, and then fall off. I have always been led to believe that at/near that maximum, the engine is running at peak efficiency. To get the most efficiency, the most energy conversion, you want to operate in that region. Sure, you are using more fuel mix to achieve that power, but you are "moving" faster, also. If the engine is getting better energy conversion, then it would need to use less fuel to maintain a given amount of energy necessary to keep the car moving. I presumed that a modern EFI would be able to coordinate and keep track of the minimums necessary to attain the called-for power demand.

A lower speed might use less fuel mix from one point, but an engine running more efficiently faster would also not need as much fuel. I point to the effect commented on this forum about how a higher temp thermostat can lead to both more winter heat, and a bit better fuel economy. Engines run more efficiently hotter. For a given RPM, a hotter engine will get better energy conversion, and use less fuel to get a given energy level, meaning better fuel economy.

So, an engine can run more efficiently at higher RPMs than lower RPMS. For a given drive ratio, an engine running more efficiently at a higher RPM might relate to a non-linear use of gas to speed relation. That is to say, a doubling in RPM would not necessarily require a doubling in fuel consumption. You would be able to go farther on less fuel by going faster. Again, presuming that you are operating more efficiently in the higher RPM range.

Now, factoring in wind resistance. As you increase speed, you eat more HP trying to overcome pushing the air out of the way. This means less HP available to push the car to a given speed. A higher profile vehicle will more quickly hit the point where pushing the engine just can't get you any more speed without using a disproportionate amount more fuel to get there. Lowering the profile will limit the amount of HP robbed by wind resistance. Of course, I am singing to the choir on that one.

If you were to subtract from the aforementioned powerband the horsepower "robbed" by wind resistance, which is exponential with respect to speed, and therefore RPM, considering a given drive ratio, you would get a peak in power (and efficiency) *somewhere*, but before the engine-only peak. This would be the point where the system runs optimally. Any higher rpms and you need a greater proportion of fuel per output. Any less, and you also go the other way, as you are using less fuel from keeping off the accelerator, but gaining less energy per unit fuel a the same time.

A lower profile car would have the wind resistance build slower, and would reach peak efficiency at a higher RPM than a higher profile car. Thus, at a higher RPM (greater speed) it would be more fuel efficient than the higher profile car.

Or am I nuts?

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Old 10-26-2011, 10:49 AM   #17
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Default Re: Fuel Economy Tips

Quote:
Originally Posted by dbetz View Post
Didn't know that ethanol cut 3-5MPG.
David
Ethanol in gasoline is a scam. (Even a bigger scam from a political point of view, but this is about mpg, not politics)
Ethanol has less energy compared to gasoline, so , it isnt even possible to get as good mpg using it. Talk to someone with an E85 vehicle about the mpg using it. They will usually try it once, and never again.
I did an analysis using it on my car, and it was consistently 3-5 mpg less every single time.
So lets say you get 30mpg on gasoline versus 27 on E10.
Do the math for 10 gallons used.

30*10=300 miles 10*$3.50 gallon = $35.00
27*10=270 miles 10*3.40 gallon = $34.00
Now you need to drive another 30 miles to catch up to the 300 for gasoline
which is about just over another gallon of that crappy E10. So you add another 3.40, and it has now just cost you $37.40 to go the same distance as having used regular gasoline.

So, you have now just spent more to go the same distance, and not to mention all the subsidies and crap from a political view, you can see that Ethanol is a scam.

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Old 10-26-2011, 11:13 AM   #18
AlexofNazareth
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Default Re: Fuel Economy Tips

The whole point of fuel economy is to keep the engine out of its power band. Yeah, if you keep the engine at 2400 rpm, where the max torque is at, it'll be easier to drive up hills or drive against a strong wind, but will do nothing about improving gas mileage. The higher the rpm, the more fuel is being used. Its as simple as that.

Last edited by AlexofNazareth; 10-26-2011 at 11:22 AM..

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Old 10-26-2011, 11:36 AM   #19
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Default Re: Fuel Economy Tips

Quote:
Originally Posted by dbetz View Post
As for the tires, I was always under the impression that softer would lead to more traction, but also greater rolling resistance. Kind of like max or over inflating them leading to less traction, but greater road efficiency. Whether or not it would account for the full drop off I cannot say.
All true. But my point was that my tires were so old (over 10 years) that while they had the same treadwear rating as the new tires, they were significantly harder from 10 years of UV exposure. I'm happy with the new tires- the reduction in road noise is worth the mpg penalty. Not to mention I no longer hydroplane if someone spits out the window onto the road.

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Old 10-26-2011, 11:56 AM   #20
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Default Re: Fuel Economy Tips

Quote:
Originally Posted by AlexofNazareth View Post
To optimize economy, you want the engine to be at a lower RPM. And how does that make sense, "a smaller profile car having a maximized MPG at a higher speed"?

The slower you go, the lower the engine speed will be, the less wind resistance there will be and the better mileage you will get(within reason, i.e. <50mph. Thats for any vehicle.

+1, this is more or less how overdrive works in a tranny.

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