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Old 11-22-2017, 06:25 PM   #1
DrvLikHell
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Default What speed for absolute maximum MPG?

I've tried searching for this several times, but there are so many threads about gas mileage and MPG numbers that I can't find what I'm looking for. Either that or it's never been discussed?

The title says it all, I'm trying to figure out what speed is the best for absolute maximum gas mileage. All of the EPA ratings and such state that highway mileage is greater than city mileage. Obviously, this is because city driving usually requires stop and go driving while on the highway you can simply cruise at a steady speed. However, my city/town is extremely spread out and there are many times when I get better mileage driving "in town" than I do on the highway. This is likely because in town driving, for me, can consist of 10 or 20 minutes (or more) of 45 mph cruising with only one or two stops for traffic lights.

So 45 mph is more fuel efficient than 70 mph, but what's the most efficient speed, or how can it be determined? I have a BAFX OBD2 Bluetooth adapter and the Torque app, but Torque seems to give lower fuel mileage numbers than when I divide miles driven on the odometer by fuel used at the gas pump. Plus it's hard to watch the real-time MPG numbers without nearly running over mailboxes. What do you guys think?

Btw, I have a 1996 SL2 5 speed with a hollow cat and no airbox resonator.

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Old 11-22-2017, 08:53 PM   #2
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Default Re: What speed for absolute maximum MPG?

From what I have always read, 40, to about 45 yields the best mpg.
By then the torque converter is locked in.

That being said, light as a feather will boost anyone's gas mileage at speed. But, above 50, you will head down the gas mileage slope dramatically with each 5 mph increment.

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Old 11-23-2017, 12:05 AM   #3
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Default Re: What speed for absolute maximum MPG?

It's a 5spd...if it's moving at all the "torque converter" is locked in :P

I got 2mpg better at 3000rpm 70mph in a '96 SC2 than a '97 SW2 at 2600rpm at 70mph (the SW2 had a MP2 transmission, the SC2 had stock). 38mpg vs 36mpg. Probably could have been higher if I wasn't driving up/down a bunch of mountains on the way to work like I am now (and getting 29mpg out of a '99 Forester 5spd).

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Old 11-23-2017, 12:21 AM   #4
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Default Re: What speed for absolute maximum MPG?

0 mph.

- duh

What did I win?

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Old 11-23-2017, 12:24 AM   #5
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Default Re: What speed for absolute maximum MPG?

- You did say "absolute maximum" in your specifications.


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Old 11-23-2017, 08:24 AM   #6
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Default Re: What speed for absolute maximum MPG?

Depends on the final gearing and such but the lowest speed at which top gear and torque converter lockup can be maintained without lugging. On the Saturn "S" series cars it comes out to about 40 MPH on flat ground with no head wind. Will vary up or down a little depending on slope or wind.
If it's a manual again the lowest speed you can maintain top gear without lugging and not having to downshift.
These people that do the extreme MPG thing will also tell you in a manual the engine is most efficient at WOT and low RPM. No pumping losses past the throttle plate. One of the many reasons a diesel is more efficient in MPG than a gasoline engine.

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Old 11-23-2017, 06:34 PM   #7
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Default Re: What speed for absolute maximum MPG?

Hmm, funny this thread appeared as i have been wondering whats a decent speed to squeeze out the most fuel economy.

I have been trying to see how much gas i can save recently so i have been coasting and keeping my revving below 2500 RPMs. Recently have been practicing a method i read about called "pulse and glide" which is a short pulse of speed followed by a glide back down to whatever speed you started at (try 35-45) and repeating the process. Since the TCC locks up around 40 mph or so, that means id be spending more time around my efficient speed instead of just under it. Though if traffic is around, ill try 35-40 instead

I think ill try comparing how it does with/without P&G. Could help my city mileage though.

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Last edited by Jonasan308; 11-23-2017 at 06:39 PM..

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Old 11-23-2017, 07:42 PM   #8
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Default Re: What speed for absolute maximum MPG?

low and slow
I try to get into 5th gear asap (2000 SC1) - usually around 30 ~35 mph traffic permitting. The I tend to keep the tps number (scan gauge monitor) as low as possible usually single digit or just into the doubles.
this keeps me in the 38pmg range +/- 1 or 2 (warm months)

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Old 11-23-2017, 11:02 PM   #9
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Default Re: What speed for absolute maximum MPG?

Quote:
Originally Posted by Jeeprz! View Post
- You did say "absolute maximum" in your specifications.

Actually... at 0 mph with the engine running you're getting the absolute worst fuel mileage because you're burning fuel and going nowhere.

Quote:
Originally Posted by underthehood View Post
Depends on the final gearing and such but the lowest speed at which top gear and torque converter lockup can be maintained without lugging. On the Saturn "S" series cars it comes out to about 40 MPH on flat ground with no head wind. Will vary up or down a little depending on slope or wind.
If it's a manual again the lowest speed you can maintain top gear without lugging and not having to downshift.
These people that do the extreme MPG thing will also tell you in a manual the engine is most efficient at WOT and low RPM. No pumping losses past the throttle plate. One of the many reasons a diesel is more efficient in MPG than a gasoline engine.
I tried doing the ultra low RPM thing by shifting early and mileage got worse. When I let the engine rev normally it's better. About the slowest I can go in 5th gear without lugging is between 30 and 35 mph. At 30 mph it's a little too slow to do any kind of reasonable acceleration without bogging the engine, while 35 is no problem unless I need to accelerate quickly.

I don't get the WOT at low rpm idea. That just causes my car to bog until the revs climb and then I end up in triple-digit speeds. You'd have to have a pretty weak engine to be able to keep it at WOT at a low RPM without accelerating... or be hauling a very heavy load.

Quote:
Originally Posted by Jonasan308 View Post
Hmm, funny this thread appeared as i have been wondering whats a decent speed to squeeze out the most fuel economy.

I have been trying to see how much gas i can save recently so i have been coasting and keeping my revving below 2500 RPMs. Recently have been practicing a method i read about called "pulse and glide" which is a short pulse of speed followed by a glide back down to whatever speed you started at (try 35-45) and repeating the process. Since the TCC locks up around 40 mph or so, that means id be spending more time around my efficient speed instead of just under it. Though if traffic is around, ill try 35-40 instead

I think ill try comparing how it does with/without P&G. Could help my city mileage though.
I also read about pulsing in an article about hypermiling. Mostly it applied to hybrids that use the electric motor for accelerating. I figured it would be ineffective in a regular gasoline car because you'd always be accelerating when on the throttle, and never cruising. Plus in a manual transmission car like mine, you'll slow down faster due to better engine breaking where an automatic will coast better (I'm assuming yours is automatic). I'll be interested to hear your results because, while coasting, my app tells me I'm getting ridiculous fuel mileage.

On the topic of overall fuel mileage, I read one article that said economy cars are most efficient at cruising, so get up to cruising speed without delay to maximize cruise time. Many other articles say the opposite, to accelerate slowly to use less fuel. Personally, I think it's somewhere in between for economy cars like ours. Obviously, you don't want to do WOT away from a stop sign just to get to 45mph and cruise, but it also doesn't make sense to spend half a mile accelerating slowly since, while accelerating, you're getting far less than optimal fuel mileage.

Quote:
Originally Posted by laser3kw View Post
low and slow
I try to get into 5th gear asap (2000 SC1) - usually around 30 ~35 mph traffic permitting. The I tend to keep the tps number (scan gauge monitor) as low as possible usually single digit or just into the doubles.
this keeps me in the 38pmg range +/- 1 or 2 (warm months)
That's what I try to do, but I only get low 30s at best. My last calculation was 33.8, and that's the highest in a while. Lots of cruise control at 45mph this week and no highway, but mine is a twin cam vs your more frugal single cam.

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Old 11-24-2017, 05:06 AM   #10
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Default Re: What speed for absolute maximum MPG?

If I'm not mistaken, the highest torque is the best rpm to shift into the next gear. Revving the engine above max torque (in this case 2400 rpm) wastes fuel; more rpm = more fuel use. With specs given, best rpm shift would be at 2400 rpm minimizing fuel use. Higher rpm uses more fuel. Observe displayed speed at 2400 rpm in each gear. Whatever 5th gear speed shows at 2400 rpm should be best fuel economy unless you can drive at lower rpm in 5th gear (lower rpm uses less fuel). Staying below 2400 rpm and matching traffic speeds should return best overall fuel mileage.

SAE Net Torque @ RPM:114 @ 2400

SAE Net Horsepower @ RPM:100 @ 5000

EPA Fuel Economy Est - City (MPG):29

EPA Fuel Economy Est - Hwy (MPG):40

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Old 11-24-2017, 09:21 AM   #11
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Default Re: What speed for absolute maximum MPG?

Here's the bottom line. Going for max mpg will drive EVERYONE (even blue haired ladies) behind you bonkers to the point you may find yourself at the wrong end of someone's fist. Drive sanely but stay with traffic. Best for yourr car and your health

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Old 11-24-2017, 09:30 AM   #12
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Default Re: What speed for absolute maximum MPG?

^+1
All these people going for max MPG with their SOHC engines and they get the same, or lower, than I got over multiple tanks with a DOHC.

My dad won't go over 45mph on 50mph roads and 60mph on 65mph roads. Always complains about low MPG, at one point we had pretty much the same truck and mine got better MPG despite driving faster with a bigger heavier truck. Mine's a '00 Chevy 2500 with 6.0L, his was a '04 Chevy 1500 with 5.3L. He even downgraded to a '06 Tacoma this summer to try to get better mileage and while I'm sure it worked...it wasn't by nearly enough to be worth paying $16,000 for an '06 with 130k miles on it.

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Old 11-24-2017, 11:37 AM   #13
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Default Re: What speed for absolute maximum MPG?

From personal experience I can say the SL SOHC beats the pants off the SL2 DOHC versions. My daughter (oldest) back in 2010 bought a 2002 SL which I used for a couple months to "shake it down" and make sure I had everything done to it before she took it with her.
It regularly would give me high 30's where my SL2 would under same conditions return only low to mid 20s and mid 40's where the SL2 would return high 30s
She still says it averages nearly 37. And believe me she does NOT take care of it the way I took care of the SL2. I cringe at how that car is cared for.
However the SL2 could run away and hide from the SL so I would gladly trade that for MPG LOL

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Old 11-24-2017, 03:03 PM   #14
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Default Re: What speed for absolute maximum MPG?

Quote:
Originally Posted by fdryer View Post
If I'm not mistaken, the highest torque is the best rpm to shift into the next gear. Revving the engine above max torque (in this case 2400 rpm) wastes fuel; more rpm = more fuel use. With specs given, best rpm shift would be at 2400 rpm minimizing fuel use. Higher rpm uses more fuel. Observe displayed speed at 2400 rpm in each gear. Whatever 5th gear speed shows at 2400 rpm should be best fuel economy unless you can drive at lower rpm in 5th gear (lower rpm uses less fuel). Staying below 2400 rpm and matching traffic speeds should return best overall fuel mileage.

SAE Net Torque @ RPM:114 @ 2400

SAE Net Horsepower @ RPM:100 @ 5000

EPA Fuel Economy Est - City (MPG):29

EPA Fuel Economy Est - Hwy (MPG):40
That's a very useful bit of information about shift points. It looks like you quoted numbers for the single cam, though the dual cam has its initial torque peak at about the same place, while its max torque is at 4800.

Quote:
Originally Posted by underthehood View Post
Here's the bottom line. Going for max mpg will drive EVERYONE (even blue haired ladies) behind you bonkers to the point you may find yourself at the wrong end of someone's fist. Drive sanely but stay with traffic. Best for yourr car and your health
This isn't LA, I live in Florida, old ladies out number the thugs 10 to 1, easily. It's not uncommon to find someone going 25 in a 35. Which is highly annoying to me because that's 4th gear speed, not 5th, so it's wasting gas.

For the record, I definitely get less overall fuel mileage than an equally maintained single cam car. Laser3kw said he gets around 38 mpg in his SC1. I've never gotten 38, ever.

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Old 11-24-2017, 07:09 PM   #15
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Default Re: What speed for absolute maximum MPG?

SAE net torque is listed as maximum values at whatever rpm is associated with it. I do not think there is a minimum torque as it isn't a talking point. Obviously minimum torque is engine off and goes up as engine rpm rises and ends as max torque, in this case 114 ft lbs@2400 rpm. Max torque is the most power used for loads. HP usually lags behind and is a bragging point but has no real bearing in fuel economy discussions. Racing where fuel is ignored except for how many laps can elapse before a tank runs dry and this usually means all out high rpm driving with the worst fuel economy. If you have the numbers for the DOHC, the values simply apply at the other rpm numbers but the ideal shift point is at max torque@ listed rpm. At 2400 rpm, the engine is just at mid range in its rpm band with peak torque@2400k rpm, barely anywhere near max rev limit of 5k. The highest torque at 2400 rpm would be considered the best time to shift without raising rpm higher since higher rpm simply means more fuel used per moment of time, reducing overall fuel economy. All this is presuming driving on flat roads where engine loads are not increasing or decreasing as in uphill or downhill. The dynamics of driving to achieve best fuel economy is never mentioned relative to a person's area that can be extremely poor because of steep hilly, mountainous terrain to completely flat roads for miles. Each person no adding terrain details leaves out why mileage is greater or lesser than others. With several variables, all are considered. If in hilly terrain, shifting at higher rpm would be interpreted a the car momentum loses speed quickly while shifting to higher speeds/higher rpm dictates safety over fuel economy. On flat roads, the ideal shifts are right at posted values. Slight variations in terrain and shifting 100-1000 rpm from specs will alter fuel economy so each person finds their ideal sweet spot to maximize fuel economy. I believe the specs for torque is the baseline to start with.

If you noticed, I discuss rpm and torque first, leaving out mph. Speed follows torque when maximum torque is reached before maximum hp. Whatever speed is associated with each gear at maximum torque relates to how fast a vehicle is moving. As anyone interested fuel economy, including myself, staying out of speeders and occupying the slowest lane ultimately determines fuel economy without disregarding driving safety. If I'm not mistaken, serious hypermilers may ignore driving safety by insisting on drastic low acceleration and maintaining speeds below posted limits to achieve maximum mileage at the cost of overall driving safety to everyone nearby. Some places have minimum and maximum posted speeds to prevent slow and high speeds outside this range.

Last edited by fdryer; 11-24-2017 at 07:19 PM..

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Old 11-25-2017, 12:36 AM   #16
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Default Re: What speed for absolute maximum MPG?

Since I have an SL2 with the dual cam engine, as I stated, torque specs are a little different at 122lb/ft @4800 rpm. However, the torque curve has an initial peak around 2400 rpm, then falls slightly until around 3200 rpm where it begins to rise slightly until its max at 4800 rpm. Here's a graph I found elsewhere on the forum:



While taking the torque curve into account during shifting is a good idea, for dual cam cars, waiting to shift until maximum torque is achieved isn't the best idea for fuel economy.

Also, as I mentioned, I live in Florida, which is pretty flat. Average elevation in my area 20 feet above sea level. However, that brings to mind a factor almost as big as hills, and that's altitude. Being at low altitude with dense air will definitely have an effect on fuel economy opposed to a person at 7000 feet elevation with pretty thin air. Air temperature also plays a big part, and since Florida is quite hot for 10 months out of the year that probably doesn't help my overall fuel mileage. Having thought of that I'll be interested to see if my mileage improves while we get to enjoy temperatures in the 70s and low 80s finally.

There are debates as to whether cold air or hot air is best for fuel mileage, (remember Smokeys Fiero?) but I won't start pondering that here since I was only curious about a specific speed that produces the best fuel mileage while cruising.

From the consensus, it seems that speed would be as slow as you can go without causing the engine to chug or bog, which could be as slow as 35mph depending on conditions. I'll reset my fuel data in Torque and let it calculate a few of my boring drives.

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Old 11-25-2017, 01:40 AM   #17
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Default Re: What speed for absolute maximum MPG?

Keep in mind all these people that say go as slow as you can in top gear for best MPG are getting LESS MPG than I got at 70mph in a stock, unmolested SC2 in the mountains.

During the oil embargo the national speed limit was 55, which IMO is still too slow for best MPG.

According to this Wikipedia artical, "Engine efficiency peaks in most applications at around 75% of rated engine power," this means the best RPM is when it's making about 93hp (124 * 0.75), which on the DOHC's around 2700rpm according to your dino chart.

At the end of the day you'll have to experiment with your daily route to find what works best for you. Pick a "goal" RPM for the week and track the results. Try to use the same station, pump and fill method over a couple tanks for best results, different pumps turn off at different times and if you leave the nozzle hanging out some it'll fill the tank more before shutting off.

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Old 11-25-2017, 02:36 AM   #18
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Default Re: What speed for absolute maximum MPG?

Try using the dohc torque values, same shift at (approx) 2400 rpm, not 4800 rpm.
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Old 11-25-2017, 09:04 AM   #19
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Default Re: What speed for absolute maximum MPG?

Quote:
Originally Posted by fetchitfido View Post
Keep in mind all these people that say go as slow as you can in top gear for best MPG are getting LESS MPG than I got at 70mph in a stock, unmolested SC2 in the mountains.

During the oil embargo the national speed limit was 55, which IMO is still too slow for best MPG.

According to this Wikipedia artical, "Engine efficiency peaks in most applications at around 75% of rated engine power," this means the best RPM is when it's making about 93hp (124 * 0.75), which on the DOHC's around 2700rpm according to your dino chart.

At the end of the day you'll have to experiment with your daily route to find what works best for you. Pick a "goal" RPM for the week and track the results. Try to use the same station, pump and fill method over a couple tanks for best results, different pumps turn off at different times and if you leave the nozzle hanging out some it'll fill the tank more before shutting off.
After all of the engineering data the secret to this is to operate the engine at the band of maximum efficiency. The trick is to be able to know in real time what this efficiency value is. Long before we were blessed with these electronically controlled marvels the answer to this was discovered. The device is a mechanical vacuum gauge that reads in inches of mercury vacuum. You can find these as dash mounted instruments. You can read this from the OBD-2 if you are happy with about a 1.5 second delay which makes the info rather out of date and useless. These are sometimes sold as performance meters or economy meters and gave red-yellow-green bands. By watching the vacuum gauge you will see when you get max efficiency for the conditions you are driving in.

The RPM bands for the DOHC engine between the higher gears is 800 RPM so if you play this torque peak game you will drop 800 RPM when you shift which will result in a very low vacuum condition until you recover most of the RPM after shifting. The DOHC mileage is highly tire size dependent and for max MPG the 185/65 or 195/65 tire will have the best results and the 195/60 the worst all else being equal. The 195/65 is a non-standard tire size and the mass marketers will not install it on your car.

Generally the DOHC will produce the best mileage with shifts at 3000-3200 RPM and moderate acceleration. This keeps up with traffic and avoids large throttle openings.

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Old 11-25-2017, 09:50 AM   #20
Waiex191
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1999 SL2
Default Re: What speed for absolute maximum MPG?

When my second kid was born, my wife resorted to a daily drive to get both kids to nap. Puttering around Monroe and Easton, CT on a bunch of 30-40 MPH roads she consistently topped 40 MPG. I topped 40 on a drive home from Syracuse once. Held 55 all the way back.

A bunch of geeky engineers (I was one of them) at Sikorsky had this conversation way back. There is a bunch of drag that is not speed dependent, like bearings and such. If you look at the power required to jam a car (or helicopter) through the air, it starts to get significant around 40. So, you don't want to go 5 mph because the bearing drag is no better than at higher speeds. You may as well go faster as you are increasing the M in MPG without taking a hit on G. Aero drag increases with the square of speed. Ideally your engine will be in a gear where the engine is efficient​ at he speed you are traveling.

...
Bryan Cotton
'99 SL2, owned since new
5 speed manual
Rebuilt at 204,067 September 2017
Engine, subframe, diff pin mod, brake lines, headliner, alternator, and so on!

Last edited by Waiex191; 11-25-2017 at 09:58 AM..

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