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Old 10-27-2001, 06:28 PM   #1
Tim94SC1
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Dizzy Tire size and odometer reading

I've posted quite a bit lately about my gas mileage woes, but there's something I'm trying to figure out here...Like I've mentioned before, I'm running 15" SC2 wheels with 195 60 R15 tires which are the stock size for these rims. My car came with stock 14" wheels with 175 70R14 tires. Thanks to info from someone on this board, I found a tire calculator that tells me that my tires are 2.4% too slow, if my speedometer shows 60, I'm really running 61.4 mph. It also shows that "Revs./mi" (revolutions per mile?) are 853.0 for the stock tire, 833.0 for the new tire. It has no info for the odometer reading. Is anyone good with figures to help me figure out what the difference on the odometer will be, based on these figures? Since I'm trying to figure my mileage, this is critical.

Thanks a lot
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Old 10-27-2001, 07:12 PM   #2
Heartbeat - '95 SL2
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The difference at the odometer is the same 2.4% error. So, to calculate the correct mpg:

1) calculate mileage as you normally would, i.e. full tank trip odometer mileage divided by number of gallons to refill tank. This is the inaccurate mileage.

2) to adjust for the correct mileage, take the inaccurate mpg and divide by 0.976, i.e. 1 minus .024, or 100% minus 2.4%.

So, if you calculate that your getting 20 mpg, you're really getting 20.49 mpg.

Don't forget, you've also increased the width of the tires, which also affects mileage, if you're comparing mileage before and after the tire switch. There are no hard numbers for this, but I've heard it can be a few mpg difference. Even with this taken into account, my car has always had the stock 195/60-15's, and gets 25 city, 35 hwy.

Try the open hwy mileage test I suggested, if you want to eliminate stop light traffic as the cause of your bad mileage. Also, read Ssicarman's post asking if your engine is getting up to operating temp. He's basically saying that aside from the temp sensors, if the thermostat is stuck open, the car may run fine, but if cold, is running richer than it should. I believe I posted before that I get horrible mileage due to this when making short stop n go trips.

Also, it's much easier if you continue to post related questions in a single thread. That way all of us can review what's been covered before. This is what, the 3rd or 4th thread? Too confusing for me to remember if it's the same person, or different people, and what I've answered to who.
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