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Old 10-03-2012, 04:39 PM   #21 (permalink)
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Quote:
Originally Posted by ALS View Post
I did this with my old Volvo and got better fuel mileage. I went from 195/60/15 to 205/55/16's. The reason I got better mileage was the slight increase in tire diameter which translated into lower rpms on the HIGHWAY. Any increase in wheel tire mass or width will get you lower fuel mileage in a daily driver.

In the Gen III's the 17's will cut your mileage about two to three miles per gallon over the stock 15's.
I have wondered on this, with the Prius specifically. The Prius has a CVT, so (at least in theory) the RPM's should be related to load, not to speed. Does anyone know the answer to this question "in practice"?

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Old 10-03-2012, 05:00 PM   #22 (permalink)
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Replaced the Firestone Turanza EL400 tires with Firestone Ecopias on our 2008 Prius and on the drive home from Firestone I noticed the mpg meter tick upwards a few tenths. Let's see what the next few weeks will show.
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Old 10-03-2012, 05:12 PM   #23 (permalink)
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Did you adjust the computer to tell it you had new tyres ?
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Old 10-03-2012, 07:52 PM   #24 (permalink)
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The 17's vs 15's prolly didn't change your mileage noticeably. The 17" rim/tire combo was most likely much taller than the 15" rim/tire combo giving you an "apparent" lower MPG reading when in actuality it's about the same.
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Old 10-03-2012, 08:40 PM   #25 (permalink)
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Tire diameters

I don't see that you've corrected for tire overall rolling diameter between the wheel and tire sizes you are comparing. You may have significant odometer and speedometer error/ changes as a result of the wheel and tire swaps that you should correct for. I agree that a smaller/lighter wheel and tire will often lower rolling resistance. Also, narrower tires probably will have less drag. Tire Rack publishes accurate tire diameters for each tire they sell.
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Old 10-03-2012, 10:01 PM   #26 (permalink)
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Tire Diameter

I see a few of you asking about tire diameter. Yes it's true, the diameter changed, by 1/2 inch or so. Not a significant difference in circumference (~2%), and certainly would not account for much of the change.
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Old 10-04-2012, 11:48 AM   #27 (permalink)
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Diameter

Yes, but 2% is nearly 2 mpg in the 60+mpg range you're talking about.

Here's an easy way to correct for odometer error. Drive ten miles by the mile markers on an Interstate and compare your beginning and ending odometer readings to to the ten miles. If your odometer lost 1/10th mile in ten miles (not uncommon), when you calculate you mileage multiply the miles driven by 1.01 and divide the answer by the gallons used for your tankful. If your odometer reads high for the ten mile test, subtract out the extra miles when calculating your mileage.

Another problem with measuring mileage, especially with very high mileage cars is getting consistent fill ups tank after tank. With modern cars with evaporative emissions systems you're not supposed to top up the tank. What I try to do locally is always fill my car at the same side of the same pump and use the auto shut off (no further topping up or rounding off). A tenth or two gallons of gas one way or the other can give big swings in mileage on your Prius.
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Old 10-04-2012, 03:27 PM   #28 (permalink)
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Quote:
Originally Posted by campisi View Post
The 17's vs 15's prolly didn't change your mileage noticeably. The 17" rim/tire combo was most likely much taller than the 15" rim/tire combo giving you an "apparent" lower MPG reading when in actuality it's about the same.
Glad someone else noticed this - I was on vacation when this posted, so I could read it, but not comment. The taller wheel/tire combo will cause under-reporting of actual mileage (and lower speeds, too. This means some gain could be the lower resistance from traveling a little slower...). Certainly the width and compound (rolling resistance) will impact the mileage, though. You should do some math based on the wheels/tires on there before vs now, so you can get a rough estimate of the actual mileage, assuming the car from the factory came with 15's (so it was calibrated for 15's). I'm curious to see what the larger combo actually did to your mileage.
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Old 10-04-2012, 06:56 PM   #29 (permalink)
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According to TireRack.com, the Ecopias I got are 24.5" in diameter, the same as what I measured on the 17's (17's were much lower profile). They have 10/32 of tread, meaning that when both sides are considered, there is 20/32 of tread, or over half an inch. I am going to go ahead and say that the 2% difference that may exist is negligible in this case, especially since we are talking a 20% difference in FE (not 2% or 5%). I will measure actual circumferences and report back, but don't expect anything earth shattering.
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Old 10-04-2012, 07:14 PM   #30 (permalink)
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Wow! Those must have been REALLY low profile - can you post some pics of that? I'm honestly curious about how something like that would work on a Prius. To be only a 2% difference they must have been rubber bands. Sounds truly obnoxious - not saying that to be a jerk, but because I'm getting a ridiculous mental image.

What's the aspect ratio and size on the current tires vs the old ones?

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