Changed tire size

According to TireRack the Integrities have a circumference of 76.65" (827 revolutions per mile) and those Kuhmos have a circumference of 78.54" (807 revolutions per mile.) 1-(807/827) = 2.4% difference. So if the MFD said 55mpg it should say 53.7mpg now. Unfortunately it says 47mpg which is the equivalent of 48.1mpg if you believe that all these numbers are accurate. My MFD reads about 3% high, so if I swapped for slightly larger tires (fewer revs per mile) it would be pretty much dead-on.

 

Note as well that new tires tend to give poorer mileage for the first few thousand miles you put on them, due to higher rolling resistance. So even if you correctly calculate the mileage you are getting, it should be lower than what you were getting with the old tires.

Use a GPS to get a good MPH reading comparison with the car. That will give you the correct "real" difference. The calculated difference isn't always accurate.

 

Is there a way to program the computer for tire diameter? Since the regular and touring models have different tires, there might be a way to specify tire diameter so that speed and odo will read correctly. Or maybe they load that in at the factory and there is no way to change it by the owner? Just seems like there might be an "easter egg" for this.

 

Here is a handy calculator for tire size: Tire Size Calculator

 

<div class='quotetop'>QUOTE(abq sfr @ Aug 4 2007, 03:47 PM) [snapback]490639[/snapback]</div>

No. The Touring 16" tires are lower profile than the standard 15", to make them the same diameter. This has been discussed before, and apparently, there is no adjustment in the software or hardware available from Toyota.

 

<div class='quotetop'>QUOTE(brick @ Aug 3 2007, 08:58 PM) [snapback]490248[/snapback]</div>

Brick,

The original Integrity tires were probably 185/65R15 (non touring) with a revs per mile of 855. If his Original tires were fully worn, the tread would have lost about 0.25 inches which accounts for another (0.5/24.5) 2% of change in MPG reading.

JeffD

 

<div class='quotetop'>QUOTE(bshef @ Aug 3 2007, 04:23 PM) [snapback]490213[/snapback]</div>

These 2 sites will help you figure out how much larger, or how much smaller your diameter
has changed due to tires being changed.

www.1010tires.com/TireSizeCalculator.asp?action=submit


MileageCharts\Tires\Tire size calculator.htm [http://www.miata.net/cgi-bin/tirescgi]