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Discussion Starter · #1 ·
so im just curious....

with 35" tires added (4.10 gears), and Speedo NOT Recalibrated....

what is true MPG when computer shows 15 mpg?

also at 55 mph.....what is true speed?

im guessing 7-10% on both?
 

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I went from 29 to 32 and at 55 I am doing 61 according to GPS.
 

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buy or borrow a portable GPS to see what your speed is.

As for MPG, I dont think there is going to be a direct conversion. Just calculate it the old fashioned way. Write down mileage at next fill up, then at the fillup after that, not how many miles you have driven since you filled up last and divide the number of gallons used to fill up by the miles you put on.
 

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Discussion Starter · #4 ·
buy or borrow a portable GPS to see what your speed is.

As for MPG, I dont think there is going to be a direct conversion. Just calculate it the old fashioned way. Write down mileage at next fill up, then at the fillup after that, not how many miles you have driven since you filled up last and divide the number of gallons used to fill up by the miles you put on.
that math wont work....

with the larger tires.... your "miles driven" will show less than you actually traveled do to the larger tires.. it will give yu the same number your dashboard is telling you doing it your way.

just was curious...not a big deal... thought maybe someone had a formula.
 

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that math wont work....

with the larger tires.... your "miles driven" will show less than you actually traveled do to the larger tires.. it will give yu the same number your dashboard is telling you doing it your way.

just was curious...not a big deal... thought maybe someone had a formula.
doh! how true you are. BUT with some additional math you could correct for distance driven knowing the circumference of old tires and new.
 

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The circumference of your tire is (35/32) that of standard. Thus all linear functions of distance traveled, such as MPG and MPH, are calculated using the same ratio.

More precisely, the ratio is (New tire diameter)/(Standard tire diameter).
 

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Discussion Starter · #8 ·
The circumference of your tire is (35/32) that of standard. Thus all linear functions of distance traveled, such as MPG and MPH, are calculated using the same ratio.

More precisely, the ratio is (New tire diameter)/(Standard tire diameter).

so then about 9.4%


32 x 9.4% = 3.008 + 32 = 35" tire

I guess that makes sense : /
 

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doh! how true you are. BUT with some additional math you could correct for distance driven knowing the circumference of old tires and new.
The circumference of your tire is (35/32) that of standard. Thus all linear functions of distance traveled, such as MPG and MPH, are calculated using the same ratio.

More precisely, the ratio is (New tire diameter)/(Standard tire diameter).
so then about 9.4%


32 x 9.4% = 3.008 + 32 = 35" tire

I guess that makes sense : /
Or you could quit being cheap and buy a procal or flashcal :tomatoes:
 

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Put your information in that link and you can easily compare to see the difference at 55mph.


You didn't list the year of the vehicle, nor if it is auto or stick (that information is part of the calculaton because of the 5th/6th gear ratios)....but assuming it is a +2012 auto.

5th gear at 55mph.

35's will turn 1797 rpms
32.1's will turn 1965 rpms

Your speedo is off by roughly 9% in that scenario.
 

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If you have a JK with the 3.6, you could use the gearing chart to figure it out. Just figure out what your RPM should be with your gears and tires, then go drive on the highway. Match the RPM then compare to your indicated ground speed, then simply do the math. This would at least get you into the ball park.

With your tires and gear, you should be at 2288 RPM with an auto, or 2197 RPM with a manual.
 

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