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-   -   Calculating actual mileage/MPG with oversized tires and stock gears (http://www.jeepforum.com/forum/f15/calculating-actual-mileage-mpg-oversized-tires-stock-gears-1717786/)

Holder350 10-24-2013 06:33 AM

Calculating actual mileage/MPG with oversized tires and stock gears
 
Make sure my math is correct.

Say your odometer reads 287mi and you just put 16 gal of fuel in your tank, your mileage would be 17.93mpg BUT you have 31" tires (or larger)

Stock tires on an XJ are 235/75R15

So doing the math to get that in a standard size would be

Code:

235mm X .75 = 176.25mm/25.4 (mm in an inch)= 6.93"

6.93" X 2 sidewalls= 13.86" + 15" of wheel = 28.86"

Now it gets interesting.....

We have to find by what percentage the new tires are larger.

31"-28.86"= 2.14"/.2886 (this is one percent of the original tire size) =7.4%

So actual mileage would be 287mi + 7.4% = 308.2mi

Code:

308.2/16= 19.3mpg
Now, some will scream that I used diameter instead of circumference....and apparently they didn't pay attention in geometry.

28.86 X 3.14 (pie.....:drool: )= 90.62

31 X 3.14 (p...i...e..... :drool::drool: )= 97.34

97.34 - 90.62 = 6.72/.9062 = 7.4% <- amazing isn't it.....ratios are the same because the circumference of a circle depends entirely on its diameter times a tasty pastry containing a filling of sweet ingredients.

So if you had say 33" tires on stock gears....

Code:

33"-28.86" = 4.14"/.2886 = 14.34%

287mi + 14.34% = 328.15mi


WXman 10-24-2013 06:48 AM

That's awesome, except you're using listed diameter and not real world measurements, so all your stuff is off.

Best thing to do by far is use GPS to verify the true difference in your speed vs. what the speedometer shows, then use that percentage to add to what your odometer is reading when you fill the tank.

hustler905 10-24-2013 07:53 AM

The math does seem unnecessarily complicated and likely to show you an inaccurate number. I used mile markers on the freeway on a long drive.

e.g. Went from 108 to 208, but my odometer showed a difference of 110.

Holder350 10-24-2013 01:45 PM

I agree that real world measurements would be different, but those measurements could be found fairly easily with a real world measuring device.(in that case I agree that circumference would be a much more accurate)

I still think its close, and defiantly over complicated haha!

Guess I was bored and my mind got to wondering.......happens way to often

Sent from my phone where correct grammar is difficult.

Dukcaln 10-24-2013 08:45 PM

31 inch tires on stock gears for a 96 zj, what will the difference in miles be on the odometer? my stock were 225/70/16 @ 710.5 revs per mile. The new tires are 31/10.50/15 @ 685 revs per mile. So if I travel 200 miles what will the odometer read for stock vs larger tires? If stock was 200 miles, then the larger tires will read less miles on the odometer, right?

Dukcaln 10-24-2013 09:52 PM

So I think I figured it out. My odom reads .8 for every actual mile I travel. So 200 miles on the odom is actually 250 miles traveled. So my last tank I figured 11.11 mpg. But since it is .2 off, it was actually 13.8 mpg. All city! Makes me feel better especially since I am lifted and have 31 inch tires.

Holder350 10-25-2013 12:07 AM

225/70/16 calculates to 28.4"

31-28.4 = 2.6/.284 = 9.15%

So take your odometer reading plus 9.15% and that's your actual miles traveled.

200 odometer miles = 218.3 actual miles

Sent from my phone where correct grammar is difficult.

WXman 10-25-2013 07:26 AM

See, every tire runs different. One 31x10.50 will be slightly different than the next 31x10.50. Also, the markers on the highway aren't perfect. They are installed by workers who measure off a mile at a time and there's lots of room for error.

I'm telling you guys... the easy way to do this and eliminate error is to use GPS and see what the % difference is between true and speedometer. Then add or subtract that % as needed to the odometer reading when you fill up the tank. If you do it this way, it takes all the measurement and guesswork and variances out of the equation.

misfit77 10-25-2013 07:52 AM

http://www.miata.net/garage/tirecalc.html

Helps with all my tire size questions

S8NNG8 10-25-2013 08:01 AM

I just round it off to 10%. 200 miles on 31's would be about 220+-. The XJ was also calibrated to a 225/75R15, not a 235.

Holder350 10-25-2013 08:33 AM

Quote:

Originally Posted by WXman (Post 17139378)
See, every tire runs different. One 31x10.50 will be slightly different than the next 31x10.50. Also, the markers on the highway aren't perfect. They are installed by workers who measure off a mile at a time and there's lots of room for error.

I'm telling you guys... the easy way to do this and eliminate error is to use GPS and see what the % difference is between true and speedometer. Then add or subtract that % as needed to the odometer reading when you fill up the tank. If you do it this way, it takes all the measurement and guesswork and variances out of the equation.

I use a GPS also on the 98 with 33s and have found my calculations within a couple miles over the course of a tank. (I use 14%)

Also didn't realize 225/75/15s were actual stock size.

Sent from my phone where correct grammar is difficult.

mschi772 10-25-2013 09:14 AM

Quote:

Originally Posted by Holder350
Also didn't realize 225/75/15s were actual stock size.

There were other stock sizes, but the stock speedo gear is most accurate for 225/75r15. 215 and 235 both appeared as stock depending on year and options. Some had 16" wheels, and I'm not sure what the stock tire size was there.

The speedo gear is SO easy to remove and change. Why bother doing all the math when you could just pop a proper gear in there to make you odometer and speedometer accurate again?

Sent from my phone

Dukcaln 10-25-2013 02:26 PM

my stock tire size per my sticker on the door was 225/75/16


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