2trackin

trying to calculate my mileage after an upgrade to 31" tires.
Stock being 225/70/15 (27.40 in diameter) to a 31x10.5x15 (30.97 in diameter). That is an increase of 13.03%. Could I then assume that each rotation is carrying the vehicle 13.03% further than the previous tire?
and that I should add 13% to mileage shown on the odometer?

If so, assuming my spedometer/odometer were correct with stock size tires. I have gone from an average of 17.34mpg with 225's to 12.05mpg with 31" mud terrains (temporary and really really crappy no name bias plys).
I also would like to add that the 17.34mpg was at stock ride height and a mixture of mostly city and some highway. Where as with the 31"s current on there, its all city and never above 45mph. But alot of stop and go.
I would have liked to be able to test accurately with highway mileage, but I do not trust these tires above 45 not to mention they make you go wherever on the road they want you to.
I'm hoping that the 31" km2's I'll be throwing on soon will help out some.
I wouldnt mind hearing of any small mods that may help fuel mileage at all.

Firehawk068

I would say, find a good stretch of road( interstate works best)
Find a mile marker, and zero your odometer. Travel about 20 miles or so to another mile marker, then check your odometer, and adjust your milage calculations accordingly.
I use my GPS.......you could borrow someones to check miles also.

2trackin

For the interstate run, ill have to wait for my new tires. But I may try out the gps first so I can get an accurate number with these tires as well.

Wiclassic

first link tells you which speedo gear you need based on tire size and gear ratio, second link is from Quadratech, Just find the year of your Jeep and the proper number the first link gives you, install and no more guessing


http://liftyourxj.com/tweaks.html#Speedo

http://www.quadratec.com/products/pr...ears&submit=Go

TheJerm

Its not 13% off cause my 33's were 13.5% off. You need a GPS to be able to calculate your speed then you can figure out the % difference. There will be error b/c say 10% off at 10mph is different than 10% at 70mph. I noticed a .5 mpg increase going from hand calculation to replacing the speedo gear.

2trackin

I did the exact measurements. It was 13% difference. Read my measurements and do the math yourself. The difference of your 225s to your 33s very well may have been 13%. Different tire manufacturers sizes will very slightly among different brands, and even tread designs.

TheJerm

My gf runs 32's from the JK and I run true 33's which are basically brand new. She had 225x 70 previously and I had 235x75. We used GPS's and hers is 12.5% off and mine was 13.5% off. This was calculated be comparing the GPS to the dash. Back when I ran 30.5's, I was off ~5% off IRCC.

Just checked your math, you forgot to divide by 2. 27.4" to a 31 is 3.6" bigger in diameter but the calculation needs to be made by the radius of the tire. Or else your "double counting" the 1.8"

cornrush

This is great I've been looking for a thread like this

no rdplz

In short, multiply your odometer reading by 1.112 then divide that number by the amount of fuel you put in. If you want i can give you the math for finding that number later as I'm currently at work.

JCKustoms

If you have 3.55 gears, I would say you would be off by 6% at all speeds. So if your speedo reads 25 you are going 26.5...at 55 you will be doing 58.3... And so on... But like others have said, double check with a gps because all 31" tires are a different diameter! :-)

2trackin

During one complete revolution, A 27.4" tire will travel 86" and the 30.9" will travel 97" that is 13% further. Therefore my spedometer and odometer will be 13% slow.
IE: if I traveled 100 miles with the 27.4" tires. I traveled accurately on the odometer 100 miles.
Then if I put the 30.9" tires on, drove that same 100 miles without any calibration for larger tires, I would have actually traveled 113 miles, while the odometer would still read 100 miles.
I think you may have mistook 2*pi*radius and pi*diameter.

This calculation is relative to calculating fuel mileage prior to recalibration of the odometer.
Thus, by adding 13% to the mileage my odometer shows before dividing gallons used into miles driven, I have my new fuel mileage or "mph"
All of this being said, given the original speedometer was accurate.

Dan91

I forget the exact size I decided stock tires are but I think I used 28" for my calculation. With 33"s it ended up being approx 18% larger, so that's what I multiple by for mileage. For estimating my speed, I just use 20% cause its easy to figure out in my head.

TheJerm

I'm sorry but I really think your wrong. Having ran 30's, 32's, 33's and 35's in the past, I would say you 6-8% off, not 13

JCKustoms

Use a GPS...it doesn't lie! lol

JCKustoms

You are correct! You can't just add the difference from the tire diameter...