calculating milage with larger tires

JeepinMaxx · **Posted:** Wed Jul 02, 2008 11:02 am

I calculated my mileage yesterday (using odometer and actual gallons pumped) and it came out to 15.5 MPG for the tankful.
BUT - since the odometer is calibrated for stock tires - how do I figure out the REAL mileage since I'm running 32" (235/85/R16)? I know that the speedometer is off by 5 MPH at 60PMH (actual via GPS is 65).

Is there some sort of calculation I can make?

JL Rockies · **Posted:** Wed Jul 02, 2008 11:33 am

I have the same size tires and I have calculated with GPS that the factory ODO is off 15%. So when I fill up I add 15% to the ODO and then divide.

ObiHann · **Posted:** Wed Jul 02, 2008 11:59 am

Would it be possible to get the computer reset so it is accurate?

Cobalt-60 · **Posted:** Wed Jul 02, 2008 12:09 pm

ObiHann wrote:

Would it be possible to get the computer reset so it is accurate?

Yes it would. It is called a Pinion Index Reset for your BCM. See here:

http://www.lostjeeps.com/forum/phpBB3/vie ... 29&start=0

and

http://www.lostjeeps.com/forum/phpBB3/vie ... highlight=

Dave08 · **Joined:** Fri Nov 02, 2007 4:17 pm **Posts:** 250

If your KJ came with 225/75/16's it should be 8.3% off so multiply by 1.0839 which would make 15.5= 16.8

JeepinMaxx · **Posted:** Thu Jul 03, 2008 11:41 pm

Thanks for the info and links

Measured the rev's per mile (based on the actual circumference of the tire at rest is 98.5") I got 643 - that brings the mileage up to ~17l.8 mpg on the KJ with 32's. I still think upgrading the gearing would help..

I have 4.56 with 33's on my YJ and get ~ 18 mpg on the highway :lol:

JL Rockies · **Posted:** Fri Jul 04, 2008 3:04 pm

Dave08 wrote:

If your KJ came with 225/75/16's it should be 8.3% off so multiply by 1.0839 which would make 15.5= 16.8

assuming the ODO was right in the first place.

Dave08 · **Joined:** Fri Nov 02, 2007 4:17 pm **Posts:** 250

JL Rockies wrote:

Dave08 wrote:

If your KJ came with 225/75/16's it should be 8.3% off so multiply by 1.0839 which would make 15.5= 16.8

assuming the ODO was right in the first place.

Assuming he was only checking to see changes in mpg from stock to now

JeepinMaxx · **Posted:** Fri Jul 04, 2008 11:33 pm

Dave08 wrote:

Assuming he was only checking to see changes in mpg from stock to now

Just trying to figure current MPG's with the stock odometer reading and 32's - don't care about the MPG with the stock tires...

Pote · **Posted:** Tue Jul 08, 2008 6:07 pm

JeepinMaxx wrote:

Dave08 wrote:

Assuming he was only checking to see changes in mpg from stock to now

Just trying to figure current MPG's with the stock odometer reading and 32's - don't care about the MPG with the stock tires...

when I got larger tires, the shop had a computer program built into their checkout system that told me the new rpms of the tires and the new speed on the speedo. for example he told me that with my original reading at 60 mph, I will be driving 61.5 with my new tires.

tjkj2002 · **Posted:** Tue Jul 08, 2008 6:08 pm

Pote wrote:

JeepinMaxx wrote:

Dave08 wrote:

Assuming he was only checking to see changes in mpg from stock to now

Just trying to figure current MPG's with the stock odometer reading and 32's - don't care about the MPG with the stock tires...

when I got larger tires, the shop had a computer program built into their checkout system that told me the new rpms of the tires and the new speed on the speedo. for example he told me that with my original reading at 60 mph, I will be driving 61.5 with my new tires.

That's assuming your speedo was correct in the first place.

daspes · **Posted:** Tue Jul 08, 2008 9:31 pm

tjkj2002 wrote:

Pote wrote:

JeepinMaxx wrote:

Dave08 wrote:

Assuming he was only checking to see changes in mpg from stock to now

Just trying to figure current MPG's with the stock odometer reading and 32's - don't care about the MPG with the stock tires...

when I got larger tires, the shop had a computer program built into their checkout system that told me the new rpms of the tires and the new speed on the speedo. for example he told me that with my original reading at 60 mph, I will be driving 61.5 with my new tires.

That's assuming your speedo was correct in the first place.

Which would probably be a false assumption...

rangerd77 · **Joined:** Sat May 03, 2008 10:48 am **Posts:** 512

you guys should try running what i run, the wider the worst. but the streeters are on there now just for a limited time only. but i really dont see how some of you guys out there wheel with those skinny booty tires. just doesnt make sence to me

i ran mine dry the other day when the TSL's were on. 19 1/2 gal. tank ='s 215 miles
and today i did the same thing with the the TOYO's 19 1/2 gal. tank ='s 320 miles

so yes i am glad that in that way that i have those right now with the price of fuel. but for anything alse the TSL's go back on.

USPLibby · **Joined:** Thu Jul 28, 2005 12:29 am **Posts:** 377

I had the dealer fix my odometer, they type in the new tire spces, after I put on 265's. It seems pretty accurate, I have been keeping perfect time with the roadside mileage markers.

Tokyojoe · **Posted:** Thu Jul 24, 2008 12:04 am

Maxxis Bighorns 265/75/16 - coming out at 31.8" and calculations say 637 rpms for a 265/75/16 = 31.6" tire. The Maxxis are slightly bigger. Any idea if this will be good enough? Anyone know where to get it down to the right size?

cmohr · **Posted:** Thu Jul 24, 2008 12:42 am

Tokyojoe wrote:

Maxxis Bighorns 265/75/16 - coming out at 31.8" and calculations say 637 rpms for a 265/75/16 = 31.6" tire. The Maxxis are slightly bigger. Any idea if this will be good enough? Anyone know where to get it down to the right size?

From memory taking Static Load into account, a 265/75/16 should be 652 revs per mile. Is the stting top key into the ECU as a "Pinion reset" to correct your speedo.

You cant just use the overall diameter, as when you have them underload (on the car) that Radius from hub centre to the ground is shorter than hub centre to top of tyre, thus a smaller circumference.

Have a look here, not "THE" same tyre, but, specs are close enough to make a comparison, Look at the Revs at 45mph coloumn.

http://www.bfgoodrichtires.com/specs/mu ... /1152.html

calculating milage with larger tires

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