I recently upgraded my stock 29 inch goodyearHT's to Dean 315/75/16 mud terrain sxts (35s). Im wondering if anyone know the mileage difference on the odometer, mph and mpg... Or how to calculate this?

 

Of you have a navi, turn it on and compare the two.

 

When I changed to 35's I seen about a 10% offset (speedo said 60 and I was actually 65-66).. When I went to 37's this jumped to almost 20%

 

I think the faster you drive, the further off it is, so you can't really say the difference is a solid number, like 5 mph. If you have a gps of any kind, it should tell you how fast you are truly driving.
Check out the AEV Procal. It will correct this, as well as some other options, and its a lot cheaper than a full blown programmer.

 

I agree. The higher the speed, the more the speedometer is off.

 

Thanks for the input. Im gonna have to wait a couple months on the programmer. Until then is there a fairly accurate way to figure the difference on the odometer and mpg?

 

Yep. Get the circumference of your old tires and the new ones. Take your new tire circumference and divide by the old one. This should give you a value similar to 1.12 (example only) and from that your mph should be 12% faster than what it reads.

 

Well your mpg just take your miles divided by the # of gallons you fill up with?