Just put on a set of 265/75/16/D BF Goodrich AT's and I have some questions? According to the attitude monitor my speedo is off by 2 mph. My question is what do I multiply my total # of miles traveled to get an accurate # of miles to check my MPG. About how much will this throw my mpg computer off? Also are the bigger tires going to effect my mileage a great amount? They look alot better than the stock 245's. What is the best level of juice to run in for mileage? Thanks for any and all input.





Jason

Okay, here's how to figure it out regardless of tire size.


It's a simple ratio. Figure out the total size of each tire/wheel combo you want to ratio (in this case the stock, and the 265/75/16s).


To figure out the total size take the wheel size, plus the tire size. Wheel size is easy, in this case 16". Tire size is easy to, but requires a conversion factor (mm to inches). Take the 245 (stock), and divide by 25.4 (mm/inch). That gives you 9.646. That is the tire width. To get the height, take 9.646 and multiply by the ratio (70 for stock), and then multiply by 2 (for the tire on both sides of the wheel). This is 9.646 x 0.70 x 2 = 13.5


Next take the 16" for the wheel and the 13.5" for the tires and you've got about 29.5". That's the reference tire for you.


Do the same with your 285/75's and here's what you get. 16" + (285/25.4 x 0.75 x 2) = 32.8"


Now to find out what you need to multiply your stock speedometer by to find out actual speed, just divide your reference by your new tires. In this case divide 29.5 by 32.8 = 0.89867 (I didn't round anything off for that one).


Here's a quick chart for your particular case.


Actual Speed Speedometer will read
25 22.5
40 36
55 49.5
60 54
70 63
100 90


Hope it helps,
DennisEdited by: baimpala

Also note that this does NOT take into account any tire growth due to rotational forces on the tire, or shrinkage due to sidewall flexing.


Dennis

I add 3.5% to my miles traveled, by multiplying 1.035 by the trip odometer reading. I have been running 265/75/16E Bridgestone Dueler AT Revos, since two hours after driving my truck off the dealers lot. MGP will go up about .5 after the calculation. I typically get 16 mpg around town and 19 on the highway.

Thanks guys for the input. Guess I should have paid more attention in school huh? Thanks again





Jason