I've searched and everything I find talks about the speedometer. I've got a question regarding the odometer. I recently upgraded to 315's and my speedometer is off which I expected. My question, is there a way to actually calculate what my milage should be? Say my trip mileage says I drove 140 miles, what's the actual mileage traveled.

I just purchased a speedometer calibrator from a fellow member and from what I've read this will get my odometer back on track. If I'm wrong please let me know.

Plug in the numbers here: Tire Size Calculator - Compare Tire Sizes (current tire is the stock one, new tire is the 315)


Get the number for %error on revolutions per mile, drop the minus sign..

Ex: 5.2% = 0.052 (your number will be different)
Multiply your Odometer number by 1.052 = Actual distance traveled.

MPG= Actual distance traveled / Gallons used.

Mine is about 4-5mph off but it doesn't mess with your odo/trip.. If that was the case we'd all have falsified information on how many miles on are on our trucks.

Thanks Ripcord, I obviously didn't spend much time paying attention in math class growing up.


Revolutions per mile (km)
Current Tire(265): 637.23 (395.96)
New Tire(315): 586.93 (364.7)
Difference: -50.3 (-31.26) -7.9%

So if my trip reads 140 miles am I multiplying it by 1.079? I'm not sure why my difference is a negative number. If I forget about the negative i get 151.06.

Its negative because there are less revolutions per mile.

Are you sure you put the right numbers in? When I did it for stock (265/70/16) and 35's (315/75/16) I got like 11.something.

I'm not sure if I'm correct but if I start my 31" tire next to my 35" tire and let it roll one revolution wouldn't my 35" tire travel a greater distance in one rotation versus the 31" tire? How would that not effect the odometer if the computer/sensor was not calibrated for a 35" tire?

I'm pretty sure I input the numbers correctly. 267/75R16 were my original tire size however I did not check the door jam label. I recall seeing 225 or 245 as the original tire size. 315/70R17 are my new tire size. If I input 315 as my current tire size and 265 as my new tire size I get a positive value.

265/70/16 is the stock size, not 75.

Other than that, youre good to go. And no, larger tires will not change the ODO readings, making it inaccurate..

@ryan, you are absolutely correct. Last week, my sending unit in the fuel tank started acting up, so my fuel gauge doesn't work properly. As such, I've been calculating fuel consumption by the odometer. The other night, it was late, I was tired and just wanted to get home, so didn't get gas when I should have. I thought "Meh, it's cutting it close, I'll make it." Nope, ran out of gas halfway home. Luckily, I was able to coast to some farm property we own, and we have gas there for tractors and such. Point of the story is, yes, larger tires affect the odometer. I sat down and did the math just to be sure. I had traveled something like 15 or 16 miles more than the odometer read on that specific tank of gas

Thanks for the help. I really wish my trip was off because my gas mileage is so terrible around town. I really need to re-gear. I've got about a 1/4 tank of gas left and I've gone 140 miles.

I keep confusing myself by how I write.. When you change to larger tires, that change is not reflected in the ODO & Trip readings.. Which is why you have to calculate to find the error that is now present between the ODO readings and the actual mileage driven. Hope that makes sense.

And you need a tune up, read the buyers guide post #7.

You're still going the same distance no matter what tire size.. your revolutions per mile is the only thing that changes.

Yes but you odometer is calculated by the revolutions your wheel or hub or whatever makes. It doesn't actually know how far you've driven, it just knows how many revolutions the hub has made and calculates the mileage based on the revolutions and the stock tire size.

I could be wrong but that's how I've always understood it.

If my 4runner thinks(calibrated) for a 225/70R16 tire and I have a 315 tire. how is it not off? Even revolutions per mile would be off.

Thats correct