Bigger tires and odometer mileage

Hello Everyone,

My first Toyota truck was a '78 (almost my age now) 2wd when they were just called Toyota Trucks. My next one was an'86 4x4 single cab with the bulletproof 22R. I sold that when it had 350,00 miles and it was running fine. I had only replaced the clutch and brakes. That one is still running and hauling firewood in the mountains of southern New Mexico. Then a '96 Tacoma Extra Cab with the V6. Then I had a 2006 Tacoma Sport 4x4 briefly. I didn't like it because it was so big and would get hung up in the frequent gullies I have to cross. Now I have a 2000 Tacoma Extra Cab 4x4 with the 3.4 V6. It is just about perfect in my opinion and only has 250,000 miles. On all the Tacomas I've had I replaced the recommended P225/75/R15 tires with LT 235/75/R15 tires. Almost all the ones I've seen have the bigger tires installed. They make my odometer run about 10% too fast, and the speedometer too. So all these trucks have about 10% fewer miles than the odometer reads, right? Or there is there a bigger error, something like Compound Interest? Is there an easy fix for this? I mean, they still go a hell of a lot of miles, but I am curious.

 

Welcome to TacomaWorld

 

Hypertech inline speedo calibrator. Unsure if they have them for 1st gens or not

 

Actually, it's the opposite. Overall, I think the effect here is much less than you're thinking it is and personally, I'd just know what the difference is so you're informed but other than that basically ignore it.

A larger diameter tire will cover more distance per revolution than a smaller diameter tire. Since your tires are larger than stock your speedo should read LOWER than your actual speed. And your odometer would also read lower than actual mileage.

Second, where's the math that got you to a 10% difference? There's no way that's correct. That tire size difference is pretty minor, mathematically the circumference is 2.1% different.

If you do the math your speedo should now be slow by 0.4mph at 20mph, 1.1mph at 50mph and 1.5mph at 70mph. On my truck (which is a 2nd gen) a 1" larger diameter tire actually made my speedometer more accurate. I tested it with GPS and with the larger tires the speedo and external GPS match dead on.

If you do the math on mileage, it's basically the same situation. It now takes 699 revolutions to go 1mi vs needing 713 revolutions with the stock tires. That difference of 14 revolutions is equal to ~104'. So, for every mile that your odometer racks up you're actually going 1mi plus 104' - which is ~2% difference.

I think this is relevant for speed so you can be sure you're not going faster/slower than you think you are. But I wouldn't worry about mileage. Mileage is a good gauge only because thats kinda all we have. It's not actually an accurate measurement at all of a vehicle's condition. It doesn't factor in how those miles were put on, how many revs the engine has been through, how many cold starts...etc.

 

Thank you very much for your considered and informative reply. I think my first thought on the subject was that yes, it makes sense that a larger diameter tire would cover a greater distance at the same number of revolutions as a smaller one, everything else being equal. But when I reset my trip odometer to zero at the start of a highway that has mile markers, when I reach the tenth mile marker the odometer reads 11. when I reach the 20th mile marker it reads 22. This happens on all the highways I've tested. None of my trucks have been new when I bought them, so who knows what rear-end ratios they were originally equipped with. But I would think the only different rear-end ratios would be for the different engine and transmission options, and I have no reason to believe the previous owners would have changed any of that. One I bought from my sister and I know she didn't and she bought it new.

Anyhow, I will take your advice and "basically ignore it."

Thanks again.

 

This math hurts my head. All I know is when I upsized my tires, it corrected the 3mph variance between my actual speed, and the speed shown on my speedo. Be fore upsizing, I was actually 3mph slower than my speedo was reading.

 

Linear relationship, not exponential. Increase in circumference from stock to bigger tire is what the odometer is short.

Maybe the vehicle didn’t have bigger tires its whole life, so may not impact the odo 100%.

 

The math is not complex.

It is simply based on the circumference of the tire. Circumference = Diameter * 3.14.....

A ratio can be set up between the small and big tires.
Cbig=Dbig*3.14
Csmall=Dsmall *3.14

3.14 will cancel from both to leave just Circumference and diameters.

Or it simplifies further to be just a simple ratio between the diameters.

Example: moving from a 31" tire to a 33" tire. 33/31 = 1.06. The 33" tire is 6% bigger circumference than the 31" tire. Meaning the speedo and Odo will read about 6% low.

 

Tire calculator at the bottom of the home page