Tire size, speedometer, and math

Fact 1: While rotating my tires the other day I noticed that the spare tire is not the same size as the four on the ground. The four on the ground are 260/70-16 (which I think is stock), and the spare is 260/75-16.

Fact 2: The speedometer consistently reads higher than actual speed as compared to a GPS.

So, I did some math.

260/70-16 have a height (H) of 260mm x .7 x 2 + 16, which works out to H = 30.33". Given that height they have a circumference (C) of H x pi, or C = 95.28". We'll come back to that later.

260/75-16 have a height (H') of 31.55" and a circumference (C') of 99.12"

A mile is 5280 feet, or 63,360 inches.

The circumference ratio of the 75's to the 70's is C'/C = 1.04 (watch for this number to come back up).

Tire circumference ties directly to the number of times a tire rotates per a given distance. The 70's will rotate 63360/C = 665 times per mile. The 75's will rotate 639 times per mile. I think the higher number of rotations per mile will result in a higher speedometer reading.

When the speedometer reads 70 mph the vehicle is actually going about 67 mph. 70/67 is about 1.04. (I told you that would come up again)

My theory is that some time in the past a previous owner put 75's on the truck and had the speedometer recalibrated for that size tire. Then someone put the stock tire size back on, but didn't recalibrate the speedometer.

Do you agree with my logic?

If so, can I recalibrate my speedometer myself or do I have to take it to someone? If I take it to someone who, and how much does it cost?

 

I think you mean 265, I'm not sure what you are referring to when you say recalibrate your speedo. You mean have it show the actual MPH for the tires you are running? Or show the actual mileage on the vehicle after you calculate how long you've been running 'X' tire for?

 

Dammit. It is 265. Now I have to redo the math.

As to the speedometer, I meant the speedometer, not the odometer. I want to know how fast I'm actually going.

 

With the correct width the ratio changes to 1.034. Close enough that I'm sticking with my theory.

 

This link will save you a lot of math:

http://www.tacomaworld.com/forum/tirecalc.php

In order to correct the speedometer, you have to swap out the driven gear. They come with all different numbers of teeth, and you can change the number of teeth to correct what it reads. The speedometer assembly bolts into your transfer case on one side toward the rear near the output.

 

Before you get too worked up over this error you've discovered, it is important to note that Toyota built in the error so it would meet a federal requirement for the stock speedo/odo not to read too low.

As I understand it, under federal rules they are allowed to be under by up to 5% and are given no leeway for being over.

The end result is that the 1st gen Tacoma speedo/odo has an error very much like what you have discovered.

FWIW, on my truck, when I put on larger tires, from stock 31s (265/70-16) to 32s (265/75-16), the 32s made the speedo/odo agree with the GPS speed readout.

 

I appreciate it, guys.

I'm going to see if I can find the gears that are mentioned here, and see what's in my truck now.

If I can easily and cheaply get it more accurate I would like to.

 

Exact same here.

 

lots of good info here: http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=46&s_kwcid=AL!3756!3!50155054213!e!!g!!tire%20size%20explained&ef_id=VDK5AwAAAbO8HPsg:20141007221615:s

I always used to think it was height /width /rim size but its actually width /height /rim size

 

It's actually height (in millimeters)/ratio of width to height - rim size.

The width is 265 millimeters. The distance from the ground to the bottom of the rim is 70% of 265. Double that because there is tire above the rim, too. Convert to inches and add the wheel diameter (16). That gives you the height.