Can A Change In Tire Diameter Be Accurately Calculated ??

[billt]

Here's the deal. I need some help with the math involved with this. My truck came equipped with P-235 X 15 tires from the factory. I don't know the rolling circumference of that tire. After they were worn, (around 22,000 miles), I replaced them with 31 X 10.5 X 15 tires. Obviously these tires have a larger rolling circumference that has effected both odometer and speedometer accuracy. I have had that size tire on the truck ever since.

My truck now has 152,000 miles on it. Which means for the last 130,000 miles it has registered fewer miles, and a lower indicated MPH because of the larger diameter tires. Is there a way to calculate the difference, so I know with some degree of accuracy, how many actual miles my truck has on it?

[MikeBjerum]

Tire manufacturer's have spec sheets on their tires giving the full dimensions of inflated tires.  You can easily adjust for wear by deducting the amount of tread depth lost.  Following that it is simple math to come up with percentage of change + or -.  Be sure and use the actual manufacturer data, and not generic figures for most accurate figures.

[Solus]

Ok...I guess we can assume your speedometer was calibrated to mark a mile driven by counting wheel rotations based on a "standard" or "assumed" wheel  radius (which is used to calculate circumference)

Find out what that assumed/standard circumference and we can find out how many times the wheel rotates per mile.  Or maybe the calibrated wheel rotations per mile is documented someplace.

With that, we can determine how many times your wheel rotated to get your current mileage and convert that to the miles of the old tires...

BUT we would need the radius (you can measure that with a ruler easily) of the old tires, both when they were new and when replaced...since that radius will change and an approximate average radius can be calculated. 

With out those numbers, we can't do much to determine what the actual mileage is

This is all theory...I've never done it so have never verified it

[MikeBjerum]

Since you have already changed the tires out you can't use the old General Motors standard calibration method:

On a long straight section of smooth road, put a chalk mark on the tire and the road, drive the car forward a minimum of ten tire rotations (the more the better) and stop with the mark at the exact 6:00 position, mark the road, measure the exact distance between the marks, and divide by the number of rotations.  We did this to calibrate speedometers back when they had gears in the transmission.  We could change out the driven gear, and there were several choices. 

One of our mechanics was good enough that he could go down the road with his watch and the mile markers to get close before we started the exact.  He would go out on the highway, ten minutes later the car was on the lift, he'd come in with the old gear and tell me he needed XXXXX (color designation for the different gears), and head back out.  He knew the tooth counts by memory and could usually nail it in one or two tries.

[Timothy]

Mathematically..

You didn't mention "aspect ratio" so figuring a 235/75/15 tire is approximately Ø29" or about 7% smaller than the 31" tire, you can use that as your percentage of mileage calc.

130,000 x .93 = 120,900 add the 22K for the total.

Other than that, does it really matter? 

edit..  the average beginning depth of a tire tread is between 11/32 and 13/32 and won't really have an appreciable effect on distance traveled over the life of the tread.  Most states consider a tire worn out at 3/32 or about .093"..

[Sponsor]

[mkm]

I don't want to think about the math right now, but I do have a question. If I understand correctly, you put a larger tire on, and your speedometer now says you're going slower than you really are? My truck basically has the metric version of 35" tires. My speedometer says I'm going somewhere between 2 & 4 mph faster (depends on speed) than I really am. That has been my experience with all larger sized tires.

[tombogan03884]

Have a cop clock you with his radar gun , f@#k the math.   

[Timothy]

Here's a question for you guys and gal..

My wife burned 30K of rubber off her tire one winter and never moved an inch...

How?

[MikeBjerum]

Have a cop clock you with his radar gun , f@#k the math.   

Or, just drive around.  Our locals have radar set up in school zones, construction, and just for the fun of it.  Most of them match ... Damn!  Just look at your navigation system ... If you have one.  That's all I use for speed anymore.

[Solus]

If Bill wants to determine an accurate as possible calculation for h is trucks actual mileage,  he is gonna need a lot more info than an accurate speed reading for his current tires.