Calculate Tire Radius (or diameter) Changes Due to Speed

[4xWriter]

Let me get to the meat first. We will be needing new tires soon and I plan to upgrade to a slightly taller 16 inch tire to correct a built-in speedo error (see below). I'm reviewing revs per mile on the existing tires as well as some taller alternatives. In order to make my calculations more accurate, I wonder if there is a rule of thumb or calculation for the amount the tire diameter (or radius) may increase at speed? I have noted that the revs per mile advertised for the tire (apparently based on static loaded diameter) differ from what I have calculated for the actual tire on the car (compensated for wear)

Our 2000 Honda Accord V6 has read 5 mph high at 60 mph since new. I've gone round with the dealers, who say this cannot adjusted via reflashing the ECU (unlike many other mfr), at least at a dealer level, There were two tire sizes available on our model, one was slightly taller, and it appears both the cars were calibrated to that tire. Unfortunately, it's on a 15 inch rim and ours is on a 16-inch alloy. There is a 16 inch tire that looks to be about right.

BTW, we have been waiting all this time to make the correction because these Michilen XSE tires just will NOT wear out. Nearly 70K now and they still have 4/32+ tread!

 

[FoMoCoMoFo]

I like to measure the rolling circumference of the tire on the car.I think if you do that at the right pressure, load and temp the rest is not worth worrying about.
variations in temp. and wear over the life of the tire will probably create more speedo error than speed related diameter change.
Good luck

 

[4xWriter]

FoMoCoMoFo: You are very likely correct. I have just unleashed my pedantic alter-ego on this. The numbers for the new, slightly taller tire, plugs in almost perfectly. Great handle, BTW!

 

[NickE]

As a TSD ralliest it is well known that old bias ply tyres will have lower revs per mile as speed increases. This is much reduced with Radials.

Specifically if you wnated to research it check out: The Great Race, a time/speed/distance type of event run on vintage equipment and using only a speedometer and clock.

You might also want to research TSD rally, we generally measure distance in 0.001miles and time in 0.01min, thats precision and accuracy.

(5ft-0.6sec)

 

[Bribyk]

Race tire data will have the loaded and rolling diameters but I doubt you would be able to get that info for street tires. Then you also have to account for % slip, etc...