Tire size calculator

I was messing around with this Tire size calculator and came up a few wonders.

Base stock tires: 175/65R14
Sport stock tires: 195/55R15

Is the Speedo and Odo meter on base model and sport model has the same calculation? Assuming everything started with Base and Sport is just add ons. If so, according to the tire size calculation link above, "When your speedo reads 60mph, you are actually traveling 61.3 mph." But what about the Odo? When your odo reads 60miles driven, are you actually traveled 61.3miles??

 

should be the same

 

i would assume the calculations for the odo and speedo are made within the ecu, i would think that the ecu in the base and sport have compensated software for the tire size difference.

 

As long as the overall diameter of the tire and rim together are the same for both base and sport models. It's why the tire profile % is lower on the sport model....it compensates for the bigger rim size....so it doesn't throw off the speedo which is linked to the odo.

 

according to that site there is an overall diameter difference

Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
175/65-14......4.5in... 11.5in...23.0in.......72.1in..........879........0 .0%
195/55-15......4.2in....11.7in...23.4in.......73.7in..... .....860........2.1%

 

While the calculation might say that there are so many variables in actual vs specs that you can only say the calculation is + /-2% accurate.
Worse, the actual traveled distance and speed depend on the measured ones by test because the miles traveled per tire rotation is specific to each tire. And you can check those in tire rack tables showing tire specifications under 'revs per mile'. The reduction is due to the deflection of the tire, as the tread of the tire squashes as is pressed into the road, The actual rolling diamer of the tiire is less than the calculation by size.
cute, eh. Now you whttire choices is not a simple calculation.
cheers.

 

sometimes but more often by a direct connection between displays and wheels via a rotating cable and even then when the input is to the ECUWheel rotatio counters are a late model improvement because the counting sensors have to be there for the ASC.

 

Tire diameter also changes as the tire tread wears down, by a few percent. Tires of the same nominal size from different manufacturers also vary slightly in size. The overall accuracy of the speedometer and odometer are at best within a couple of percent, and that can't really be avoided so long as they work by counting wheel revolutions. Luckily, that's plenty of accuracy for any practical need.

A GPS can, at least in theory, give speed with quite high accuracy. Whether a given GPS receiver actually does this is another question entirely. (Speed can be derived from the satellite signals essentially independently from position.)

 

Actually, the rolling radius of tires as thetread deph changes doesn't change very much, about 1 percent at most.

 

Maybe I'm missing something, but with a radius of 12 inches, 1% is about 1/8", and as I understand it most tires have around 1/4" usable tread depth. That would seem to me to be about a 2% change over the life of the tire. Is there some odd physical phenomenon with the way tires deform as they roll that minimizes the effective change, or...?

I guess you could stat that as ±1% from nominal if you take half-worn as the nominal diameter, but that would still be an approximate 2% change overall.

 

when you compare the rolling radius when the tread ribs are squashed the actual tread depth is only 1/8" greater at full tread compared to 1/10th inch when tire is worn out. the 1/8" is often quite generous; calculations are fine but not always equal to real world.
try converting the revs per mile to diameter and then comparing to diameter with no tread left. might be interesting.