Because the tire is 2 * PI radians around. And the circumference is
2 * PI * R.
Punch some buttons on your calculator. For percent changed the radius works exactly as well as the diameter which works exactly as well as the circumference when comparing speedometer/odometer errors.
A high school student in algebra class should be able to explain why. Because we are lacking HS algebra graduates I'll explain.
The only difference between circumference and diameter is that diameter is multiplied by PI to get circumference. When comparing two tires the constant PI can be factored out from both sides of the equation.
Mike, determining tire height with radius suits better for those who want more academic conversation and more "convincing" formula. They have the joy of multiplying the radius by two to get what we know without multiplying, "how tall the tire is", diameter.
Approximately 11/27/03 14:11, rnf2 uttered for posterity:
Depends on how fast it is turning and the load. However given two tires from the same manufacturer and model differing only in size, isn't too much of a stretch to presume that these would be more or less equal, allowing the larger diameter to be a reasonable WAG for speedometer effect. With the disclaimer that at higher speeds the circumference grows depending on construction etc. etc. and that even the tire measurements given by a manufacturer have a disclaimer that they are averages and apply only in reference to the rim width noted, etc. etc. etc. Or more simply, either a change in radius or diameter may be used to calculate the percent of speedometer nominal effect given that the other factors all cancel out. Except for amount of air. And atmospheric pressure. And pizzas consumed by the driver and passengers. And fuel load. And speed. And temperature. And fudge factors. etc.
Still waiting for anything I have been wrong on instead of childish, unfounded name calling. Of course that's what those with inferiority complexes about ignorance do, I have found in my travels. Feel free to relay any example to email, as I will not be returning to this board, since knowledge seems to make some people upset (read as jealous?) here.
Don't feel bad about upsetting Jason.. this is a quote of his from alt.trucks.chevy..
"I must travel on to more intelligent places, as some fools here are intimidated by knowledge, and I so despise people getting bent out of shape when they read actualities. I prefer everyone to just get along. Best of luck, all with of your trucks, and keeps those tires turning!"
This was as a result of a discussion in the aforementioned newsgroup.. that discussion was about tires too.. worst part is that he may end up in one of the two Jeep newsgroups..
-- History is only the past if we choose to do nothing about it..
I work at a tire shop and can say that for Example, you have a 235/75R15, and a 215/75R15, that the 235 will be TALLER!!! It would in fact be quite a bit taller, by a couple Inches. Your moving up
2 sizes, since you skipped a 225/75R15. If you had a 215/75/R15 before and wanted to keep the SAME Height but get a little wider, then a 225/70R15 would be the tire to get. Or Wider yet with a 235/65 or 60 R15 Depending on the Brand of tire since they do very from different Manufactures.
I have P275/45R20 on my own truck. The Height is quite close to the Stock Chevy P245/75R16" wheels that were on in. Much Wider tire but not a whole lot of sidewall. Ignoring the Wheel Diameter or either 16" or 20", you can see that the first number is going up and the Second Number is going down. So while the height is the same, the tire is wider by quite a bit. If I needed to replace the stock 245 tires, but want to keep the height but a little wider, again I'd go with a P255/70R16. The height the same, the Width a bit wider.
If someone says there want to go up from a 215 to a 235 and don't give the second # then I and anyone else will assume that the second number is the SAME. In which case the tire WILL BE A LOT TALLER! A couple inches is a lot. Your jumping up 2 tire sizes.
MotorsForum website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.