Gotcha. Wasn't sure on whether it was diameter or radius at the time.

 

Thank you all for the links and information!

It's funny that you say that, because we have a few speed checkers installed along the roads in town, and they always tell me I'm going about 3mph slower than my speedometer says. I thought maybe those checkers were off, but this would makes sense. Thanks, Nuke!

 

With respect to speeds vs tire diameter . . . be at least a little skeptical of the online calculators. They're good enough for most folks' purposes, such as estimating the effect of changing tire size. But they aren't nearly good enough to sort a one or two mph speedometer error out of 40 to 60 mph true. Nor are they good enough to use in estimating how well a roadside speed monitor was calibrated.

Here's why . . .
Every one that I have ever seen calculates about 3% to maybe 3.5% fewer revolutions per mile than the tire mfrs themselves provide in tabular form (that you can find at places like TireRack.com), because the developers of these pages always manage to overlook the fact that rubber is not a particularly rigid material.

20,900 divided by tire diameter in inches will give you better answers. The online formulas all use 20,168 instead.


BTW, if they're Pirellis, they're 235/55-17. The 18's were BFGoodrich.


Norm