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So, is there evidence that the trim levels in North America are calibrated differently? (185/55/R15 is only 0.7mm off 175/65R14 -- completely negligible since a little tread wear makes as much difference.)
If I get a Fit it'll most likely be a base (actually the middle level here in Canada, which I speculate is equivalent to the US base). I'm not likely to change the wheels, at least not until the factory tires have worn out, but I'm curious to know, if I stick on my old snows next winter, how far off they'd actually be. In real life of course I can wait until I have my next car and check it against highway mileposts.
. . . . .
If you have tires of "width / aspect R rim", the rolling radius in mm is
(12.7 × rim) + (width × aspect ÷ 100)
For example, for the stock 175/65R14 the rolling radius is
stock = (12.7 × 14) + (175 × 65 ÷ 100) = 291.5
You would want to keep the rolling radius of any replacement tires within about 8mm of that at worst.
If you have selected a rim size and tire width, the ideal aspect is
(stock - 12.7 × rim) × 100 ÷ width
For example if you want 205mm wide tires on a 16" rim, the ideal aspect is
(291.5 - 12.7 × 16) × 100 ÷ 205 = 43
so you'd get the closest, 205/45R16.
If you want to know how far off a change will put your speedometer, get the rolling radius of the stock tires and the new ones; your actual speed is
speedometer × new ÷ stock
For example, if you're reusing the 155/80R13 from your '77 Civic (rolling radius 289.1mm) and the stock tires are 175/65R14 (rolling radius 291.5mm), when your speedometer reads 100 you are actually doing
100 × 289.1 ÷ 291.5 = 99.1
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