Wheel diameter question

[91Z28]

For the purposes of using MPH/RPM calculations, how do you determine the diameter or circumference of your wheel? I have 40/18s.

[Eugenio_SS]

assuming it's a 275/40-18
40 means sidewall is 40% of 275mm. = 110mm
you have the wheel + 2 sidewalls for the overall diameter.
2 sidewalls = 220mm = 8.66"
18" + 8.66" = 26.66" which is 1" more than stock.
stock is 25.66"..., so your speedo is off by 3.9%
you are going 4% faster than the speedo shows.

[91Z28]

Thanks

[1KBird]

For the purposes of using MPH/RPM calculations, how do you determine the diameter or circumference of your wheel? I have 40/18s.
Check this out. 1010tires.com/TireSizeCalculator.asp Don't forget the www up front. I can't post full links yet.