18" Rims and the Speedometer: How much is it altered?

[Scotticus]

After purchasing a set of RX-8's, and putting some 225/40/18 tires on, how will my speedometer be affected? I'm assuming that it will read slower, meaning the car will be more ticket-prone. Oh good.

For example, if you were to be clocked at 60km/h, how fast would your speedometer be reading at that time? 55km/h or so?

Thanks

[Malcolm991]

I dont think there is a difference!

[yearoftherat]

IIRC, there will be a dif of about 5% in speed

[gabbygenier]

get a gps and find out:chuckle

Edit. found this
http://www.torontomazda3.ca/forum/showthread.php?t=30630&highlight=speedometer

[yearoftherat]

Yeah thats the link I was looking for...

[Olestra]

I always use this:
http://www.1010tires.com/TireSizeCalculator.asp

Stock Tire - 205/50R17
Section Width: 8.07 in 205 mm
Rim Diameter: 17 in 431.8 mm
Rim Width Range: 5.5 - 7.5 in
Overall Diameter: 25.07 in 636.77 mm
Sidewall Height: 4.03 in 102.36 mm
Radius: 12.53 in 318.26 mm
Circumference: 78.75 in 2000.2 mm
Revs per Mile: 829.6
Actual Speed: 60 mph 100 km/h
Speedometer Difference: -
Diameter Difference: -

Tire 1 - 225/40R18
Section Width: 8.85 in 225 mm
Rim Diameter: 18 in 457.2 mm
Rim Width Range: 7.5 - 9 in
Overall Diameter: 25.08 in 637.03 mm
Sidewall Height: 3.54 in 89.916 mm
Radius: 12.54 in 318.51 mm
Circumference: 78.79 in 2001.2 mm
Revs per Mile: 829.3
Speedometer1: 59.9 mph 99.9 km/h
Speedometer Difference: 0.036% too slow
Diameter Difference: 0.04%

So the answer is you picked the best tire size and you have practically no difference in tire diameter.

[Scotticus]

Nice! Thanks guys. I had searched a few threads, but only found guesses.

[Marsh]

Keep in mind that your speedometer is already off from the factory. My 18's are MORE accurate than my factory wheels were and are a good deal more accurate than my 16" snows. According to GPS my speedo is less than 2% out with 18's.