SlickRSM

I have a stock setup with 225/45/17 tires but as of Tuesday im changing over to an aftermarket wheel setup that is 225/35/20. Im a little confused some sites such as...
http://www.discounttire.com/dtcs/infoTireMath.dos
says im going 68.2mph when my speedo says im doing 65mph after the new wheel setup

BUT another site
http://www.1010tires.com/TireSizeCal...?action=submit

says im doing 57.1mph when my speedo says im doing 60mph

SO which is it? Faster or slower when im doing a certain speed with my aftermarket setup on my Lexus IS 250 AWD?

slee18

iTrader: (1)

more then likely your going faster. It depends on how much larger your diameter of the tire is compared to stock. by the looks of it because you didnt increase the width you are going faster.

Scott C

What really matters is the circumference of the tires

Stock circumference: (225/45-17)
Width: 225mm
Aspect Ratio: 45
Sidewall height: (225 x .45) = 101.25mm
Convert to inches: (25.4 mm/inch) = 101.25mm/25.4mm/inch = 3.986 inches
Wheel diameter: 17 inches
Overall diameter (17 inches + 3.986 inches + 3.986 inches) = 24.97 inches
Circumference is (Pi * diameter) = 3.14159*24.97inches = 78.45 inches

New circumference: (225/35-20)..... I'll skip the annotations and do the direct math:
Sidewall Height: 225 x 0.3 = 67.5mm or 2.66 inches
Wheel diameter: 20 inches
Overall diameter: 25.3 inches
Circumference: 79.52 inches

The new tire/wheel setup is 1.07 inches bigger in circumference. This means you will be going faster than indicated on the speedo. The approximate change is (1.07 inches/78.45 inches) = 1.36%

Meaning, at an indicated speed of 60 MPH, you will actually be traveling 60.8 MPH.

This not a big delta as most cops give 5-10% in speed for differences in tire sizes from mfr to mfr.
The best way to verify is to get a GPS and compare against the speedo.

ohno

iTrader: (5)

^ Thanks for the info..