How fast am I going? (aftermarket wheels)
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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?
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?
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Join Date: Feb 2009
Location: TX
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![Default](https://www.clublexus.com/forums/images/icons/icon1.gif)
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?
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?
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.
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