well my car was from the factory with 17" wheels so i guess the speedometer was calibrated for that wheels size...well i have since upgraded to 20" wheels so since the speedometer reads the revolutions from the wheels, if both the 17" wheel and the 20" wheels had the exact amount of revolutions per minute, than the 20" wheel will produce a faster mph, correct?...im saying this because a cop pulled me over and said i was going 94mph and i am sure my needle was nowhere near that....i would say my needle was around the 85mph mark at that time so could the wheels size difference cause a appx 10mph difference in speed?

 

no....wheel size doesnt matter.....only diameter of the tire....you where going 94

 

lol, i think you're just trying to get an excuse to take to court

i've heard that larger diameter wheels make a slight diff, but not 10MPH+

more like (+-) 1-2mph

 

If the rpms are the same between the 17" and 20" then the speedometer would read the same.

Check this site out and see what the difference is.

http://www.1010tires.com/tiresizecalculator.asp

If your wheels/tires have a larger diameter than your stock ones, you will have a higher indicated speed than what your actual speed is and vice versa.

 

So the other day when I was doing 210mph with my 20s... it wasn't true??

 

lmao. i dont think you'd be in too good of shape after going that fast in a mustang

 

But don't cops radar have a +/- of 15mph or maybe its just 5mph idk

 

it's +- 5 for your speedometer and +- 2 for their radars i believe

 

^^^^ Thats why you should not travel more than 7 miles above the posted limit.

 

...ouch 94mph ticket has gotta hurt