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According to my friend's speed laser and my gps, the speedometer on my car is 3mphs off

I think going from 18 to 20 wheels caused this

How am I suppose to fix this ? Do I go to the tire store where I bought my wheelz from ?
 

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3mph? You don't worry about it. On most cars the factory speedo error is more than that.
 

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According to my friend's speed laser and my gps, the speedometer on my car is 3mphs off

I think going from 18 to 20 wheels caused this

How am I suppose to fix this ? Do I go to the tire store where I bought my wheelz from ?
That's because the outside diameter of the 20" tire is larger than the tire on the 18" wheel. Its all part of using bigger wheels, unless you compensate with a reduced tire sidewall height (a.k.a. aspect ratio), or get a tuner that can program the speedo to correct for the change in tire diameter.

The quick fix is to just slow down 3 MPH.
 
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Yes, it's the tire size change. Your tire dealer can't help with the problem, they have no way to change anything with respects to the car's computer.

You might check with the dealer for a flash to compensate or look into programmer with the option to tune for different tire sizes.
 

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We're gonna need someone with the math smarts in here...

If the car measures the rotation from the center of the hub and thinks you went one mile but a larger tire is used then you'd have actually gone further then it thinks... Meaning more actual miles driven then registered... Smaller tire meaning less actual miles then actually driven... Larger circumference = more miles driven then registered.... Seems right to me... Lol
 
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