Depends on what size tire you started with.

 

I am running 33" tires and hooked up my GPS that told me that my speedometer was showing almost exactly 5 MPH slow at all speeds.
FOr now I just keep that in mind when passing a state trooper, but I've been thinking about getting a Speenometer Calibrator.
I'm sure there are many out there but I found one at good ol' JC Whitney (not sure how well they work).
http://www.jcwhitney.com/speedometer...202y2007o620j4

 

That's an odd result as if you are 5mph off at 60mph then you should only be 2.5mph off at 30.

 

Your speedo is off by a percentage equal to the difference of the old tire diameter vs the new tire diameter.

If you go from a 32" to a 35":
32/35 = 0.9143
1-0.9143 = 0.0857 or 8.57% difference

Indicated 60 you would really be going:
60x1.0857 = 65.142

Indicated 30 you would really be going:
30x1.0857 = 32.571