Originally Posted by JKralph
your wheel size won't affect your speedometer, but a change in tire size will. If you got a smaller wheel, but put the same size tire on that wheel, then your speedo will be the same.
Just so the OP is clear; What JKralph means here is that the over all tire diameter is the same.
For example if your over all tire diameter is 32" on an 16" wheel, your speedometer wouldn't be any different than if you had a 32" tire on an 18" wheel.
However, if you had 255/70R18s and then went with 255/70R16s then you'd have a discrepancy in your speedometer.
255/70R18 would be approximately a 32" tire.
255/70R16 would be approximately a 30" tire.
So, going from the first hypothetical tire to the second would give a speedometer error of about 6%. Or, when the speedometer was at 55mph, the actual speed would be about 51.7mph.