yup, kat is right... if your speedo says 60mph then your going faster than that. your factory mph sensor is on the transmission. so if you add more diameter your going to go farther per rotation.

i always REALLY exaggerate these things to make them make sence. for example. if you put monster truck wheels on your VX, even when your VX said you went 1 mile, you would of gone more like 5 miles. so your moving at a faster rate. similar, if you put go-kart wheels on it, your would barely move at 10 mph on your speedo.