it wouldn't be off by a set MPH
it would be off by a percentage.....
i switched from 225/75/R14 to a 245/75/R16
its about an 10.5% difference
so i just do the math when driving down the road.... 60 mph X 1.10 = 66mph
so if my speedo says i'm doing 60 i'm actually doing 67
if it says i'm doing 65, i'm actually doing about 73
use a tire calculator online to figure out what percentage its off by, then just do the math on the fly