If it is off 10 mph at all speeds, changing the ratio will make the problem more complicated. You are pretty lucky to have a constant offset error. If you find that hard to deal with, why not move the needle ahead to correct it.

1951 GMC 1 Ton Flatbed -- It is finally on the road and what a great time I have driving it!
1951 1 Ton Completed

My Chevy Master 4 Door is on the Road!