going from 37 to 36 is a little less than 3%. there's about a 5% difference between 19 and 20. you probably can't hit it exact but being with in 1% is pretty good for the old speedos. if folks would do the math on this stuff a lot of guess work could be tossed out. the hard ones to do are the '65 and earlier were there are only 6 pinions to choose from and there's about a 5% spread between them all.