a 20 tooth with a 3.73 rear by itself won't be accurate. any axle ratio lower (higher numerically) than a 3.55 will require a ratio reducer. there was never any combo's made that was perfectly accurate. the easiest way is a 17 tooth with a .80 ratio reducer. 19 and 20 tooth were used mostly in 3.23 gear cars. 21 tooth was for 3.55 rear and 3.91 gear cars used a 21 tooth with a .80 reducer. I use an 18 tooth with a .90 reducer for a 3.23 gear and 28" tall tires. speedo is close but not perfect.