You don't need a Speedo, just a good tach and an Excel Spread sheet to calculate your speed.
mph = rpm x tire diameter / differential gear ratio x 336/ transmission gear ratio
and for the run of the mill Mighty Max
Diff=3.909, 1st gear=4.33, 2nd gear=2.355, 3rd gear=1.509, 4th gear=1, 5th gear=0.827
Tire Diameter is calculated with, tire diameter = 2 x section width x aspect ratio / 2540 + rim diameter
Mine is 225 section width, 60 aspect ratio, and a 15 in Rim diameter for 25.63 Tire diameter( 225/R60-15)
check here for pix of the 2 blocks I've been thru and various progressions of the builds over the years.
http://mmeierle.com/