I just got my 1948 6400up and running with the 2 speed rear end functional. 13' grain bed dump. I took the sides off and am currently driving as a flat bed.
i put 9.0x20s on the back with the rear end in low until last weekend.
So if i am not mistaken low is 8.10 and high is 6.13. I have a freshly rebuilt 261 and sm420.
I ran the numbers on a speed calculator considering the 39 inch diameter rear wheels.http://www.csgnetwork.com/rearendgearcalc.html
My speedo jumps around quite a bit and doesn't work in low so I have used the WAZE gps speedometer on my phone. To my surprise my calculated speeds match up with the GPS. here are my mph calcs in 4th gear 1.1 that i have found to be accurate.
rpm low hi
2000 28 38
2500 36 47
3000 43 57
3250 46 61
I can tell you that 45mph feels good. It steers nice with the 9.0x20 highway tires on the fronts. the rears are agricultural traction tires that make some noise but feel great also. 55mph on the two lane highway is ok. The hydrovac brake booster works and provides plenty of stopping power.
Remember that these 6400 do not have shocks just leaf springs so there is some bouncing goin on when empty over expansion joints, dips and such.
Just my 2 cents. Have fun, I really enjoy mine.