Go to web page http://la.znet.com/~x1937/Flx.htm .
Skip down to the part about the optional rear end ratio. On a straight away it will easily out step a deluxe sedan.

On paper it looks like the 4.75 ratio would have given it a high top speed.RPM at 60 almost same as car with 3.73 and 6.00 X16 tires.In the real world, due to the weight and wind resistance the top speed would have probably been slower than with the 6.43 ratio.And the engine would have been working-almost lugging.It was probably recommended for flat country only....School buses back the had a govorner set for about 40 MPH.With those tall gears it would be lucky to maintain 50 MPH with out a govorner.Would be equal to pulling a big Air Stream trailer behind a 1937 Master.The engine speed with a 6.1 ratio (info from 1939 truck data book) would be 3000 RPM at 50 MPH.About 3500 at 60.....which would have been about its top (ungoverned) speed with a tail wind....engine would have lasted a lot longer if driven correctly with the 4.75 though.

Strange, the 4.75 and 6.43 ratios are not listed by Chevrolet .Wonder what rear end they used....A two speed would have been a better choice for inter-city use.