I would imagine that your rear is in low range. My '52 3 ton will run 60+ but it has a .83 overdrive on 8.25-20's. The '46, while still not running 100% will do 45 on 7.50-20's with 1:1 rear. You should be able to get 55 with no problem.
Scott


1947, 1950, & 1952 Chevrolet 1/2 ton
1952 GMC 450 series Cab Over Engine
1946 Chevrolet 2 Ton