I have a 1948 3/4 ton Chevy truck that has all of the original parts, which is why the top speed is around 45 miles per hour. I would like to increase the speed to at least 55 miles per hour to be able to drive it on the freeway more safely. I was told that I can achieve this by changing the rear axle with another vehicle’s rear axle. Is this true? and if so, which vehicle’s rear axle will allow me to do this? If this is not true, how can I achieve the increased speed without changing too much? I would like to keep it as original as possible.
I am probably not the first to tell you but many who own early pickups share your problem. Let’s face it early haulers were designed to do just that. HAUL. Low gears were needed to haul heavy loads with relatively small displacement engines. Common gear ratios for 1948 Chevrolet ¾ ton pickups were 4.57:1 and 5.13:1, and while these low gear ratios did the job serving as a torque multiplier they did sacrifice top speed in doing so.
You mentioned wanting to keep the vehicle as original as possible, so we’ll rule out the possibility of upgrading to a more modern overdrive transmission and focus on the least invasive upgrade.
Your truck shares the same rear differential as the 1967-1972 ¾ ton Chevrolet truck. And automatic transmissions versions of this truck came equipped with 4.10:1 gears, and a few of my sources claim 3.90:1 gears were available. In either case the upgrade should easily get you to your target top-speeds and the rear differential can be swapped out without having to remove your rear axle housing, or even the need to jack it up, given its floating axle design. While personally I have never made this upgrade my sources claim the entire process can be completed within a couple of hours.
Best of luck with your project!