My 51 3800 4-wheel pickup hauls a camper most of the time. Loaded, the outfit weighs 9020 lb. The '55 235 truck engine turns 2900RPM at 55MPH in direct drive.
It has a Crower "RV" cam and seems happy at that RPM. Years ago I added a 3-speed "Brownie" auxillary transmission. It has a 24% under drive, a direct and a 17% overdrive. In overdrive the engine turns 2800 at 60. That makes it a nice highway rig. I can almost keep up with the semis.
It has the "helper" springs and needs them. That was a RPO (regular production option). It also has the hydro-vac brake booster which was also a RPO. It needs that also.
For full disclosure the auxillary or overload springs as they are sometimes called came from a wrecking yard many years ago and aren't the stock leaves. They are 2-1/2" wide rather than 2" and came with 5 or 6 leaves. I use only 4. The truck handles pretty well but I'm at the weight limit of the 255/85X16 tires. There's 6,000 lb on the rear axle. Next step is 19.5 wheels and tires.
