Tractor trucks were used to tow semi-trailers up to 37,500 lb (17,000 kg) with 15,000 lb (6,800 kg) on their fifth wheel.
Semi-tractor/trailers have to stay on relatively flat ground, and are not rated for full off-road use.
On improved roads they could tow up to 55,000 lb (25,000 kg) with 25,000 lb (11,000 kg) on their fifth wheel.

Tractor trucks normally towed a 12-ton, 2-axle trailer.
There were stake and platform, van, low-bed, and tanker bodies.
Six-ton, 2-axle expansible vans and 6-ton, single-axle vans were also used.
Tractor trucks were used to tow semi-trailers up to 37,500 lb (17,000 kg) with 15,000 lb (6,800 kg) on their fifth wheel. Semi-tractor/trailers have to stay on relatively flat ground, and are not rated for full off-road use. On improved roads they could tow up to 55,000 lb (25,000 kg) with 25,000 lb (11,000 kg) on their fifth wheel.
Tractor trucks normally towed a 12-ton, 2-axle trailer. There were stake and platform, van, low-bed, and tanker bodies. Six-ton, 2-axle expansible vans and 6-ton, single-axle vans were also used.
M54 5-ton 6x6 truck