It's less about increasing the payload on the 350 and more about reducing the payload on the 250. The 250 (edit: 250
Tremor) is built identical to a 350.
The 250 is "derated". In other words the GVWR is artificially lower. This is done for one of two primary reasons (or a combination):
- Lower registration costs. Some states have significantly higher registration costs when you cross certain thresholds. The 250 skirts under some of those common thresholds.
- Maximum non-CDL towing capacity, since a CDL is required (with exceptions) once you pass 26,000 pounds combined GVWR. Lower GVWR on the truck means you can have a higher GVWR on the trailer without exceeding the magic number.
But mechanically the 250 and 350 Tremors (specifically Tremors) are identical. Why they are built identically when non-Tremors are not is presumed to be a majority about cost savings in manufacturing, potentially with the larger tires playing a factor.
Worth noting that it is possible to order a 350 with a derate package to bring the GVWR down, but I suspect that would be necessary with non-Tremor trucks to guarantee you get the beefed up suspension and axle.
For non-Tremors it is the overload spring! And there can be a rear axle difference depending on how the 250 is optioned (350 gets the beefier axle by default, 250 it only comes with certain packages IIRC).