Its not just for the speedometer, the speed sensor is an extremely important sensor to the PCM, without it, the engine will not run correctly without getting the chip changed, and even then, it'll have problems with stalling out under deceleration, with emissions controls, fan controls, and the highway mode function.
A cummins diesel is a fantastic engine, but they are very heavy, and are pretty tough to shoe-horn into these trucks, there is a lot of suspension work etc that needs to be done so the truck handles properly. 6.2 / 6.5L diesels are great for getting good mileage, but without spending a fair bit of money on one, they can be real slugs.
The new LSx engines, esspecially the lower displacement 4.8Ls and 5.3Ls get excellent mileage, similar to what a 6.2L gets if geared properly, and driven nicely, with way more power available if needed
