If the diesel's operating range is from 1400 rpm to 2900 and the gasoline engine might be expected to operate from 1400 to say 4200 rpm. At 1400 rpm the gasoline engine won't make much power but it needs to run slow if its going to get descent fuel economy when cruising empty. For this reason you might want both the gas and diesel to have similar gears for cruising empty down the highway. When pulling a heavy trailer up a hill the diesel will be happy at say 1800 to 2900 rpm where as the 8.1 liter gas engine is going to need 3500 rpm before it starts making some ponies. So now if the gasoline engine is going to be expected to pull like the diesel in low gear its going to need a 7.25 ratio rear end to compete with the diesels 3.73 ratio.
