[Big Dave]

I don’t much believe 55 MPH unless he is like a lot of these Cummins fans who claim outrageous mileage – by driving 45 MPH max., downhill.

The big diameter wheels are counterproductive unless you are doing nothing but running the super-slab. Every time you accelerate from a light you have to accelerate those four big flywheels.

To me, starting with a half-ton truck, I’d use 235 tires, a 3.08 or 3.23 rear end and T-56 tranny with the 0.5:1 overdrive. We’re talking 1150 RPM @ 70 MPH. With the operational flexibility of the T-56 (a close-ratio four speed with two overdrive gears) a 4BT3.9 eight valve Cummins, jacked up to 200-250 HP would give excellent performance. Add a few aero improvements and real-world 35 MPG would be in reach – driven at the posted limit.

The twelve valve Cummins engine weighs over a half ton. Pickups are hugely nose-heavy anyway and a half-ton truck would probably not be loaded enough to counteract that huge engine weight. The 4BT3.9 weighs about 700 lb – roughly the same as a Chevy big-block. This would result in a much easier to handle truck. The twelve valve would eat up 10 HP just to run the power steering pump to fight that weight.

Maybe “Diesel Power” would like to arrange a long-distance MPG run. Say LA to Jacksonville next summer. This way you get past the ‘snapshot’ MPG a ScanGuage gives you and you get to check out performance over a wide range of terrain. I’d be game.