55mpg diesel truck.....800 horsepower.

[JQmile]

I just got word a buddy swapped a 12v Cummins into a 1/2 ton truck, along with some 22 inch wheels, a 3.27 rear gear, and .62 overdrive. That puts the big diesel just barley above idle at 60-65mph, and he claims a unreal 55mpg on a 105 mile roadtrip. Also it put down 698rwhp......of course at that power level the truck does smoke a bit black Still super impressive, and a claim I am anxious to verify.

[Frank Lee]

Yes, that is unreal.

[Cd]

I saw a huge jacked up diesel truck that ran the quarter mile in something like 10-12 seconds. What dropped my jaw was that the truck also managed "20 plus MPG".

I never, never would have thought a truck could be so fast and fuel efficient at the same time.

I would believe 25-30 MPG with your buddies truck ...TOPS !

Now the disclaimer : The truck that I was talking about was in DieselPower magazine, so it was probably stretching the truth on the MPG figures ( How could you prove it anyways ? )

[JQmile]

LOL I write for Diesel Power Magazine.

Wonder what truck it was.....we have a few that do pretty well, as long as you have a balloon foot. Last year's Diesel Power Challenge winner got 19.96 mpg through mountain roads, and still ran mid 11s on all terrain tires. I've gotten 27mpg with no OD in my truck, wonder how it would do with a double overdrive?

[Duffman]

Ive gotten over 24 on my truck 2x but that doesnt make it a 24mpg truck. What your truck is, is the average off all the good as well as the bad trips.

[Big Dave]

Yeah. I believe that claim as far as I can throw it. Maybe if he never drove faster than 35 MPH.

JQ, if you write for Diesel Power, perhaps you saw the spread they did on my truck.

[wagonman76]

I can believe that if you take a high torque engine and gear it so youre barely working it to get down the road, you can get better mpg. I have found with more than one of my cars that a 3.1 engine will give 2-3 better mpg than the same car with a 2.8 engine (3.1 is just a stroked 2.8). There was also a guy who put a 3800SC into a Citation and said that 35 mpg was easy if he kept his foot out of it, but he built it for performance. After installing the mpguino in my Trans Sport van (3800, 3.06 final drive), I was able to get 27-28 mpg just one day of going to work and back. Im sure going downstate and back would yield even better. If I were to take the powertrain from the van, put it into an a-body which is 900 lbs lighter and much less wind profile, and swap in a 2.84 final drive, I bet I could get some pretty high numbers.

[JRK5150]

Hey JQmile, any chance we will see your buddies 55mpg truck in Diesel Power? That's a story I'd LOVE to read! Please get more info from your buddy. 55mpg sounds a bit high, but a light truck with that engine just over idle at highway speed... maybe.

[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.

[Duffman]

1982 Chevette diesel 36 hp at the wheels, 56.2mpg.

OP's signature really puts it in perspective. 55 MPG is VW tdi country.