just out of curiosity, as i was driving home today, a question popped in my head, what makes a 12 valve 6bt5.9 so fuel efficient? i have a couple of leads to maybe why they are, but i just wanted to know what you guys think

 

What do you get for mileage? I also have a 1995 auto 4X4 and I am getting 16 mpg. But you must be getting great mileage with the 2wd and 5 speed manual.
I think that they get pretty good mileage because of the smaller displacement, the diesel characteristic that sends excess fuel back to the tank, and since you have a good combo, the 5 speed manuals allows shifting when the diesel torque demands the shift.

DK

 

It depends on the gearing too- I have heard of 4x4's with 3.54 gears getting 24mpg, tho I get between 14 and 17 in my auto with the 4.10. The standard will get better mileage than the auto because the auto has inefficiency inherent in it's operation.
I think a lot stems from the design of the engine- diesels with long strokes produce lots of torque at low rpm which allows tall gears and low rpm cruising. Low rpm always means reduced fuel consumption for the same amount of work. Most PS engines are shorter strokes and higher rpms as are the Duramax. The B series is not small displacement- 5.9 liters is 360CI, that's the same as a Chevy 350 Small block.

It's not a diesel characteristic to return unused/excess fuel to the tank- that's common practice for most fuel injection systems, gas and diesel. It allows the injection pump to have enough fuel at high demand but only use what it actually needs for the conditions at hand without having to vary the line pressure at the IP for rpm conditions allowing a simplier IP design.

Personally, I think the B series is the epitome of mechanical design and simplicity for efficiency. Too much computer control and intervention can detune the engine, plus there is very limited emissions controls on the 12V which is almost always power and efficiency reducing.

 

i have a 95 2wd i dont know what gears i am getting 18 city 20 hwy consistantly

 

im not sure on the gears but i get 22 mpg on the highway at 1700 rpm on fifth gear

 

The 24v's are just as fuel efficient if not a tiny bit more due to higher injector pressures, more valves, and electronic timing.

 

i'm avg 15.5 in and out of town towing sometimes and mine has 3;73's

 

Im pretty solid at 16 mpg with my 95 4x4 with 3:55 and 33" BFG Mudder tires.
I can get up to 17 on the highway and as low as 13 if Im pulling something heavy.
But as a daily driver, Its pretty reliable at 16mpg.

 

I got a little over 22 on my last tank

Usually sticks in the 18-20 range

15 or so when I pull

 

I usually get 18-19 to work and back

a good 20 open road empty

14 with my camper (4000#) doing 70mph & 17 doing 60mph

 

5 speeds and 3.54 ratio.......

 

21 all day long, almost all highway driving. if i keep it under 2k rpm i see about 23.

 

Been a few months since I drove the old girl but she was knocking out a consistant 19.6-20 MPG mixed driving with a good amount of WOT driving. Definatly has potential for more MPG if I ever get around to swapping turbos and getting the pump dialed in. I just stabbed the pump in with a few crude adjustments when the 215 pump was installed. Cant wait for deployment to end so I can tinker away of my 260k old girl so more!

 

18-20 with 4.10s but i got 33" bfg at's so the gear ratio is higher than 4.10 prbobaly around 3.27 since i cain't find my gear calculator right now.

 

Nope, across the board they have been less fuel efficient.
When they were coming out with the 24v, Cummins said it would be more efficient and quieter...it was neither.