[Joeaksa]

Originally Posted by look 171

So tell me, i have a stock mid 90s Cummins with 90K on the original motor and tranny. I have not been able to get more then 15-16 mpg on the high way doing 70mph. I would be happy with 19 or 20. after speaking to a couple of diesel machanics, they both said to leave it alone and just put fuel in it and drive. No mod necessary to get better milage or power to these earlier trucks. I haven't looked into it since. No additives in the tank but some injector cleaner once in a while. The truck has no problems. I have head about marval's mistry oil in the gas tank to lube the injectors, but have not done so. What do you guys think?

Try putting some two cycle oil in your tank for a couple of fill-up's. I use just a bit less than 1 ounce for each gallon. My truck has a 38 gallon tank and I let it get down to 1/4 or so, so stick about 20-25 ounces (good sized gatoraid bottle full) of two cycle oil. Said to be good for .5-1.0 mpg and much better for your injector pump. West Marine has it on sale right now, so might check there if you have one close.

If you have an automatic then thats about all you are going to get. What rpm are you showing at 70? 2200 or so I am guessing? Back it down to 65 mph and that should be around 1800 rpm and your mileage should come up. What kind of tires you running? I have 10 ply aired up to 70-75 psi and that helps rolling resistance.

The earlier trucks are not computer controlled like the 1998-2002 and newer versions are, so will never get as high a fuel mileage but I would hope you can bump it up a bit.

Mine loves about 1600 rpm (top of peak torque) but thats 58 mph and too slow for me on the highway even if I do get 28-29 mpg there. Looking at a gear vendors overdrive unit to give me more gears so I can run lower rpm (top of the torque curve) at whatever speed I want.