97 6.5 bad mileage

Robert Burnham

Junior Member
I have a 97 3500HD with a 6.5 turbo diesel. The engine has 38000 miles on it. I am getting 11.2 miles to gallon highway and about 7.5 miles to gallon city. I have had it to a dealer and they find nothing wrong. It has a new air filter, front tires and alignment, tire pressure is ok, etc. Anyone out there have any ideas, my last truck had the same engine and got 24 miles to a gallon on the highway. I have thought about a exhaust system but they don't seem to do much for mileage. thanks

John DiMartino

PlowSite.com Veteran
My dad's 94 6.5 turbo got 15 mpg on its best day,and it was only in a 2wd 1500 truck.If your previous truck got 24 mpg,and was a 3500hd 15KGVWR-you should never had sold it,without witnessing it,I dont believe that mileage is attainable with that truck,and gearing,unless your going downhill all the way.

Robert Burnham

Junior Member
John, my last truck was a 94 3500 with an auto trans, not a HD, the only modification it had was a K&N air filter. It honestly got 24 mpg on the highway..


Long Island, NY
ok, I know someone who has the same setup that you do, and he said he took it to a dealer and they put some kind of chip in it, or reprogramed the computer or something electronic like that. Look into it and try it out. Good luck.


Senior Member
des moines iowa
fuel milage

We have a new 2000 3500 4x4 and were getting 6 miles a gallon to. After trying to figure out why the guy who was driving the truck was getting such poor milage I decided to drive it myself for a while to check it out. What I found out by accident was the back tank was pumping fuel to the front tank via a faulty hose/connection. IE it was pumping it out on the streets very slow and right onto the pavement. Checked with another competier here in town regarding there 3500 that was being fitted same time as we bought ours (Different dealerships, same truck equipment supplier) and they called back this morning and have found the same faulty connection and were pumping gas out on the street also. The connection we found was the line running into the front tank was cracked right before the pass into the tank. So gas would slowly leak out onto the pavement as you were driving the truck. Don't know if this will help but might take a look underneath. I noticed mine while running the truck, had raised the bed to grease the hoist while I was fueling up. Put the gas into the back tank first and it started pumping to the front tank since it was empty. Surprise as I noticed my boots were suddenly wet and smelled like gas.......