We have a '96 FLD120 flattop, 60Series Detroit 430/470hp, 10-sp Eaton & 4.11's pulling flatbed. Driver says he likes to run around 70mph and it turns 1,700rpm at that speed. I was worried about the mpg and had planned to trade out the gears for 3.55 or 3.70. A month ago we had it in the shop getting an A/C dryer and charged up and got an ECM printout and it showed Avg fuel useage of 6.9mpg :shock: After seeing that, I think I will leave well enough alone. Someday when one of the gears fail, I may swap out to a 3.70....maybe.

Man, funny you should post that, I looked at a truck yesterday at the Peterbilt dealer in Baltimore. It was a 96, 379 model with a Detroit engine. 430 hp at 2100 rpm with 4:11 ratio. I asked the salesman if the 4:11 would eat up the fuel and he said it wouldn't with the overdrive transmission. I think it was a 13 speed. I believe he said it should be around what you are getting.....6 to 7 mpg. I wasn't sure whether to believe him or not but I guess he wasn't BSing me.

When I worked for Stevens, I drove a T2000 with a 435hp Cummins N14 direct 10 speed and 2.85 rear end and 22.5 lopro's. It was governed at 68 mph but would have turned at roughly the same RPM as your truck at 70 mph. With grossing between 70k and 80k on the average, I always averaged over the company minimum of 6.2 mpg, usually 6.5 or better.

Ick
 

I have a 99 F/L Classic with the 500 Detroit. It had a 10, direct, I think and did well...about 6mpg. The trans suffered a hemmorage of some sort so I threw in an old 9 over that laid out back of the shop for a decade or so. Killed this Detroit. It won't pull a hill. It takes a lot of work to start off on a grade without spitting out the drive shaft and worse of all, the mpg is about 4.6 now. On the bright side, if it were still the 70's, I could run 100mph plus!!!