98 Freightliner c120.
What mpg average should one expect running southern midwest,no mountains?

Detroit 12.7 500,
Sleeper 70" Condo,Wheel Base 240"
Transmission Super 10,Tire Size 11R24.5LP,
Suspension Air Ride Frame Steel,
Axles Tandem Interior Signature,
Rear End Ratio 3.90

Don't know for sure but you should be able to see at least 7 or so with that setup.

The number one factor in fuel economy is YOU, the driver. I managed to get 7-7.5 out of a cat 550 with an 18 and 4.11 rears. When I had a driver in the same truck it got 4.5. (reason number 72537 why I don't have drivers anymore)

I had a very similar setup.

I think you can get at least 6.5 as long as you don't push it too much.

Ty for the replies.I didnt think it would get that good of mpg.

the truth.... i get 7.2- with some hills & 8.5 m.p.g. on flat. that is cruise set at 62 m.p.h in a 70 m.p.h speed limit area carrying 46,000 lbs. doing the same runs at near top end 69-72 mph i get 5.7- 6.0 m.p.g. & that is on a mil mi cabover with a 11.1 detroit. your driving makes a huge difference.

Here's an interesing formula that can help determine the engine's RPM at cruising. If you know the recommended engine speed in rpm, the calculation will tell you what speed you should run at.

To do this, you should know: your transmission top gear ratio, your rear axle ratio, revolutions per mile of your tires, desired cruising speed. It may take awhile to find out your tranny top gear ratio, and rev/mile of the tires, but once you have them, you can pinpoint exactly the RPM you'll get at the particular speed with your axle ratio.

Example: 13 speed Eaton transmission with 0.73 top gear ratio, 4.11 rear axles ratio, 22.5 tires with 495 revs/mile will have the truck cruising:

56 mph at 1,386 rpm
58 mph at 1,435 rpm
60 rpm at 1,485 rpm.

I did this calculation for a Cummins engine, and they recommend to be between 1,400 and 1450 rpm for maximum fuel economy.

Formula:

RPM equals CRUISING SPEED IN MPH times TOP GEAR RATIO times AXLE RATIO times REVS/MILE divided by 60.

Yup around 98 is about the best year for detroits that get good fuel mileage. I know someone with a 500 detroit and he gets 7.5-8 regularly.

We had a '01 Sterling with that setup. 18 speed + 3.70 rear IIRC. It got 5.5 running up and down I81 between Baltimore and Alex Bay. Grossing 80,000lbs southbound and 35,000lbs northbound.

I'd say around 6.5-6.7, depending on how you drive it. hats killing your mpg's is the 3:90 rear ends. You'd get over 7mpg easily with a higher ratio (higher overall gearing)