Fuel Mileage Seems Low

[willymutt]

We went and looked at a truck yesterday for our rockcrawling team. It was a '03 FTL M2106 with a 275hp 3126 and a FSO-8406A tranny. The rears were 4.11's with LP22.5's. The truck has a 26' box with an 8' attic over the sleeper/cab. I had a palm pilot with the cat software on it and we took it for a drive to see what fuel mileage was. The average for the life of the truck was 7.31 with an average speed of 38 mph. This was with idle fuel taken out. When we were driving it down the road at 65 mph it was 1875 rpm and getting around 7 - 8 mpg. This seems really low to me and nowhere near what I see other people getting with expediters. Is there something we are missing, or is this actual mpg? We were hoping to get 9 - 10 with it to justify the cost.

Any experience would be greatly appreciated.

 

[davekc]

That stated economy seems about right for your setup. For one you have a 4:11 rear end, and secondly, that attic over the cab is a major air drag.
A rear end with that drive and aerodynamically deficient specs will get you your stated results.










Davekc
owner
23 years
PantherII
EO moderator

 

[Tennesseahawk]

I was driving an 04 International 7500. You'd think the smaller truck and engine (320hp) would be better in fuel milage than a big truck. Not so. We got 7-9 mpg on straight thru fill ups... no idling. Just drove my new used 01 Century home from Jacksonville, FL, and got 8.1 with idling one night. Would've gotten 9 or so if no idling. Time to tweak it and get 10. Also making an effort to keep it around 65-67 mph in the fast states.

One other thing... you have a high rear end (not you personally), like I did with the International. These trucks aren't built for the long haul duties. They're more for dump trucks, tow trucks, city delivery, etc. I'll never say never, but I doubt I'll ever own another non-class 8 truck.

-Vampire Super Slooth Trucker!!!

 

[willymutt]

What would be a good rear ratio? When I sold trucks, most of them were in town and had 4.56's. Would 3.90's make that big of a difference? We would really like to have a class 8, but want something with 26' box so we can fit two rigs. Our price range, $15 - 20K, limits us also. This one was nice with the attic, but I think you are right that it adds a lot of drag. What is everyone getting with this type of spec? Seems like everyone claims 9 - 10 on the board here, but I don't see much difference in the specs.

 

[arkjarhead]

IMHO, you get your best fuel mileage when your rpm rides at about 1500.others may disagree but that is what i've found.

 

[Tennesseahawk]

You also have to take into consideration what kind of engine you have. You'll want the rear end to mesh with the RPM of the engine. Like Ark said, 1500 sweet spot. I mentioned lowering mine from a 3.79. I was told to wait till I change the engine. But having driven it for 1000 miles, I think that's low enough. I'll know for sure once I have a load on.

-Vampire Super Slooth Trucker!!!

 

[willymutt]

The problem I see is that a 3126 falls on it's face at 1500. That will make driving hills suck. I know a C12 or Series 60 like to be at 1500, but that is a totally different engine.

 

[Broompilot]

On my KW with 3.7 rears 6 spd auto sleek sleeper top, 9-just under 10 was average. RPMS at 65 just under 1,600 60 seemed to be 1450 to 1,500 depending on up incline or flat.