Peterbilt 378. 3.5mpg. What can I do?

Have a 1996 Pete 378 day cab with 3406e cat 475hp
21,500 rear axles with 4.65's
14,000 front axle
18 speed.

Currently runnng local runs around 20 miles each way.
Calculated my mileage today and noticing that Im only getting 3.5mpg.
I only run between 60-65mph

Theres got to be something that I need to look at to improove this.
Any thoughts?
My air filters are clean.

Would a fan clutch that runs 100% of the time cause this?

 

If you really have 4.65 gear ratio with a cat e model it’s really geared to run 55 mph or slower. Also a fan stuck on will hurt a bit

 

Thats what is marked on my rear axles. Says 4.65 wrote right on them. Running with the 3406e with 475hp.
Just curious but what would it be used for running at 55mph or slower? Why have it geared that way?

Anything else I can look at to help with the mpg?

 

Well the fan being engaged all the time is certainly not helping you.

The short hauls aren't exactly helping either.

You can re-gear the rear axles but honestly it'll probably be a long time before you see a return on investment. Not worth it IMO unless its time to rebuild the diffs anyways.

 

A truck like that is geared more like a dump truck. Local work and sometimes traveling on highways.

It is geared low like that so that it is easier to pull heavier loads and be able to crawl off road.

A good usage for a truck like that would be pulling a low bed or going into landfills.

It is a different spec than an over the road truck.

Tall 24.5s would help a little and the 18 with .73 od would help also.

You may already have both.

 

Yea going to tall rubber if it doesn't already. We do mostly highway miles with both of our trucks.

The 93ch i drive has 3.91 on lo pro 22.5 and 65mph was 1600-1650 with .73 it needed a rebuild so we built an old school dbl .62od (14609B and 14613 aux) brought us down to 1400 at 65. Imo for you its prob cash better spent elsewhere.

The other mack we bought was a heavy haul/texas oilfield truck with 4.42 on tall rubber and its at 1600 at 65. Same as 3.90 on small rubber in the .73 slot.

If you plan on keeping it a while you could go that route. The T2180 is on its last legs and thats the only reason were building one of those dinosaurs

 

My suggestion: get a more aerodynamic truck

 

That's not gonna help much on a 20 mile haul

 

I tried.

 

What were you thinking when you bought this truck? I love cats but that motor is not a fuel saver and you got the wrong gears ,and you haven’t even fixed the fan yet. hopefully you can get this all straightened out before you go broke!