Peterbilt 378. 3.5mpg. What can I do?

I have a couple concerns, besides the fan needs fixing for sure.

How can you have 21,500 rear axles?
Is that a single drive axle with a tag axle?
Should be 38K 40K 44K 46K tandem.

There's something not right if you're running 65 mph @ 2300 rpm with a .73 OD and 4:65 gears.
I had a 66 KW with a single over 13 spd, (.81 or .83), with 4:88's on tall rubber and that truck went 65 mph at around 2100 rpm.

One thing you can try to improve mileage along with running 55mph if you live in a wintery place is park the truck outside overnight and next morning when your tires are cold air them up to 95 or 100 or 105 lbs, whatever you run them at, before you drive it, trailer tires too.

Airing up a warm or hot tire in any climate is a guessing game.

 

That fan alone is probably sucking up 0.5 mpg.

 

You have an 18 speed transmission, correct?

Most 13 and 18 speeds have two overdrive gears above direct gear, which is a 1:1 ratio, which means for every engine revolution, the driveshaft makes one revolution.
Your OD gears are .81 and .73, so the engine turns .81 or .73 revolutions for every full revolution of the driveshaft, depending which OD gear you are in, engine turns slower, truck goes faster in OD than direct.

Get your tire gauge out and check your pressures, but do it when the tires are cold, see if they're all equal or make them equal.
If your tire pressure is too low or they're all at different pressures your fuel mileage will suffer, especially in cold weather.

I would try 100 psi for starters on trailer and drives, and maybe 105 on the steering.
You need to find your happy place, too much or too little air pressure has it's own set of drawbacks other than fuel mileage, like ride quality and tire wear.

 

An afterthought.

You're a little green, not a bad thing as long as you're willing and able to learn.

You're truck is set up for heavy loads, off road or mountain duty, going by the way it's geared, so I ask you, is there a mystery button in your truck that you have no idea what it does, perhaps a red knob on the gear stick bellow the shift knob, or maybe an extra set of air lines or wires coming out of the floor near the shifter that don't go anywhere?

There's a slim chance, (really slim), your truck night have a 2 speed differential that you might not be aware of, in which case theres a very slim chance your truck may go faster than you're aware, would be a great bonus, but the odds are stacked against it.

Most 2 spds I know of have a lower ratio than 4:65, deeper than 4:88, but you never know.

 

Your tach may be off, as well as the speedo. Check your speed with your phone's gps ap.

With those tall gears and short tires, even with a double overdrive 18, it sounds like it was spec'd for heavy haul/off road use.

Doesn't sound like you are going to get a good mpg with that setup. Drive slow is probably your best option without spending money for new gears, as that will take some time to pay for itself.

My 2 cents.

 

With an 18 speed I believe 16th gear would be direct, as in a 1:1 ratio, input on trans turns same speed as output. 17 and 18 are overdrive, meaning a 1:.73 or so ratio. I put shaft turns once and the output will turns 1 and about a quarter.

2300 rpm is screaming. I'm not a guy that's afraid of a few rpm but 2300 is a handful. There's many people here that think anything over 1700 is out of the question. But generators and ag equipment run 2100 all day.

 

And tire size won't make a big difference. Some yes but not huge