Peterbilt 378. 3.5mpg. What can I do?

Regear it. Changing tire size will not alter gear ratio as much as you need. Plus added weight and rotational weight. Neg effect. 3.23-3.73 will be ideal. At the chance of picking up 2mpg. Gears will pay for themselves fast. Tall rubber in this day and age is only useful when you need ground clearance. Tell us more about your operation.

 

In 1996 the speed limit was 55 mph. Your truck is geared for 55mph roads, not 65. Your cranking to many rpm’s for fuel mileage. You probably ain’t gonna get any mileage doing 20 mile runs anyway

 

There is in my opinion your problem. Get that rpm down to less than 1500 at Hwy speed and the mileage should come up considerably

 

Holy smokes.. you can certainly run at 2300 but yeah you are going to be drinking fuel

 

Yes the fan running does have an impact. 2300 rpms is by far the biggest problem

 

I run some short and heavy, 90-100 gross. 12.7l 13 speed and 3.58. Loaded 50% and do 5.3-6.5 normally. The weather is the variable.

Your geared for 150k plus.