Peterbilt 378. 3.5mpg. What can I do?

24.5 tall rubber (not lo profile) will help, but probably not enough to make financial sense. If you need new tires anyway, and can get 10 - 24.5 wheels at a reasonable price, I would do it. If your current 22.5 are in good shape I wouldn't spend the money to switch. You have to figure out the cost/reward payback ratio. $3,000+ worth of wheels and tires will buy a lot of fuel, and it would take a long time to recoup that expense on your current run.

 

Go play with eaton roadspeed calculator. As I said. Unless you will benefit from tall rubber in ground clearance which is understandable in some applications. The added weight, cost., and rotational mass with not be beneficial nor provide the gains you want. While all the issues you have need addressed and are harmful to your mpg. Again it is not going to provide the gains you need.

 

What Lester says, you have 2 OD gears, nearly every 13 and 18 speed has 2 overdrive gears.
If there was no trans between the engine and driveshaft the ratio would be 1:1.

16th (or 8 high) is direct 1:1
17th (or 9 low) is .81:1
18th (or 9 high) is .73:1

Going to 24.5 rubber would gain about 3 or 4 mph.

Play with this for a while.
I guessed your tires are about 40" tall which would give you about 60 mph @ 1700 rpm in top gear. (.73 ratio)

Transmission Ratio RPM Calculator | Spicer Parts

 

May lower RPM but takes more jam to turn those tires. He might not see any gain from tall 24.5 at the end of the day.

 

In your situation not going to make much difference. (Air pressure)

Running your Emodel cat on the governor (2300rpm) is your problem.

Either slow down to 55 or change your rear gear ratio. Or buy a different truck.

 

Doing 20 mile runs not much makes finical sense return wise unless you’re changing the part anyways.

 

schlumberger has 367 petes on 11r22.5 and they turned a 1950 or so all day long and the older trucks did 2100 at 58mph 425/1650 isx12 gov was set 2300 . they wouldn't gov them like that then.

Most steering wheel holders don't realize lugging puts more stress than cruising at a higher rpm. On the E-7s 1400-1450 is about the lowest ill hold it on a pull. They're slow shifters too so you gotta wind em right up about 1900 for it to fall back in at 1400

 

Let us know if it rolls a little easier.

 

Our triaxle has 4.56 gears and 8LL (.74) on 24.5s and it will still run 70 down the highway at 1800 rpm, our 96 tractor has 4.33s and a pos 15 speed (.79 od) that runs about the same, and gets 5.5 pulling ~100k 10 miles and running back empty 10 miles (dump), our spare truck does have 12/22.5s and is on the 2100 limiter at 68 mph with an rtlo 13 (.73 od)

rubber would be the easiest to start with, along with getting the fan fixed