355 to 308 ratio?

[JimT]

(04-16-2022 )earnies2 Wrote:  I just can't seem to discipline myself to run 55 when I'm getting passed by prime and swift in Oklahoma where the speed limit is 75...

What do those other trucks have to do with your truck? your delivery schedule? your fuel costs? ... nothing! Who cares if they pass you? It's not a race against other trucks. Run your business based on your needs, not based on the actions of others. If you need to go faster to keep a certain schedule and your rates reflect the higher costs you will incur in fuel and maintenance then do what you need to do. But any time you can slow down and still get the job done then that means more profit for you in the end.

As for your original statement, you didn't specify what RPM, gear choice and boost levels go along with both fuel mileage scenarios. That is, when running 55 getting better fuel mileage what gear are you in, what's your RPMs, how much boost are you using? Same for the higher speed/lower fuel mileage. I suspect those details will reveal why the difference in fuel mileage and will help to find a solution.