X15 questions

[Camstyn]

I have a 565/2050 in a 2020 Pete 389, 260 wheelbase with 78” midroof, 3.73 rears pulling light van loads (courier freight, licensed for 55,000lbs but most miles are near empty)

Lifetime average is 6.3, 6.0 on winter fuel and 6.6 on summer fuel. Tried turning it down to 505/1850 but it gets the same MPG as factory setting, and drives the same, just less power on reserve at the end of the pedal which it never gets anyways.

Curious if anyone knows if the 500/1850 Efficiency actually gets significantly better economy than the performance when driven the same way. I did my linehaul run with a rental International LT day cab last week which has a 525/1850 performance series and it was way better than mine in fuel, like almost 20%. I know aerodynamics play a small factor (keep in mind I don’t drive faster than 57mph and this was a day cab), I’m a little baffled as to why it was so much better on fuel than mine. Tare weight is lighter and it had slightly better rolling resistance tires but same winter type treads. Rear end ratio and transmission same as mine.

I’ll be buying a new truck in the next year or so and this was a bit of an eye opener for me. A 20% fuel savings would be significant. I can do this run with a day cab if I want to.

You might be asking why I bought a truck like this to do this job. Resale value. The truck it replaced was an aero 579 with Paccar MX13 @ 500/1850 and it was no better on fuel than this truck, and I could barely give it away when I was trying to sell it.