X15 questions

[JimT]

Only things that jump out at me are the size/wheelbase. Daycab will be a little lighter then a sleeper. If you can run a daycab then you're looking at about 150-160" for a single axle (which sounds doable with your weight) and about 185" for a tandem axle. You combine good aero up front, a shorter wheelbase and a small gap between the cab (with full roof fairing) and trailer and you can probably gain about 1mpg. maybe more if you switch from a tandem to a single axle. (not sure what you have or if it's doable for you).

As for the engine, there could a some fine tuning that might help. Besides the tune there could be a few physical differences between your engine and the one in the rental that might contribute (speculation on my part). Also, check for boost leaks, EGR tuneup, exhaust leaks.

Then there's the rest of the driveline. You didn't mention specs for the rental so it's hard to compare. I was having a chat with a friend the other day about speed and fuel mileage. He noted that sometimes he can get better fuel mileage by driving faster then 62mph. I pointed out that the engine RPM can play a big role. Depending on your rear end and trans gear ratio's you may find that slightly faster speeds or a different gear yield different results. I for one found that in my truck 63mph in 13th gear was better then 62mph, but in 12th gear 62mph was better. Also, again in my truck, 1350, 1450, and 1550 rpms are sweet spots. Even as little as +/- 10 rpm and I can see a difference in fuel usage. (based on repeated testing with cruise control set with same weights and roads). My truck does good in 12th and 13th gear, but if I drop below 12th gear (direct) then I see slightly lower fuel mileage. There's a chance that your truck might perform better at a slightly higher speed, like 58mph or something else up to about 62-63mph.

Finally there's driving style. You sound like you have a handle on that part and I'm getting too tired to type much more. But for others reading this, driving style or habits are probably one of the biggest contributing factors to good or bad fuel mileage. Keeping boost levels down, letting it gain speed downhill (with tranny out of gear if conditions permit) and letting yourself slow down a bit going up the hills. There's plenty more to driving style, but you probably get the idea.