Fuel economy

I had a 2001 Classic with a 565hp ISX. It got 5.88 mpg, so yours sounds right to me.
Certainly can get better on some days, but the wind kicked the heck out of it.

 

Are you happy with 6mpg? With some adjustments to your driving and some upgrades to the truck there's no reason you can't get 7.5+ but it's all in how bad you want it.

 

Bingo.

Those trucks are capable of 7.2-7.5 with correct specs and driving.

Change to a modern VVT turbo , add economy tuning and downspeed the rears(2.85-3.08 range), change to a 13spd and you'll be 8+ capable.

 

that's normal only way your getting better is drive like granny

 

That's a $15000-$20000 upgrade and it's not going to get you from 6 to 8mpg. Things like high flow mufflers/air filters, low rolling resistance tires, making aerodynamic improvements like roof fairings are all a lot cheaper and will get a better return on investment. Things like re-gearing, transmission swapping, and variable vain turbos are things you do when you're already getting 8mpg and want that little extra when you're already changing those parts

 

I have a long nose Pete, Cat 550, 13 spd, 3.73 and 24.5 rubber. Zero aero mods, pulling a dropdeck and average 6.8. I rarely see lower than 6.6 and see 7 to 7.2 often.

Only thing I do is keep the rpm at 1350-1400 at cruise, tach never sees over 1600 when shifting and I try to keep my boost gauge as low as I can on hills. I also noticed the boost gauge will rise when using the jake on downhills so I only use it when I'm heavy.

 

Just FYI you're Truck isn't using any fuel when the Jake's are on. The Jake's only open the exhaust valve at the top of the compression stroke. That exiting exhaust is spinning the turbo and causing the boost pressure.

 

I had wondered about that, makes sense now.