gear box upgrade

Might be easier to dump the truck and buy another one with a 13sp.

 

Swapping to a 13 just for MPGs probably doesn't make economic sense. ROI is way out there, unless maybe you already have the trans and you are doing the work yourself.

Swapping a 13 in for a broken 10 speed is a no brainer.

 

There was a discussion here somewhere on this forum. The biggest problem was size.

 

Have the turbo tested. Turbos don't last long on ISX's.

Have you done the overhead? Maintanance is 100,000 miles. Unless that changed with the newer motors.

I've found the best mileage using mobile 1. ISX's don't love any oil but mobile 1 and delo 400. 2 kw's i drove, actually had kw manufacture stickers recommending mobile 1. On the engine frame.
Most oils run hot and the engine sounds like a tank. Mobile 1 runs cooler and the engine purrs like a kitty. That's with an ISX. I don't have much experience with different oils in detroits or cats.

Check your tire pressures.

What's the spaceing between your van and sleeper? Slide the 5th wheel up to put 12,500 pounds on the steers with full tanks. Unless you've got 13,000 pound steers then move it up closer. MINIMIZE that air gap.

It's winter time. Means winter blend fuel. Mileage goes down in the winter. Mileage also varies on the fuel being used. Even the same truckstop will vary. Also, mountains vs. flatland. Where are you generally running?

If your running the west coast. 5.5 is about average. Around 6 pulling a van. i've only ran 13 speeds pulling flatbeds. Around 6 pulling a van which was rare. I've mostly ran west coast.

 

Step one in your situation, find a good ISX mechanic not a parts changer. Throwing good money after bad only makes it worse