T660 ISX FUEL MILEAGE

Dash computers figures are notoriously inaccurate, mine is out about 14%. Only real way to figure is how many miles you just drove since last fill, divided by how many gallons you just bought.
I had a 2001 Freightliner Classic with an ISX that got 5.88 mpg at 62 mph, so yours sounds about right considering better aerodynamics and going 68 mph.

 

My dad's last 2 T800's averaged about 4.5 MPG (530 HP ISX in 1, 475 HP C15 in the other). They were both also tri drives and were grossing over 100,000 lbs daily with a lot of hard pulling off highway. I'd expect to see your average would be a little higher than that since you have a much more aerodynamic truck, probably aren't running off the highway in steep mountains very often, if at all and are probably grossing under 80,000 lbs. Try and calculate the mileage out by hand like uncleal13 suggested.

 

How many miles are on it?might need a top end set or new air filters. I heard guys saying they were in the low 7 with that type of truck.what's your rpms at 68mph? My 06 9900 with isx pulling a step is 5.8 to 6.8 with 13 .

 

Before you spend any money on any repairs, put a pen and paper to it. Dash computers are never accurate.

 

Agree with all that said dash computers are way off. I drive between 65 and 80mph, don't go anywhere under 79,500lbs and I'm in the mid 6 range.

2012 T660 ISX 13over 3.08 rears on tall rubber with 550000 on the clock

 

I drive a w900l with 500 ISX. Has 13 speed, 24.5 rubber. If I turn her loose she runs triple digits. I try real had to keep it at 70. With 45K no skirts or tail it gets around 6mpg except under adverse conditions and it will drop down to 5.5. Has 145K on it. Suits me just fine.