379/ISX fuel mileage issues... looking for ideas

Donaldson M100463

 

I don't think you have 3.70's, Captain. You cruising speed with those tires sounds more like 3.55's to me.
I was on a rant about getting better mileage out of my ISX not that long ago. I added the Donaldson muffler & noticed a tenth to two tenths gain.
I ran the overhead a month prior to that & got only a minor improvement ( < 0.1 ), but it was pretty close to spec and I wasn't expecting much there. I still plan on running the overhead once a year.
For fuel additives, the only thing I've ran that has shown anything in mpg gains and idle smoothness is 2-stroke oil. Have tried power service, Howes, Lucas with no improvement. Plain old cheap@$$ Walmart 2-stroke oil at about a 1:100 ratio stopped the rough idle & showed me about a tenth mpg increase.

 

Didn't the 2006 models have the troublesome EGR valve? Your mileage is (was) worse than my 400 ISX with 9 speed and 3.55 or was it 3.58 gears in the pumpkins. Didn't have quite the miles on the engine (about 120,000m less) but was a 6.5mpg+ engine and that was at 67mph and with mountains.

 

I have 07 t800 Kenworth with 70 in mid roof sleeper I haul camp shacks and Mobil homes all wide tall and long great for catching wind the unit loaded weights 88000 lbs or better I maintain on average of 6.1 mpg running at 1450 rpm to 1550 rpm about 105 km/h

 

Give Pete the last 6 of your VIN and they will tell you what your ratio is. If your mufflers don't have any holes in the body of the muffler i would save my money.

 

But, stock mufflers are notoriously inefficient. They don't put on the best mufflers, only the best price they can get on a supplier bid. The Donaldson high flow could make quite a difference. Also, keep in mind that the baffles in stock mufflers collect a lot of soot and can really restrict exhaust flow. Heck, we're only talking a few hundred dollars here (two stacks, less for one) that would easily be recovered in a month or two on better fuel mileage.