ISX 5.3 mpg is it normal??

you really can't use that as any sort of comparison. Sure you beat the truck but you were still bobtailing. I can get 8mpg bobtailing my 1982 BigCam II.

 

1: slow down, every mile per hour over 55 reduces you're MPG rating by 0.1 MPG so if you're getting 5.3 running at 68 you should get 6.8 running at 55. the trick with the cummins is to keep the engine rpm's as close to 1450 as you can while cruising.
2: get the trailer as close to the back of the tractor as humanly possible, wind won't jump more than 30 inches, and optimal is 24 inches or less.
3: shift progressively get into the highest gear as quickly as possible, and don't idle the truck while loading, fueling, sleeping, eating, showering or otherwise goofin off, the worst fuel mileage is a 0 mpg. and 1000 rpm LOL. cummins website as a program that will allow you to type in you're ratio's and settings to see if you are optimizing you're motors capabilities and also has tonnes of fuel saving tips for operators.

 

somethin else you might try is to get a wheel alignment, 3 axle if using someone elses trailer 5 axle if using your own. When I bought my truck I was getting less than 5mpg. Got my axles aligned and cruise at 60-63mph nets me 7+mpg in a 2k Classic with a NON governed 515hp Cat and 13 speed with the same rears with 30k in the box. Might not completely solve your problem but every single .1mpg is money your your pocket.

 

Up the horsepower to max. leave roadspeed alone. instead of rears,go with taller tires(assuming you have lowpro). do complete alignment.Wax truck(sounds crazy,Iknow) Keep tires gauged to specs.Check aerodynamics.Lucas fuel treatment has always saved me double its cost.Howes also.Check exhaust,maybe go with less restrictive.

 

I'm sorry gents if I come across a bit stupid, i'm an Irish owner driver and I do refrigeration work all across Europe , I have a couple of questions for you, is that the average fuel consumption over there? About 7 mpg? Over here that would be unacceptable , I get average 8.9 mpg out of my scania r580 pulling 44 tons (97,000 lbs) I'll get well over 10 mpg if I have a light load. Is fuel cheap over there? Here it's terribly expensive maybe that's why ye are not too bothered about the mpg, and can I ask why you leave the engine running all night? Is there no cab and block heaters on them or maybe no air-conditioning ? Please excuse my ignorance but I'm dreaming of owning one of those big American rigs and I am thinking about importing one for lowloader work I got coming up but I don't know if I could justify those fuel bills, or maybe peterbilt will fit a scania v8 for me hehe, thanks guys, any advice is much appreciated, keep on truckin

 

Isn't a US gallon different than a Imperial Gallon?

 

I'm not certain I'm doing the conversion correctly, but I believe 8.9 mpg using imperial gallons is about 7.4 mpg in us gallons. Also based on what I could find on the net the truck speed limit in Europe is much slower than the speed limits in the U.S. So when those factors are considered it may explain the discrepancy in fuel mileage numbers.

 

Dont drop it to 400HP. Trust me those hills are going to kill you. I have a Volvo d12 395HP and things aren't too fun. Keep it in the 435+ range

 

Driving speeds are WAY higher here and wind resistance takes its toll on mpg. This is something you don't deal with in Europe driving stuck in traffic. To give you some idea: my average speed on all miles on my truck is 95 km/h. Which is way above your speed limits, right? And this is average speed, including all the traffic.

There were those Renault Magnums with that Route 66 adventure/advertising gig and from what I have heard from drivers who talk to those drivers - they were really scratching their heads seeing their super duper Euro rig getting 6 mpg (US gallon is different than imperial) driving in western states.

I doubt your Euro engines are more efficient. They're just smaller in capacity and hp

 

Run at 65 mph for a while like the others have said.