ISX 5.3 mpg is it normal??

I would agree, that with 3.70 rears, only option to get decent mpg is to slow down some. One mentioned putting on taller rubber, but after you buy new wheels and tires that have a higher rolling resistance, you could regear the diffs for almost the same money. No need to reduce hp on the engine. If geared and driven right, an ISX spiked up to 700 hp will get as good or better mpg than a 400 hp engine.

I have an '06 ISX, 435 ST, 3.42 rears and 13 speed, and drive it between 62 and 65. My mpg averages in the low to mid 7's. Of course the best thing I did to help mpg was to put a steel plate block between the EGR tube and the intake manifold. That got me a minimum .5 mpg better fuel mileage.

 

Fleet Trucks with the Company I was with used to average 6.5 mpg at 65 MPH pulling Doubles with a 6x4. They averaged roughly 72,000 lb Gross. They were geared for 1450 rpm at 65 mph and the 2004EPA ISX was set at 400 / 1650.

 

i had a friend with a ISX 13 speed.. at 60mph, best he could do was 6.1mpg.. its that flat nose that gets ya and the trailer

 

Wow, that is low on the MPG's!

Yu are ahead of the game 'cause you have a 13, most of the rest have only 10 geears to play and nothing in between to split...

1. Low(er), speed
2. Aerodynamics, add trailer fairing (~1,800 installed)
3. Driving habit, get off the cruise when hitting hills/specially mountains
4. Synthetic oils, all oil/grease locations
5. Air filter/exhaust too restrictive ?

ETC..., hope this helps BUT never lower HP, you will suffer and the margins are too low and too aggravating(getting passed all the time/fighting will small hills).

Mine is Celect Plus # 500 W/3.78 # 10 speed/22.5's low pro's

 

My truck is spec'd similar to yours, and I'm seeing high 6's with heavy (76,000 to 80,000 gross) loads while running 62-63 mph. When I can keep the loads light ( <25k in the box), I get mid 7's to low 8's. I've played around with mine quite a bit to see just what this combination likes best.

The sweet spot with the ISX seems to be around 1400 rpm. If you're running light, you can get away with 1200 or so, but when you're heavy you'll start to get 'chuffing' noises coming from the intake. That VG turbo doesn't like trying to make boost at those lower rpms, and the air velocity drops & momentarily reverses direction.

If I drive 68-70, I'll see my fuel mileage drop like a rock. If I try to lower the cruise rpm, fuel mileage drops. There's a somewhat narrow window where this motor shines & gets great mileage, and you've got to be on the ball as a driver to keep it there.

Progressive shifting is actually not a good idea with this motor, as you'll get that reversion ('chuffing') if you shift # 1000 to 1200. I get great fuel mileage shifting between 1400 to 1500 while loaded, and no 'chuffing'.

If it has a lope or idles rough, try adding some 2 stroke oil to the fuel at about a quart or half gallon per 100 gals of diesel. That did the trick for mine and it did it in a hurry. Supposed to have to do with lubricity in the inner workings of the injectors. Whatever it did, it smoothed the idle considerably, and I've see a consistent .2 mpg across the board as a result. I dosed 4 fill-ups that way, and haven't done it since, but the effects have lasted.

What's been previously said about the ISX not liking higher RPMs is absolutely true. Anything above 1550 while shifting, and above 1500 cruising is empty revs, and completely pointless.

I would absolutely discourage you from lowering the HP of your motor. If anything, I would get it bumped up to it's maximum, which is most likely 500HP/1650TQ. Dropping off horsepower just means that when you're climbing a grade, you'll have to drop even more gears, which means more time spent climbing the grade, and far more revs per mile while doing so. It all equals higher fuel consumption.

Anyways, there's my $0.02 on the subject. Take it or leave it, but it's been working pretty well for me.

 

Hi guys, thanks for the replies, yup I didn't actually realise that there was a difference in us and imperial gallons, my apologies, you can drive faster over there but you got an overdrive gear I think? That would compensate a little as your engines won't be screaming for mercy at 70 mph, but we also carry Alot more weight 97000 pounds on a semi and 132000 pounds on a double. It's wrong to think our engines are smaller capacity and less horse power, the average hp here is between 480 and 560 hp, here in Ireland we have a great fondness for the 16 liter v8 and those range from 500 to 730 hp, 500, 560 and 580 are the most common (owner drivers love the 620hp and I'm sure will love the 730 more) but the new 730hp model is a 16.4 with a new Scania/ cummins collaboration fuel system that scania are promising 11mpg at 44 tonnes (I'll believe it when I see it) that's the one I'll buy next for the refridgeration work. I was wondering could I buy a US Volvo with a Volvo engine as parts for cummins could be a little difficult here, I am really looking forward to getting an American rig, as a matter of fact I'd love to be trucking in America , over here it's over regulated, it's no fun anymore, take for instants its forbidden to overtake on the motorway at certain times of the day in countries like Germany , holland and belguim , you sit for hours in what's basically a que for hours at a time, we also have tachographs here which monitor everything we do, hours, speed and distance plus our length is restricted which rules out long cabs like yours ( scania did offer long cabs but due to the inaction of our politicians ditched them from mainstream production, pity, they will still build you one but at great expense) I am going to buy a US rig fir oversize loads where I would have a permit for a long truck. Oh those magnums really aren't considered a super truck here, they reduced the price by 15,000 euro and gave a 5 year bumper to bumper warranty just to sell them, once again thanks for they replies guys I hope to hear from you real soon,

 

I know Magnums are not much, but there wasn't too many occasions for Euro trucks to drive in the Western States so it was interesting case on fuel efficiency of European motors. It make me doubt there is any advantage on either side, the difference gotta be in driving conditions and gearing.

As for horsepower, I'm really glad to hear about proud Irish truck owners, I was wondering if there are any left anymore. Was reading on BLB about how you like to accomodate thos 700h motors and all, but honestly how many of you guys still buying own trucks? Why don't you write about it, please give me some better idea. Heard stories about you crazy Irish leaning to run hard in the past, isn't electronic logging killing you financially? Are you, single truck driver/owners thing of the past already?

When it comes to your choice of truck you want to import I think Volvo 780 or rather 880 with D16 625hp is way to go. But only because Cummins is not popular in Europe. D16 has history of some serious problems here, most of them were fixed under long, 500,000 miles warranty Volvo offered as a part of premium deal those truck were marketed (the VT 880 mostly). I strongly recommend you digging into it (there are few threads about it on this forum as well) and later try working with Volvo USA to learn what job were done on the particular truck you will pick - before you buy it of course. It is serious deal you are getting into, high hp motors were dropped at some point and whole VT line as well. I don't know exactly why, but if you serious about doing this - be careful. Not trying to discourage you, you definitely have right idea!

 

Hi there, you actually need to lessen the gap between the unit and trailer, over here we run fridges with the refridgeration unit just 6 inches from the back of the cab, we keep the pin on the trailer on it's deepest setting, the closer the trailer the better so as to create as little turbulence as possible, we find the gap difference can give between .5 and 1mpg difference depending on settings.

 

Unlike EU trailers the US trailers have king pins welded to upper plate of trailers so there is no way to change a gap this way. The gap can only be changed by sliding a fifth wheel which can overload a steer axle if moved too much forward. Most common kin pin setting here is 18". If a trailer's king pin ever needs replacement a worn pin is cut off and a new one is welded back.

 

why it is not placed with screws here trailers have 66inch from king pin to front wall and 64 from king pin to front corners longitudinally and 80inch radius

in countrys where they have 3axle tractors 75-76inch from king pin to front wall and diameter of 88inch