ISX 5.3 mpg is it normal??

Man "Brotha" thats gotta hurt, Ive got a buddy that runs an 05 ISX set at 500HP like yours and is getting around 7 MPG's with 3.70's and a RTLO16913 trans.and keeps it at about 70 MPH.

 

trade it off on something a year or two older, or before whenever cummins went to emissions motors. Sorry, don't know allot about them specificly.


Lowering you HP will do nothing but make you hate the truck even more. Going to that low of a gear with a 68mpg limiter will likely ake your top gear useless. These things need to be worked out on a rpm calculator to keep your optimum rpm at your desired cruise speed.

you don't say what trans you have. you don't say what your's hauling and the type of driving OTR, local, off rd etc.

5mpg is bad for an otr van truck, it's good for an oversized heavy hauler. see what I mean?

 

Well, being that you're a newbie, according to your profile, I'm gonna stick my neck out and offer a different view. Take a look at your driving habits. I use cruise as much as possible, I rarely run over 60 MPH. But most of all, don't drive it like you're qualifying for NASCAR, especially on surface streets/slow traffic. I saw a guy take off the other day with so much torque that I thought his frame was gonna snap.....
Just a thought anyway...
BTW, I have a 400 ISX 9 speed and 3.58 rears, and I am getting an overall of 6.4 according to the ECM with just over 100K on the truck.

 

those fridges don't get great mileage. There are a whole lot less aerodynamic things out there but there is allot better too. For starters you need a little more gap between truck and trailer t clear the unit. that sure doesn't help your mileage.
I honestly don't know much about the isx's but it seems to me for every person I hear getting good mileage with them I hear one saying it's bad. Personally I would take it to someone who really knows the ecm setting well and find out what it's set at and get some ideas of what can be done to make it better. And no the cummins shop isn't the best place to do that they are all more interested in keeping it running "clean" then keeping it running well.

 

So if you are running 70mph, with a 3.70 gear, and a OD 13 speed you are turning close if not over 1600rpm. A 05 ISX with a VNT turbo and egr loves low rpm's, id say you will gain millage with a 3.36 rear over the 3.70's. Those motors hate to turn that high, these new motors are designed to run at 1300-1400rpm at cruising speed, thats how you get millage out of them, turning 1500+ cruising will net crap for millage on them.

Try slowing down to 63-65mph, i bet you will see 6mpg.

 

It would be far easier to tell you some of the things I have learned about isx if you were on at the same time as me some things to look at is your turbo actuator operating properly if you dont know what that is I can explain it to you just ask it makes a difference has the egr been replaced have the breather tubes been cleaned new air filter has your egr had all the latest updates done is the lift pump working properly this only the beginning of the things that i could tell you about isx all these things and another couple thousand worth i have adressed in the last 140,000 miles contact me tell me every dertail you can i can give you an opinion

 

if you've had to address ll that in 140k miles I don't know if it sounds like a motor worth having

 

No 5.3 MPG is not normal for a Cummins ISX of any year. Pre emissions engines would be anything before 2003. I know that is when that emission regulations started. I have heard of a few 02's with emission equipment on them. Anything above 1500 and the engine will not sip the fuel anymore it will drink it. I went on a 2300 mile round trip bobtailing in a 1997 Western Star 4964EX. The truck idled every night for 6 nights. It then idled for four days and nights. Part of the trip, about 2 hours worth I ran the truck at 1900 rpm. The truck was only off for 16 hours out of 10 days. I still got 6.8 mpg. I was not easy on that truck when I took off from stops. I had to keep up with the other truck in front of me. Granted most of you will say but you were bobtailing. Sure but most of you don't run your trucks at 1900 rpm for two hours either.