Paccar PX8 Fuel Mileage

jjoerger · Feb 19, 2011

A friend of mine is buying a 2011 KW T370 with a Paccar PX8 set at 300HP and an Allison 5 speed auto. It has a 272 inch wheel base, 12k front axle and 21k rear with a 5.57 ratio. The truck will have an 84 inch sleeper and 22 foot dry box.

The dealer has him convinced that he will get 13-14 MPG with this set up. (He is using this to help off set the monthly payments)

Does anyone have a similar set up or opinion on what type of mileage this will get?

I say 10 MPG if he is lucky.

06081956 · Feb 19, 2011

I have a freind that drives a truck like that one but it doesnt have a sleeper. Its just a 26 ft box. He tells me today that he is getting around 10 depending on your driving habits

LDB · Feb 19, 2011

Why the Allison 5sp and not the 6sp?

greg334 · Feb 19, 2011

I think it would be a 6 speed in it, the only difference is the ecm settings anyway.

nightcreacher · Feb 20, 2011

Bigest mistake in buying a truck,is trying to offset the payment by saving fuel,There are to many variables that can take a truck that is gettin 13 mpg and cause it to drop to 8 mpg.Just think how many loads at that lower fuel mileage it would take for you to go broke..Load weight,weather,mountains,these are just some of the things that can change your fuel mileage.I'm in a tractor,I figure my expenses at 5 mpg,but last year,I ran 140000 miles and my average fuel mlieage was 6.7,so in actuality, i was ahead of what I had figured.I did have loads that put me uder the 5mpg because of the mountains out west and the strong atwinds,I also had loads that I got over 8 mpg.When i bough this truck,if i figured I needed to get 7 mpg to help make the payment,I would have been behind the 8 ball
In 1983 cat came out with their B modle 400 hp motor,claming 6 mpg.A lot of guys went broke cause most of the time that motor got the same 4 and 1/2 as the other Cat motors

jjoerger · Mar 22, 2011

The truck is now being built with the following changes.
1. 6 speed auto transmission.
2. 4.88 rear gears
3. disc brakes all the way around.
4. Low rolling resistance super singles
He is having a used KW studio sleeper custom fit onto his cab and will be using an Arctic Air battery powered APU and Espar heater.
There will be an air deflector on the cab and sleepers roof.
He is going from an 8 MPG truck to this new one and hopes to be able to get 12 MPG from this one. With fuel at $4 per gallon and driving 130,000 miles a year he will be saving $21,666 dollars per year if he can get that kind of mileage.

Does anyone know of a shop that does body skirts with the built in tool boxes like you see on RV's?

greg334 · Mar 22, 2011

Is that an Automatic transmission or an auto shift transmission?

Ragman · Mar 22, 2011

greg334 said:
Is that an Automatic transmission or an auto shift transmission?

OK then, you can call me a dumbsh*t, but what is the difference?

greg334 · Mar 22, 2011

OK dumbsh*t

, the difference is that the automatic (*like an allison 1000, etc. ) uses a torque converter and an autoshift has an actual clutch.

There are two types of autoshift, one is called an Auto Shift where you have a clutch to start moving the truck and when you stop, you push it in (everything after it is moving is done by computer) and the other is called an Ultra Shift has no clutch pedal and the computer does all the work.

There is a big difference between the application of these two types and the reason I ask is if he has 4.88 in the rear and has an automatic, he may not see the 12 he expects but rather 10.

Ragman · Mar 22, 2011

greg334 said:
OK dumbsh*t , the difference is that the automatic (*like an allison 1000, etc. ) uses a torque converter and an autoshift has an actual clutch.

There are two types of autoshift, one is called an Auto Shift where you have a clutch to start moving the truck and when you stop, you push it in (everything after it is moving is done by computer) and the other is called an Ultra Shift has no clutch pedal and the computer does all the work.

There is a big difference between the application of these two types and the reason I ask is if he has 4.88 in the rear and has an automatic, he may not see the 12 he expects but rather 10.

So with Ultra Shift, the trans goes thru the gears without having to push a pedal and move a shifter, do I understand that right?

jjoerger · Mar 22, 2011

It's a 6 speed Allison Automatic.
I got an email from him today that says the factory is recommending he go with a 5.29 ratio and they are still discussing using disc brakes on the rear with the super singles. (I have no clue why that would be a problem)

greg334 · Mar 23, 2011

Yes ragman, that is right.

I would not put an allison in any of these trucks because of two reasons, one is that it is a double od trans, and the ultrashift is a better trans for this work.

Ragman · Mar 23, 2011

greg334 said:
Yes ragman, that is right.

I would not put an allison in any of these trucks because of two reasons, one is that it is a double od trans, and the ultrashift is a better trans for this work.

Thank you so much! I guess I should learn a bit more about the different kind of transmissions and how they work.

greg334 · Mar 23, 2011

The other thing that comes to mind is how the shifting can be altered.

With an Allison, it is a bit more complicated to actually program progressive shifting that gives better mileage but with the auto shift transmissions, it is actually easily programmed and does improve fuel economy.

jjoerger · Mar 24, 2011

This whole project has been cancelled. He is buying a used 2009 (bought new in 2010) Sprinter 3500 Ext with a built in sleeper. Current owner is showing proof of 19 mpg average over 75,000 miles (actual mileage) and has all maintenance records from a dealer. Got it for $29,000.
Has roof A/C and a diesel generator.