Go Back   iRV2 Forums > TRUCK CONVERSIONS, MDT's & HDT's FORUMS > MDT Conversions
Click Here to Login
Join iRV2 Today

Mission Statement: Supporting thoughtful exchange of knowledge, values and experience among RV enthusiasts.
Thread Tools Search this Thread Display Modes
Old 12-11-2012, 02:43 AM   #1
Junior Member
Join Date: Jul 2010
Posts: 5
Fuel Mileage

Hello. Haven't been here in quite a while. Anyway wondering if you could help. I deliver RV's commercially and I'm specing a medium duty truck, because I'm tired of all the headaches with unreliable pickups. So far I've had a T-270 Kenworth drawn up and will likely have a sleeper installed.
Originally I spec'd it with the PX-8 (ISL 8.3 Cummins, 330 h.p./1000 torque) due to it having rebuildable wet sleeves.
But my question is.....would there be any gains in fuel mileage by dropping down to the PX-6 (6.7 Cummins, 300 h.p./750 torque)???
Anyone have any experience with these? Fuel is a killer in this business, as well as my trucking business and I have never run these small motors before, so I am wondering what to expect.
The truck will also have an Allison auto.


Canrvpuller is offline   Reply With Quote
Join the #1 RV Forum Today - It's Totally Free!

iRV2.com RV Community - Are you about to start a new improvement on your RV or need some help with some maintenance? Do you need advice on what products to buy? Or maybe you can give others some advice? No matter where you fit in you'll find that iRV2 is a great community to join. Best of all it's totally FREE!

You are currently viewing our boards as a guest so you have limited access to our community. Please take the time to register and you will gain a lot of great new features including; the ability to participate in discussions, network with other RV owners, see fewer ads, upload photographs, create an RV blog, send private messages and so much, much more!

Old 12-12-2012, 02:12 AM   #2
Senior Member
kb7uxe's Avatar
Join Date: Nov 2011
Location: Roy, Wa. 98580
Posts: 173
My 95 Winnebago Vectra 35' Spartan 5.9 cummins, 6 speed alison gets 11.5 mpg on the hills, and 13.5 mpg on the flats towing a samurai..
But I don't think it will be as good this year as I raised the lift pump pressure from 15 to 20 psi at idle. It sure added lots of power doing that... ;-)

Dan Asbjornsen and Patty Chatman
This was replaced this year with a brand new 2017 Toyota Tundra and a 2018 FOREST RIVER WILDWOOD HERITAGE GLEN 24RLSHL
kb7uxe is offline   Reply With Quote
Old 01-26-2013, 04:13 PM   #3
Junior Member
Join Date: Jan 2001
Location: Texas City, TX
Posts: 12
I pull trailers for a living as well. My Freightliner sport chassis gets about the same milage as my dodge duall with the 4.10 ratio. Lot more comfort and safer. I also pull a 32 ft tandem dual gooseneck. I get 11 or a bit better empty. Got 9 pulling a 19,000 lb gooseneck to new Mexico.
2007 Freightliner Sport Chassis. 2006 lakota Fifth Wheel
Okie-Go is offline   Reply With Quote
Old 01-26-2013, 04:20 PM   #4
Junior Member
Join Date: Jan 2001
Location: Texas City, TX
Posts: 12
Need to figure out the way to update my signature.
2007 Freightliner Sport Chassis. 2006 lakota Fifth Wheel
Okie-Go is offline   Reply With Quote
Old 02-04-2013, 10:36 AM   #5
BirdDoggin's Avatar
Join Date: Dec 2011
Location: North West PA
Posts: 83
You should be able to get very similar fuel mileage between the motors for the weight you wish to tow by rear end gear ratio selection. The torque from the 8.3 should allow you to run a lower gear ratio (3.7? or 3.5? compared to 4.something for the 6.7), still keeping you in the meaty part of the torque curve, but reducing your rpm's enough to maximize fuel economy.

If you run unloaded quite a bit, then the 6.7 will have the advantage as it will get better fuel mileage when empty (not towing a trailer), close to 2 mpg.

You'll have to calculate unladen mileage and the fuel usage differential to get the actual cost of extra fuel for the 8.3. I would then weigh that against the added expense of maintenance for the 6.7 motor. My opinion is that the 8.3 is the better choice, at 500,000 miles that 8.3 will still affordable to maintain and that truck will be on the road ( and hopefully you've made your final truck payment), while the 6.7 will be in the shop for a turbo, injector pump....

The 8.3 may be a bit of an overkill for the weight carried for an RV'ers application, but you need to make a living, and down time will greatly out weigh fuel costs as the trucks get a few hundred thousand miles put on them. My 2 cents....
BirdDoggin is offline   Reply With Quote
Old 02-07-2013, 07:08 AM   #6
Junior Member
Join Date: Jul 2010
Posts: 5
Thanks for the info. That is exactly what I was looking for.
Half of my miles are empty, even a little over half with my mileage home on my off weekends.
But like has been said, with a Sportchassis or a medium duty truck, it will be in service for more than 500,000 km's (especially with the price of them) so that's why I was thinking the 8.3 would pay for itself even with a little higher fuel consumption.
The price of a new loaded dually pickup is getting pretty close to medium duty territory, plus I get way better service and lower prices at my class 8 truck dealers than I do at auto dealers. And I haul a lot of park model trailers and you really have to pay attention in the wind and slippery roads with a pickup.

Thanks Again.

Canrvpuller is offline   Reply With Quote


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

» Featured Campgrounds

Reviews provided by

Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -6. The time now is 10:15 PM.

Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2018, vBulletin Solutions, Inc.