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Old 12-11-2012, 02:43 AM   #1
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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.


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Old 12-12-2012, 02:12 AM   #2
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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 and Sadie the Wonder dog.
1995 Winnebago Vectra 35RQ Cummis 6BTA5.9 Allisn MD3060 6 speed. Spartan WC-2242 chassis
toad is 1986 "last of a bread" Jeep CJ7 Auto..
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Old 01-26-2013, 04:13 PM   #3
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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
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Old 01-26-2013, 04:20 PM   #4
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Need to figure out the way to update my signature.
2007 Freightliner Sport Chassis. 2006 lakota Fifth Wheel
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Old 02-04-2013, 10:36 AM   #5
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Location: North West PA
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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....
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Old 02-07-2013, 07:08 AM   #6
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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.

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