Wal-Mart buys 300 Freightliner Cascadias with DD15 engines

It currently has 42,000 miles on it. When I got in the truck, it was running 6.1 MPG. Two weeks later, the average is 6.9, so I suppose I was actually getting in the 7's with it for the average to come up like that. The truck is governed at 65 mph, running 1500 rpm at that speed. It has a straight 10 speed. After some experimenting, I discovered that the truck liked shifting at 1200 rpm in the bottom deck, and 1400 rpm up top. This engine is also cut back to 455 horsepower.

I run a lot of the northeast 95 corridor, as well as Ontario, Quebec, and the midwestern states. Southbound we are usually close to gross, while northbound we usually haul light loads (22,000 lbs or so).

I just found out today that in response to rising fuel prices, our fleet is being cut back to 62mph effective immediately. In my opinion, this will take the DD15 out of its sweet spot, probably resulting in worse fuel economy.

 

If you're turning 1500 at 65 mph, they haven't spec'd the truck correctly. Peak torque is at 1100 rpm for the 455. While peak HP is at 1800 rpm. You're not gaining anything by running it at RPM's higher than 1400.

Your company didn't pay attention, or just failed to understand the engine design and it's significance. Then spec'd as they had in the past. At 1200 rpm, this engine will not fall flat of it's ### when asked to pull out.

Detroit uses a turbo boast set-up that is comparable to the twin turbo CAT. And they love to be short shifted aka shift at 11-1300 rpm.

The fuel economy should even out around 100-125K.

As far as the "sweet spot" concern. It will be dead on at those RPM. It will take some getting used too. But when you do, you'll wonder why anyone would want an engine that had to scream to get the job done. Things just seem to move along smoother with a low rpm, high torque engine.

 

I'm not real sure on that... Detroit Diesel says the optimum cruise rpm for the DD15 is 1550. I saw a chart in some trucking mag lately that listed best shift and cruise rpm for all the major manufacturers, and changed my shifting habits based on that. According to my calculator (not the ecm readout), at 60mph I was barely able to get 6.3 mpg, where it was turning around 1350 rpm. At 65 I was over 7 in places.

 

I visited the Freightliner dealer in Columbus the other day. I learned a lot about the DD15. Very impressive. Turbocompounding seems to be a difference maker for this engine.

They recommend 1350 rpm @65 mph for the best combination of mpg and hill holding torque. That would give you over 300 rpm to hold top gear as you climb a hill. If you can really manage it, you can run even lower rpm and get even better mpg. But the risk you run is that you have that much less rpm to drop.

 

I don't doubt anything you've said. It could be a simple matter of break in also. I don't know about the DD15, but most Detroits need to see at least 75K for a break in period...some more, some less. I still think it's a matter fo spec though. You're geared a bit low, I assume you guys pull a lot of heavy loads, and may see a lot of mountain terrain. Just a guess on my part.

Here's you a page to check out with further review, specs, and RPM discussion.

http://heavydutytrucking.com/2008/01/068a0801.asp

 

Wal-Mart must've seen their ad on this site!