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ISM cummins or 60 series detroit?

I currently have a 2 trucks. My old truck is a 1999 freightliner century with an ISM cummins@370hp. I have no problem getting over 8mpg and often 9mpg in my operation. Tanker work, 80,000lbs 50% and 30,000lbs 50%. I just picked up a 2003 freightliner columbia with a series 60 detroit@ 430hp. I'm frustrated with this truck because I cant get better than 6.5mpg with it. I'm wondering if I should just get rid of this truck. The only difference in these two trucks are the rears and transmissions. The 99 has 2:79 with a 10 spd direct and the 03 has 3:73 with a 10 speed overdrive. They both run about 59-60 at 1400RPM and are driven exactly the same way. I'm going to do some trouble shooting on this detroit. Check charge air cooler, run overhead, and dyno it. My biggest question would be can I ever get this detroit to get the same or better mpg as my ISM Cummins. My ISM has close to a million miles and still gets great fuel economy and doesn't use oil and has never had a mechanical issue. This ISM is a really nice motor, nothing like the older M11. Just wondering if the detroit will ever live up to the ISM in my operation.

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My Brother has a 97 freightliner with a M-11. My dad has a 98 western star and hauling gravel for the county my brothers m-11 runs 50 liters less fuel every day. The specs are very different with the western star having 4.30 rears with a thirteen double over. The freightliner has 3.26's and a ten direct but the M-11 has been a very cheap running little engine. I would lean to the engine you know and has been working well for you. You might be better off keeping the ISM and rebuilding it with a MicroBlue inframe kit when it comes time and possibly squeezing a little more out of it yet.

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Well unless you are driving both trucks, you have no idea if they are driving exactly the same! Just the way you climb a hill and the way someone else does could make the difference in your fuel mileage. Your gearing is a very big difference in the two trucks. You have been very lucky to get almost a million miles on your ISM.

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I do drive both trucks. So I know they are driven exactly the same. The gearing is similar between these two trucks. They both run almost at the same rpm at the same speed. The only difference is ones in direct and ones in overdrive. We know the direct drive will be 3-4% more fuel efficient but the Detroit is not even close to the ISM. I also think your totally wrong about being lucky to getting 1 million miles on the ISM. What are you basing your info on? The older M11s were a troubled motor which often needed rebuilt at 700k to 800K. From my research I've found that many people with ISMs go well over a million miles. My friends family business exports trucks and they bought a fleet of internationals 70+ all of which had ISMs. Every unit had 900k to 1.2K with no engine rebuilds. The ISM is a much better motor than the M11 and they shouldn't be compared.

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I have a 03 Freightliner with a Detroit Series 60 and as long as I keep my speed down and idle time down I consistently get high 7's to low 8's. There must be something wrong with this truck for you to be getting such bad fuel mileage. Another factor might be the rolling resistance of the tires? That freightliner should easily get the high 7's driven right being empty 1/2 the time.

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I agree. Theres got to be something wrong with this engine. I need to do some trouble shooting. This detroit should do way better than 6.5 mpg. The tires are the same because I swapped them over.

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First, you would have to know if the engine has EGR or not, given that it's a 2003. Often EGR HURTS fuel economy by up to a full MPG (or more if the engine and/or EGR system have trouble).

Second, the rolling resistance of the tires and how the truck is set up can make a significant difference in fuel economy, and it looks like, based on your application, that you might have too much horsepower spec'd on the Detroit. The big thing to remember is to keep your foot out of it and your mileage should go up. Keep the engine in the "sweet spot"- 1100 to 1500 RPM.

Third, remember it's winter time, and even my mileage on my old CAT 3406E has dropped since my local fuel station switched to winter blend fuel in late October.

The Cummins ISM and the Detroit Series 60 are really good engines, it's too bad the EPA shoved EGR down the truck engine industry's throat and robbed us of fuel economy for the sake of "cleaner air."

It is possible to improve the fuel economy of the Detroit, but it will take time and effort.

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A 12.7 litre motor is bound to use more fuel then a 10.8 litre.

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,,, they are not driven the same. You are using the sixty plus extra horsepower frequently, even at the same rpm.

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Rick will be eternally grateful.
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