gas mileage with rebuilt 6.6L engine

[ktingey]

I have a 2003 6.6L duramax 2500HD. I blew a head gasket and have had that repaired along with new injectors and the valves and valve seats ground. Before all of this happened I was getting 22-23 MPG on the freeway at 70 MPH. Now I'm lucky to get 14-15 MPG. When I pull a very lightweight utility trailer empty I only get 10 MPG. The shop that did the work is telling me the injectors take a good 2000 to 3000 miles to break in and that gas stations have now switched to their winter blend of diesel and this will lower my MPG. All the numbers (injector balance rates, compression etc.) look great! The engine sounds great and runs good. I'm just very concerned about the miles per gallon. Any suggestions? Thanks.

[More Power]

I have a 2003 6.6L duramax 2500HD. I blew a head gasket and have had that repaired along with new injectors and the valves and valve seats ground. Before all of this happened I was getting 22-23 MPG on the freeway at 70 MPH. Now I'm lucky to get 14-15 MPG. When I pull a very lightweight utility trailer empty I only get 10 MPG. The shop that did the work is telling me the injectors take a good 2000 to 3000 miles to break in and that gas stations have now switched to their winter blend of diesel and this will lower my MPG. All the numbers (injector balance rates, compression etc.) look great! The engine sounds great and runs good. I'm just very concerned about the miles per gallon. Any suggestions? Thanks.

A couple of things....

1- I've only driven a Duramax 2500HD/3500 one time that produced more than 20 (and that was on a rather unique trip). Most fuel economy numbers, when driven at freeway speeds, is in the 17-18 range. SLow down, drive slower, say 60-65, and you might sneak up on 20.

2- Winter-blend fuel will decrease fuel economy a 1 mile or 2 per gallon, and colder tires and lubes in the drivetrain adds rolling resistance.

3- Don't trust the computer generated fuel economy numbers. They are sometimes close, and not so close at other times. If you changed the tire size, this could mess with the calculations. Always hand calculate the fuel economy and compensate for tire size differences if you believe there's a problem.

Jim

[AKMark]

Also,

New motors need to be broken in before they reach their maximum efficiency.

Dad's 6.2 when it was new got 4 mpg less than it did a few years later when he had over 200K on it.

My 6.5 Burb got better with age as well. Although a new set of injectors helped it even more.

My 6.6 gets 16 with mixed driving, and hardly driving more than 17 miles at one time. I have yet to take her on a freeway cruise to see what she'll do. Winter time, I expect that number to drop just a bit. I will be idling it a bit more. Also, this truck only has 91K miles on it, however it's got over 10,000 hours. It was a truck used by Conoco Phillips on the North Slope oil fields of Alaska. So it dealt with a lot of extreme climate conditions and probably idled all day every day it was in use. She runs great though! I'm considering having Kennedy tune it to see if it would increase my MPG a little, but at the same time, I'm happy with it's mileage and love the power. Got to used to old 6.2's and 6.5's, this thing seems like a rocket, especially when loaded down.