Has anybody had any luck running programmers on the duramax (in fuel economy)?
if so how much did it help? And which one did you use?

HPtuners can program the duramax. I have not used mine for that, so I can't say how well it works.

I wonder if Hptuners could work on a Mack?

Nope. I have played with the tuners for Mack and other Semi'*. The setup and software for those is usually expensive and proprietary. They can also control just about everything in the truck and do it all while the truck is running. Pretty neat stuff.

Not that there is much to electronically "tune" in a diesel engine. Never thought about the realm of "fuel economy" on a diesel. Most of us Cummins people slide the fuel plate in the injector pump back to deliver more diesel. LOL. Or just pull it off. Good for 100HP and a red hot turbo.

I had an Alzheimer moment and forgot to add BullyDog is #1 for diesel tuning. Most use it for performance and could care less about economy, but it can be used for it. The deal here is if your just after economy, going to be hard to justify fuel savings vs cost of tuner. Unless your putting on the mileage of a big rig.

you can also check out banks and ssdieselsupply.com