LOL
Another angle could be that you get to try out the diesel thing for nearly nothing in start up cost. Later you could bale with no real loss if you don't like it. If you like it you have a stepping stone to work toward the next one.

I was thinking the same thing. I had a 5.9 cummins. Loved the motor. Truck was okay. I'm partial to Chevy trucks. This will be my first duramax.

What's the best you saw?

20 is very reasonable. I drove a 2003 Duramax Dually crew cab 188k miles. It had 4" exhaust, bullydog chip and aftermarket air filter/ intake. Never got under 18-19 mpg and I drive. 80+ mph everywhere. My friend who drives very easy pulled a 42 MTI back from Florida with it and averaged 17. No one believed it except the 5-6 people who hand calculated every tank.

Truck MPG threads are NEVER qualified.

Axle Gearing
Number of trans gears
Lift
Tires
Wheels
Mirrors
Bug visors
Tonneau covers
Peformance mods

...ALL drastically can change mileage; not to mention driving style.

17.7 on the way home from the dealership......truck was empty and I was very interested to know what I could expect for mileage (never had a diesel truck before) so I drove it easy (break in) and it was about a 300 mile trip home. Truck was stock its whole life with the exception of a K&N filter and one tire size upsize on the stock rims.

If I recall the short bed crew had a 26 gallon tank and I would never get to 400 miles to a tank. 350 miles was usually the end of the show meaning 13.5 mpg in mixed city/hwy driving. The tank also seemed to hold strong for the first 1/2 then drop quickly on the 2nd half.

There was a thread on here regarding the new ones (36 gallons fuel in long bed trim) and someone was bragging about a long distance pull (Midwest to FL type deal). Noted they got 20 mpg+.....so when I inquired about how many tanks they got in their recent trip (@ 20 mpg they only would have needed 2 tanks) they remembered needing 3, completely blowing the 20 mpg fantasy.

I think the 17.7 estimate is pretty accurate. I've gotten better, and I've gotten worse. The big plus for me though, is the torque is there when I NEED it! Also, towing doesn't suck the mpg down neer as much as a gasser would. Friend has pretty much same truck with the 8.1, (probably has lower gears though) he constantly brags (actually *****es) about his 9mpg empty all the time! Pulled to LOTO last summer and averaged 15-16 pulling my 32 Victory (heavy *****). That said, I would NEVER consider it a commuter! Keep the Cobalt......


Oh, BTW, I have a 4" turbo back exhaust, EGR blocker, upgraded intake and Edge tuner. Tuner has 5 settings from mileage (1) to crazy larry (5). Never tried 5, was told the Allison would detonate after awhile..... I tow on 2.

I currently have an 08 Duramax LMM with 140k miles. For two years I had an Edge evolution race that I would keep on level 2 and a 4in exhaust from the downpipe back, completely eliminating all emissions and muffler. The best I saw was 20.5mpg on full highway to San Diego. Around town would be between 14-16. I got tired of switching exhaust for smog test so went back to stock. Now I see 12-13.5 in city and just over 17 on highway. All hand calc. Now truck is parked most of the time except for weekends. My daily is an 88 honda CRX, awesome little car.

Good info guys.
I checked the truck out yesterday and it was well taken care of. Guy kept a towel in both front seats to protect them from sun damage. I was on the way to the lake with my boat and he suggested I hook to it with his truck and try it out. Very impressed with everything. I'm very partial to white (I have a craigslist ad wanting to trade for a duramax that is 4x4, white, etc. He was aware but wanted to offer anyways) so I couldn't go through with it. If something comes up I like I'll pull the trigger. For now I'll stick with my 8-10 mpg towing.

10-12 is what you would get towing with the diesel....not sure if you would save anything.