i wouldn't say the fuel mileage is going to poop just yet,just my 2 centsMy friend that works at the local GM dealer said they are try to cut back on emission even more for the new LMM. It will take a catylac convertor that requires more fuel to cut the exhaust to meet the require emissions. That means the fuel mileage will be goin to poop.
Plenty of ppl around here use "rig fuel" or "farm fuel" which they wont be able to use anymore. Hang on your diesels!