Diesel Truck MPG

Camper, it's a 2012 GMC Sierra D/A combo crew-cab short-bed 4X4 with 373 gearing. We had an old 2003 GMC, traded for a 2011 Ram Diesel Crew-Cab, long bed with a six-speed manual and 3.42 gearing and only kept it for about nine months, fuel mileage was getting worse rather than better as it got broken in and I came to the conclusion that I didn't like backing, looking over my shoulder, with the manual and having to keep both feet active on the pedals. Just hard for an older, somewhat less than svelte guy like me. The 2012 GMC has been a dream to drive with no problems so far. Just finished another shorter trip to Grand Teton National Park to fish and the console showed 13.7 after we got back home. However, there was quite a bit of none-trailer pulling, we had to stay quite a ways from the lake, about 30 miles, so towed the boat there and back each day. Couldn't tell you about the L, whatever that is, I am not a gear-head, I just buy them and drive them.