12? sounds like there is somthing wrong with it. i get better in my full size gmc. a re-gear may be in order for the tall tires.
something seems funny with that... is the speedo calibrated to the tire size? that could mess with some MPG figuring.. I had a four cylinder reg cab and drove from Ma to Indianapolis with 1200 lbs in the back and got 23 mpg down there and with it empty i got 28 ofcourse thats alot of flat highway and normal driving speeds im no speed demon like some folks..
Nothing wrong with it. I keep up on the maint. pretty religiously. Just turned over 50,000 and did all the fluids, plugs, air filter etc. Added a block heater because I thought maybe I was spending to much time in open loop. No difference. In fact, just had it in the shop for a P303. Thought maybe it was time to do the head. It was just the plugs and they werent that old and looked great. But it tripped a misfire on cylinder 3.
The tires are no taller than the 265 75 15 that come standard on the Z71.
My mileage numbers come from the scangauge. Ive set it to adjust for the tires since the PCM is calibrated for 235 75 15. Its spot on using a GPS to match speed.
In all fairness, My regular commute is short, 3 miles each way. But even if I drive it around town all day giving it ample time to reach temp and stay there... Rarely do I get above 13mpg. The best mileage Ive gotten on the highway was during the UPOverland 2008 trip. 1300 miles in 5 days and I did not factor in the miles I spent off the highway. Best I got, IIRC, was 19.4. Thats staying at 70 on the highway.