My '01 QCSB automatic 4x4 w/ 3.54 gears and 315-75 tires gets 19-20 mpg unloaded on the highway at 70 mph, hand calculated. Never bothered to check it towing. Only modifications are a BHAF, 4" MBRP straight pipe exhaust, and a 60 hp Edge tuner.

I towed w/ gas trucks for a long time. After my first trip w/ a diesel, I never want to tow w/ a gas truck again. Even though I usually tow less than 2 hours away from home and do a lot of unloaded daily driving w/ the truck, I've never regretted getting the diesel.