I borrowed my grandfather's '05 GMC Sierra W/T (V6, auto, 2wd, regular cab) and hauled a couch and love seat 175 miles to my house. Then I drove to the mall (fighting black Friday traffic) to pick up a water heater. then to another store to return a different water heater I'd bought. Then drove the truck back (200 miles). I kept the speed at or below 65 for 90% of the trip, drafted when safe, and netted 21 mpg. I didn't think that was bad considering I drove 50 of the 400 miles in bumper-to-bumper traffic. I don't know how big the tank on the truck is, but it was still over 1/3 full after 400 miles!
I drove my Si down to pick up the truck, and drove it back. According to my Scangauge, which is calibrated conservatively for the time being (numbers were fluctuating, so I topped off the tank to the point of overflow, and I will do the same on the next tank so I can get accurate fill numbers), I averaged 32.6 mpg for the trip south. I decided I wanted to beat that on the return trip, so I tried keeping my speed under 65.
Drafting a van at 58 mph, I was getting about 35 mpg.
Drafting a Dodge Ram 2WD at 65 mpg, I was getting 34 mpg.
Driving solo at a steady 58 mph, I was getting 31-32 mpg.
Driving solo using P & G between 60-70 mph, I was getting 34 mpg.
Drafting a semi at 65 mph, my mileage shot up to 40+. I stayed behind him for the rest of the trip, and averaged 36.3 for the return leg. I'm averaging 34.5 on this tank, and hope to get 500 miles out of it before I fill up.
Wish me luck. I've been trying to get 500 miles on a tank in the Si since I bought it. My best so far is 485.
Without that traffic backup, you'd definitely have gotten better than 21mpg. That truck is newer, presumably in better condition, lighter, smaller, probably lower drag, and has a smaller engine than mine and take a look at my gaslog...