17 & 20
You must be driving a V6.
I'm driving a stripped regular cab with 6500 miles and 17 is the best highway mileage i've seen yet. I average 13 in the city. I try to keep as close to 2000 rpm and that gets me about 68-70 mph.
I did a trip to florida a few weeks ago. I started out with a fillup of mid-grade and went 307.9 miles, took 18.058 gal. That's 17.05 mpg and that is the best yet.