My 96 GMC 1500 (2wd) with 350/3.42 gear/4l80 tranny/31x10.50x15. AC always on. I average 13-14 around town and normally 18-19 on the road (averaging 75-80). I have seen 20 on a few occasions. It had 170,000 on it.
Like many mentioned and every other small block I have had. Intake gaskets. Mine went out at 100,000. I never replaced a fuel pump, but it made noises the entire 5 years I had it. water pump and alternator were the only other major failures before the ABS computer/controller went zappo and fired everything else.
I miss it!