I have a '64 K10 Suburban, 350, 4 speed, 373 gears and a Holley 4 barrel. The tires are 30X9.50 on 15" rims. Dual exhaust, with 2-1/4" pipes. This summer went out on the interstate with a friend who is a Wyoming Highway Patrolman. I had the speedometer reading a steady 60 mph for about a mile. My friend said that he clocked me at a steady 65 mph.
Filled the truck up and drove 18 miles, on a lightly traveled highway, turned around and drove the same 18 miles back. And refilled the tank. Kept the speedometer reading a steady 60 mph (ie. 65 actual mph) for 32 of the 36 miles. About 40 mph for the first and last 2 miles.
Results: I averaged 14.3 mpg. I was pleased with the results. For a truck that sits as high as this one, is fairly heavy ( compared to a newer Suburban or Tahoe ) and no fuel injection, I felt, that 14+ mpg on the highway, was fairly respectable.
