I have a 1990 C1500 Silverado, Extended Cab Short Bed, with overdrive Auto trans and 5.7l. truck has about 370k miles on it. I know that's a lot. Still runs great. When I first got the truck, it was getting about 9 mpg. I put true dual glasspack exhaust and throttle body spacer and now get about 10-11 mpg, mostly city. I put new intake manifold on it, cleaned air filter, change oil regularly, and occasionally SeaFoam it. People I ask with around the same truck get about 16-18 mpgs. How can I see about getting these kinds of numbers?