Well, I also have a 91 LS, and the sticker that was on the car said 18 Streets 23 Highway. I get 12ish on the streets, and 18 highway. It is a V8, so im suprised it gets that high. However I do have a lead foot, so thats why. I keep up on matience, and have 160,000 miles. I would suggest keeping the windows closed for aerodynamics, and turning off AC/Heater because I'm sure that affects the mileage.