if you've never seen 'mileage wars' or w/e its called on tv where the guys try to get the best mpg check it out. You guys don't seem to understand that there isnt any one factor to change and get better mileage magically. When it comes to mpg, everything is a factor! Even just having cheap tires can cost you big mpg over time! Running a 2nd battery can even improve your mileage a little too, by taking stress off of your old electrical system. Having crappy battery terminal wires can even cost you mileage bc the computer is lagging in your CCC car !
From having owned so much GM now, I'm aware that intake manifold gasket and head gasket are critical to getting good mpg. If either one/ or both of yours is shot then you can get oil changes and flush your coolant and replace your hoses and shocks all day but you'll never get over 15-17mpg. Your gaskets have to seal correctly in order to close in on 20 mpg territory
Since the G is based on stock cars driving w the windows open should increase your mileage if your rear shocks are recent. The wind catches the flat-back window like a sail and redistributes stress from up front throughout the back of the car. Obviously in a rusty car this would cause some irritating noises to happen and wouldn't be worth doing though.
Another huge factor is today's gas. Obviously if you can get 'pure gas' w/o any ethanol your G should run much better because thats the type of gas it was supposed to be running on, not this e-10 and e-15 / this 'blend' crap we have in chicago. Knowing how much ethanol/ CRAP is in the gas is important if you're concerned w mileage. Buying 'BP with invigorate' and 'shell with detergent' isnt saving you gas or money, that sh*t is for newer cars not your g-body. In a G the ethanol doesnt burn right and just ****s up your valves!
It cant be that the dual exhaust is making this sh*t mileage! I dont know if exhaust leaks cause crap mileage because i know i have some of those...
