increasing MPG

Drive with as little pressure on the gas pedal as possible, use cruise control as often as possible. consider an aftermarket air filter such as a k&n.

Have you considered just trading it in for a newer model with a CVT transmission, they really get much better fuel-efficiency. My old 1985 T-Bird got 17 mpg, later purchased a Toyota Prius with almost 50 mpg. Even if you threw $10,000 into an older car, you're never going to increase your fuel-efficiency like getting a newer car. My Subaru Outback Limited with the CVT transmission gets around 23 city, 29 highway . So, it really depends on the car and what you're trying to do.