alot of times its the tires, they are one of the most important things on the car. keep them stock size and most times you can put 2 or three more pounds over stock in them without uneven wear.
the biggest thing is the driver. i always gain the most mileage by leaving the dic in either instant or average mileage mode so you can see what the vehicle likes. if you dont have one there are a bunch of ways to get it through the obd port. i have "torque pro" for android and a odb2 to bluetooth adapter 40-50 total investment. it works on all 96+ and gives you anything you want to display
1998 Bonneville SSE
2007 Monte Carlo LT-KYB, Clarion, Polk audio, Kicker
1999 Yukon SLE-KYB, Air lift, Reese dual-cam, Tekonsha
1987 Firebird-355/th400-nothing stock