I drive a fairly innefficient 95 Mitsu Montero for my 25 mile commute to work. I carpool so I only drive two days a week. Through the past year I have been trying to maximize my mpg. In this quest I have done the following. -Replaced the 60k old sparkplugs and wires.(NGK double platinum)-Result; absolutely no improvement in driveability or mpg -Replaced the 165k old oxygen sensor. Result; absolutely no improvement -Installed 5W20 motor oil. Result; No detectable improvement. -Increased tire pressure from 30-35psi. Result; no improvement. Although the ride was noticeably harsher. -Made sure I kept my max speed under 70 mph (normally go up to 80 sometimes). Result; 1-2mpg improvment. Vehicle runs great. Always has. I guess in the end it all makes sense. Slow down to save gas. However, I thought maybe the new Oxygen sensor would help. You would think it would get weak after 10 years and 165k miles.