Running the AC or heater should make your MPG worse. The car would pump more gas to keep the car at a higher idle for extra power. At least that's how I was told it works?
My average was 16 ~ 20mpg for the last few months. I live in Florida and without AC you would melt 15min into your drive to work. I went to superlude and had the gear box fluid done for 29$. What they did was suck out the trans fluid and put in new stuff. My car is shifting from gears better now with less noises and jerkynes. Also unknown to me the front tires where over-inflated to max 40psi, funny how that noise I was hearing is going away when I turn hard left?LOL And I know they used the wrong grade of oil last time and so I got them to use the right one this time. I also used half a bottle of B-12 even after using 2 bottles of redline before it with no results.
Right now my gas guage is at halfway and it's reading 202-miles! I can't wait to see where she caps out at. I hope I can get my car back to at least 285-miles. For a while there I was getting around 220 ~ 250 per fillup. I pay 2.05$ per Gallon here and spend about 24$ at the pump.
This car will be payed off in April...