[Tripperfx3]

Setec and the guy above me both had great posts! Your both actually right, but I must ask you what car actually runs stoich (14.7)? None, because then you leave no lee way for bad gas and the such. Take the new Evo X for example. People have recorded 9.8:1 AFR's on bone stock X's. Hence why the car gets such crappy gas mileage, but at the same time it keeps the engine safe and detonation free. The only time I've seen a car run 14.7 AFR is actually at idle. This is was on my car and a few other while being tuned using a wideband (lamda) 02 sensor. Once you get on the gas your AFR's should drop. Not to mention there are cars now running ethanol and thats a whole other can of worms that I'm gonna leave closed for now lol.

To the OP they have answered your question, but this maybe a simpler one. Like mentioned the car isn't operating efficiently until it's sensors and catalytic converter and at "operating" temps. Also depending on whether your car is in open/closed loop mode, which depends on driving conditions (long trips compared to short trips) will greatly affect your gas mileage. If you can hook up a vacuum gauge and run it into your cabin (simply tee off your intake manifold) the next time you go on a long trip. Try to keep your speeds at around 65-70 MPH and keep your vacuum at 0. This will be hard due to just having bumps in the road slightly knock your foot around, but I guarantee you will get incredible gas mileage. I personally do this with my Evo and did it with my turbocharged 240sx. In my Evo, which has over 400AWHP, rather large 780cc injectors and a 255lph fuel pump I was able to do a 300+ mile trip and still had 1/8th of a tank. With the 240sx which had smaller 380cc injectors and 250 to the wheels I did the same trip and had over a 1/4 left in the tank.