[truckster]

I'm not really familiar with FiTech, but your A/F ratio should be leaner than that. You say half of your commute is 55 MPH. Is the rest of it stop-and-go?

You need to know your rear ratio. If the SPID is still there, it might have the correct ratio on it. I believe 3.73 was standard with the 307, but it could also be ordered with a 3.07 or 4.11 ratio. It wouldn't be a bad idea to open it up and see, since that gear oil probably hasn't been changed in many, many years. You'll find two numbers stamped into the ring gear; divide the larger number by the smaller number and that's your ratio.

Here's another thought - your current tire size is 2" larger in diameter than the stock tire. Have you calibrated the speedometer/odometer for your current tire size? That would definitely affect your fuel mileage calculations.

On the other hand, if you only commute 6 miles, it would take a whole lot of trips to pay for something like changing your rear end ratio.