Using FL417 and the south entrance to the airport, there is a Shell station on Jeff Fuqua Blvd. just outside the airport. Google maps reports it’s about 2.5 miles from the car rental garage, depending on whether you use the A- or B-side rental return. The gas prices there have seemed competitive to me. I used to fill up at the Hess station (now Speedway) by the BoardWalk and the gauge would still report a full tank, but with the use of telemetry that may no longer work so well. I’m always hesitant to keep pumping in gas manually to top off the tank once the pump has shut itself off, since that may back up gas into the charcoal canister of the evaporative emissions system, potentially triggering the check engine light.