I think you're on the right track with the transmission losing efficiency, there are people running much bigger cams than yours who are also getting better MPG but they're usually running taller gears with a 904 auto or some type of manual trans. I personally was able to get 17 MPG on a long road trip in my Duster, it had a 10.5:1 360 with a small 256/262/114 Voodoo cam, I also found later upon installing a wideband O2 sensor that the carb could be tuned leaner at cruise and I picked up another 2 MPG. Also was running a fairly loose (cheap) 2600-stall converter with an A-904 and 2.94 gears in the rear. But at the end of the day doesn't propane have less energy density than gasoline anyway so your mileage will always be less?
