Sorry it's not working out for you. I do hope you continue to aim for 50mpg! It's been below freezing here for the past week (snowed on Thurs) and yet I'm maintaining 52mpg.
For me 50 mpg has been absolutely no problem. Over the last ~2100 miles the display reads 59.5 mpg. I calculated one tank manually and it was 58.05 mpg vs 62.5 on the display. I'm guessing my 59.5 lifetime average is closer to 56. I have noticed a difference now that temps have fallen ~15 degrees. During 90-95 degree summer days I was up to 66-69 mpg.
Weight is a huge factor on MPG, probably much more than the 17" wheels. I have experience with this every week when traveling 360 miles each way, round trip. Outbound trip: the car goes moderately heavy (at least 400-450 pounds considering my weight) and I'm getting 48-49 MPG. I do drive considerably over speed limit (passport 9500xi keeps me clean and safe), my average speed is 67, which is pretty high considering that about 50% of the way have speed limits bellow my average, which is 65 or lower). Inbound: Car goes at least 200 pounds lighter and I get 53-54 with the same conditions. The way back and forth is pretty flat, so the only thing to blame is the cargo weight. So, just 200-250 pounds of extra cargo is resulting in a 4 MPG hit in my case.
