Before the onset of winter I blocked the grills with pipe insulating foam. I noticed a drop in MPG once freezing winter arrived. We were averaging around 35mpg around town in freezing temps. It’s hard to tell how much it helps since I have no data with the grills open in similar temperatures.
On the annual Ohio Xmas journey from Akron to Glenmont to Medway and back, we did the entire trip, 428 miles, on one tank. As we got within 3 miles of home when the last gas pip started to flash. After dinner I filled the tank and it took 9.17 gallons to fill. The tank holds 11.9 gallons, which means we had about 2.7 gallons in the tank. That means we could have gone about 120 miles before running out gas in the current conditions. The flexible bladder is the wild card, since there could have been less than 2.7 gallons in the tank.
The MFD reported 48.4 MPG for the trip, while the math indicates 46.7 MPG. I think the MFD number makes sense when you consider the times the engine was not consuming gas while in motion.