fuel mileage change with hi-test

We are on our way down to Florida from New York, via Knoxville, TN. I had topped off the gas tank in NY with only 2.4 gallons (my wife had accidentily left the car on for 18 hours and the trip to the gas station showed 1.4 MPG). We left on 12/20 and the first thing I noticed was that the fuel gauge showed one click below full even though I had topped off the tank. I think this is normal since the system doesn't respond unless you put a lot more gas in the tank. We drove 350 miles before the tank dropped down to 2 bars - indicated mileage was 41.9 MPG - a lot lower than the usual 47 - 49 MPG that I normally get on a trip like this. Outside air temp varied between 30 and 34 deg F. and we were also loaded with ~400 lbs of luggage. After stopping overnight we drove another 350 miles - 41.2 MPG. This time when I refilled I accidently used premium fuel - the first time I have ever used it.

When I started on the last leg of the trip, the fuel mileage immediately climbed into the 48MPG range and topped out at 49 when we finally arrived in Knoxville. I'm wondering whether this indicates that I need a tune-up, new spark plugs or additional fixes to get the car back to it's normal MPG. Current odo mileage is 58,000.

 

I'm sure the mountains had a lot to do with it. But first leg of the trip (NY to Lexington, VA) was reasonably flat. The second leg was like a roller-coaster. A lot of hills on IS81. But, the mileage for the second leg was only about 1 mpg less than the first leg. I'm going to add info to the log for the entire trip. We will be heading down to Weston FL on 12/29, then heading north on 3/12 with another stop in Knoxville on the way back. We'll see what variations terrain and temperature add to the mix. I probably won't be putting in any more premium gas, although it might be an interesting experiment on the return leg of IS81 in southern Virginia. Over 57000 miles my average fuel economy has been ~43 MPG. The bulk of my driving has been trips of 35 miles or less. Long trips have produced figures closer to 50 MPG (cruise usually set to 70).