1995 Subaru Legacy Gas Mileage

Over the years the fuel gauge became untrustworthy. So everytime I filled up with gas, I turned the trip mileage indicator to zero and when I drove 240 miles, it was time to get more gas. At 240 miles, I filled the car with approx 10 gal of gas. In the last 3 months, I had to fill my tank at 200 miles and put in 10 gal of gas. The car is running fine, but I does smell hot and keep thinking it's just the heater smelling this way. I opened the hood and nothing seemed wrong, but I'm not a mechanic. What possible reason is there that all of a sudden it's using this much more gasoline.

I faithfully have regular tune ups and oil changes. I have new tires purchased 1 year ago with proper inflation.

Thank you for your help,
Monika