After a recent fill up and the gas guage needle being in the right place, I drove about 60 miles and noticed the need seemed not to be in the right place- it was at the halft tank mark. Either I am leaking gas, or getting 5mph, or the guage is bad, or the sender is bad. I filled the tank and 4 gallons went in. I do not see any leaks. I have driven 50 miles and the needle is on the half way mark again?

What gives?

Jonathan