I know this will sound weird. But over the last 4-5 tanks of gas, I have noticed that I get better gas mileage at the beginning of the tank than I do at the end. I have tested this in the following way:

Every other tank I do not refill until the gas gets down below the redline. I put 11-12 gallons in and calculate mileage from the trip odometer (which I reset every time)

In between I only run the tank down about a quarter of the way, then fill it back up (usually 3 or so gallons) and do the same calculation.

I am consistently finding that my fuel economy on those short tanks (where I only use 3-4 gallons per fill) is around 25mpg in a mix of city and highway driving.

When I use the whole tank, my fuel economy drops to 19-20 mpg.

This makes absolutely no sense to me. Can anyone even propose a possible solution as to why my 240 would use more gas (leaks? fuel pump issues? etc?) in the second half of a tank than it does in the first?

Also, the 19-20 on a full tank seems pretty low for a non-turbo 1993 sedan, and any tips on how I might look to improve this would be most appreciated.

Thanks Much!