Keep a gallon in the trunk.
My old Spectra would read full for 3/4 of the tank. Then it would read 1/2 then 1/4 in about 50 miles then show empty. And you best be near a gas station
I have a spectra in mine. I noticed the same thing as you did. With a full tank you think you're getting great gas mileage. lol. Once it reads 1/4 tank I go and fill up because it's like playing Russian roulette.