I have a 1990 V8 and I am only getting about 200, so I think I have a gas leak somewhere.

 

Provided you don't drive like a madman with an out of tune engine. You are getting roughly half the mileage, this would have to be a very big leak and difficult not notice or smell and obviously a fire hazard! It might be your fuel gauge, how many gallons are you able to fill with when you are at the ā€œEā€ mark. Your fuel tank should be able to hold at least 20 gallons. I would also check to make sure that you are not dragging your rear brakes. Sticking rear calipers are very common.