i have a 95 civic lx sedan, with a similar problem. is your tank under pressure? when you take the gas cap off does air force out? thats what happens to mine, and i also have the extreme fluctuation in my gas gauge, but usually no more than 1/4 of a tank. As far as mileage, i think its just the gauge thats messed up, cause i can drive my car to E and have about 260 miles to the tank (with deliveries and fairly high rpm shifting), and then i fill it up, and it only takes 8 gallons, this is a 12 gallon tank, so there is still tons of gas, but the meter says you have none. make sure you are checking the MPG for the gas you are using. hope this helps somewhat.