I have a 2001 Honda Civic, Mileage approx 135,000 and since last week when I fill my tank the gas immediately drops to 3/4 tank. I took it to my mechanic to check for leaks, there are no leaks and he has no idea what the problem is. Have used two different gas stations in the last two weeks and have experienced the problem on both occasions.

What I would do first is fill my tank, write down the mileage, and when your gas gauge drop down to 3/4 again, fill it back up, then divide gallon's and miles this way you'll know how many miles per gallon and how many gallon's to fill back up, you will also know if gas gauge is defective or not.

U have a defective gas gage sending unit inside the gas tank, this is a very common problem. repair is to replace the gas gage sending unit.