fill it and fill a gas can write down the mileage.Take it out and drive it until it runs out of gas . see how miles it went and divide by 18 to get an accurate picture of what your mileage is. Or,just fill it and go exactly 100 miles and fill it again.Divide the 100 by the number of gallons you had to add .Either way will give you an accurate picture of what is happening.My money is on the sender or a bad ground at the sender on the tank or the voltage limiter in the dash.You say you were running hot,well maybe you weren't. The gas and temp gauge will both malfunction if the limiter is bad or starting to fail
