Perhaps if the car isn't warming up it'll give you poor gas mileage. As far as the spot under the left rear, you could have a fuel leak (which I'd be wary of). Try (even tho its -15 out) getting into a garage and taking off the left rear wheel. See if there's a rust spot on the fuel filler pipe and also take note if there's a smell of gasoline. A car that's 15 years old is bound to have problems. Call the people you bought the car from and ask them if they know of any sort of mileage problems or fuel leaks (this may be what prompted them to sell the car, afterall).