Unless you have a leak somewhere my first guess (and it's a guess) would be to check the accuracy of your odometer. If it's ticking off the miles too slowly it'll look like you're getting crap for mileage. I assume your county roads are one mile apart. Go for a drive and check the odometer to the mile markers. If you drive a mile and the odometer only shows 7 or 8 tenths you found your problem.

Save a life, adopt a senior shelter pet.
The three main causes of blindness: Cataracts, Politics, Religion.
Name your dog Naked so you can walk Naked in the park.