It could be other problems like clogged air filters ( I went from 28mpg to 18mpg, the car seemed to run fine-no black smoke either, drove me nuts until I changed the air filters out), or a drag issue ( I had rusted frozen emergency brake cable, so it was like driving with my brakes on). Air filters are easy to change, and jacking up the car to see how the wheels are spinning shouldn't take too much. I would also suggest adjusting the carbs.

Paul

Here's something I found on calculating gas mileage:

1. Fill tank completely full

2. Make note of current odometer reading

3. Drive most of the tank out

4. Refill tank to same point

5. Again make note of mileage on odometer

6. Subtract current mileage reading from previous reading

7. Divide that number by amount of fuel used

8. Presto, fuel milage