The reason for the drop in fuel mileage is the misfire. Many mnisfires you can't really hear or feel -- unless the engine is working hard under load (as when driving up a hill when it becomes more noticeable).
To check the coil, pull a plug wire off of a spark plug and place it near a metal surface on the engine. Then start the engine. You should see a spark jump from the plug wire to the engine and hear it snap. No spark would indicate a weak coil.
You can also check the coil's resistance with an ohmmeter. Here's the procedure for checking the coil:
Make sure the ignition switch is "OFF".
1. Disconnect the distributor lead and wiring from the ignition coil.
2. Connect a suitable ohmmeter as shown in step 1. Use the high scale. The reading should be infinite. If not, replace coil.
3. Connect ohmmeter as shown in step 2. Use the low scale. The reading should be very low or zero. If not, replace coil.
4. Connect ohmmeter as shown in step 3. Use the high scale. The meter should not read infinite. If it does, replace coil.
5. Reconnect the distributor lead and wiring from the ignition coil.