A couple weeks after I got my '71 Dart /6, I discovered the coil was wired bass-ackward.
The car started and ran OK but was definitely no speed demon.
There's no telling how long it was like that. The guy I bought it from really hacked up all the wiring.
It was only getting 14.6 mpg during that time. After I corrected the wiring the mileage increased to 17.8.
It's still no threat to any speed records but it runs better.