Anyone tried one of these HEI Distributors?

65DartGTConv said

I just finished my second tank of gas with the new distributor, this time with the plugs gapped at .050". The gas mileage worked out to 12.9 mpg. I doubt the bigger gap hurt the mileage so I'm guessing it's just margin of error. The two tanks average together at 13.2 which is a 1.8 mpg improvement with the new distributor.

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Finally finished my third tank of gas. This time only 12.5 mpg which brings my 3 tank average with the new distributor to 12.9. Still better than the old distributor, but not as much as I thought.

The best tank was the first tank before I increased the spark plug gap. Is it possible the bigger gap hurt the mileage?

EDIT: I remember now that the first tank had some freeway miles.