[Randy Rosenberg]

In this article, Henry Olsen states:

Race gas and the leaded gasoline that was sold at your corner gas station until the mid 1970’s was easier to tune for because a tuner could “read” the spark plugs and look at the tailpipe “color” to determine the air/fuel mixture. Today’s reformulated gasoline does not leave any “color” on the spark plug unless the air/mixture is very rich and the tail pipe color is almost meaningless.

http://www.automotiveu.com/tuningperformance.htm

Good luck,
Randy...