Whats everyone running for gears?

I drove my '67 up to fresno to get painted, cruised around 3800-4000 rpm for 5 1/2-6 hrs
3.73 gears 25.5 tires
Ran like a top till I stuck 87 octane in it, too hot a plug tested that 87 and I had brown specs on the ceramic of the plugs, couldn't feel it though.... need a colder plug for that crummy octane or need to take the timing back 2-4*

Back up gears are 3.23's