Highway RPMS ?

OK, yeah, that makes more sense now. I did a similar test for optimum WOT timing on the road by connecting the distributor vacuum advance to a hand-held vacuum pump. I would add some vacuum, which added a known amount of advance, go WOT, then hit the vacuum release valve on the pump. If my acceleration decreased after releasing the vacuum, that meant that the engine liked the additional advance. If my acceleration increased after releasing the vacuum, that meant that the engine liked the original (base + mechanical) timing better.
Here are a couple good articles I found on distributor tuning.
More Power For Your Leaning Tower: Add 14-Horsepower To Your Slant 6 - Mopar Connection Magazine | A comprehensive daily resource for Mopar enthusiast news, features and the latest Mopar tech
Gallery: Distributor Recurving for Improved Fuel Economy - Mopar Connection Magazine | A comprehensive daily resource for Mopar enthusiast news, features and the latest Mopar tech