i did a search, cuz i know i've seen this topic several times, but they're all buried.

i like autolite spark plugs, and some say 605'* and some say 606'*. i think 605'* are the "one heat range colder" plugs? what does this all mean, and how would it affect my performance and/or MPG?

One heat range colder (or two/three depending on modifications) are typically used for supercharged (or turbo) cars, in order to help avoid KR. I don't see them causing you any issues though if you wanted to use them. And yes, the 605'* are one heat-range cooler.

go whit the 605 if youre looking for performance a colder plug give a quicker spark but if youre looking for normal driving all weather plugs go stock