Hey, I thought you guys had a 55 MPH speed limit - whats this 75+MPH or even 100 MPH stuff?

The 55MPH speed limit was a federally mandated speed limit in effect from 1974 until 1995. It was determined that it wasn't constitutional for the federal government to mandate speed limits, so they told states that they wouldn't get federal roadway funding unless they made their speed limits a maximum of 55MPH.

Nowadays, most big highways are 65MPH. Some are more, some less: Maximum Posted Speed Limits

However, few people drive at these speeds. They are much too low for almost anyone. Even police cruisers regularly drive 10MPH faster than the speed limit on highways. But you could technically have your driver license revoked for going more than 15MPH over the limit, so we all walk the fine line of not getting too close to a cop who's in a bad mood that day.

Yeah, I know some states have unlimited speeds, but I thought it was back country states like Utah that had unlimited speeds, not GA.

You're thinking of Montana. It's rule was that you could drive at a reasonable speed on interstates (interstates being federally funded highways ultimately intended for military transport and aircraft runways in case of invasion, but the rest of us get to use them for automobile roads until that happens). However, in 1999, that was found to be an illegal law because it was too unclear. It's now 75 in most areas, IIRC.