I've heard the A-12 was a little faster and could fly somewhat higher than an SR-71
but I don't have any real data like a pilot's manual to back up that claim. If the SR-71 flew faster than the Mach 3.2 design limit, it could go a bit higher. However, there really wasn't any need. Like I've written before, a coworker of mine was an SR-71 RSO from 1985-90. He said they normally cruised at 3.15. It could go a bit faster but there just wasn't a need.
The A-12 had to be a hell of a ride. There's one in Huntsville in front of the Alabama Air & Space Museum and another in San Diege in front of the San Diego Air Museum. Both are beautiful birds. It's hard to believe they first flew 30 years ago.
