It's interesting reading the discussion of motor specs in the US from an outsiders point of view. I was under the impression that 17.5, 13.5 and Mod were all common in the US, if not at every club, then at least at all the major meetings. What's the actual distribution like at club tracks?
It was a little surprising to see 21.5 mentioned, but then I looked up the CRC track, and it is really small compared to what I'm used to. The smallest track in NSW right now is the one at the top of the page!
I've done some work recently to get 10.5 blinky adopted by some clubs in Australia. We haven't had any spec 12th racing until now, and as a result 12th scale has been on life support with only the requirement to run it at States and Nationals for Worlds qualification keeping it alive at all. We're getting some traction with 10.5 though. When I was trying to work out what the motor format should be for spec 12th in Aus I read about racing in the US and in the UK. It seemed the UK tracks were larger than the US ones, so I erred towards their motor format. Now we call the 10.5 class Eurostock.
10.5 is about the right speed for most everyone at most of the places I've driven it. Occasionally I worry that I might have picked a format that's a little too slow when I'm driving around a 3m wide TC track, but I've got over a decade of driving experience, and those thoughts are fleeting. At the gooped up permanent carpet track at top of screen though, the class definitely feels faster than anywhere else, they carry more corner speed and the track is tighter, and not everyone is coping as well with 10.5 there as they do elsewhere. There are a larger portion of people attempting 12th there who've never driven a pan car before, and in many cases not driven anything faster than a 21.5 TC. This is why they are introducing a 17.5 class.
So, I guess what I'm saying is that from my limited experience in Aus, track size and grip levels play an important role in determining the right spec motor. The experience level of the drivers is also a very important factor. Mod 12th was only being driven by the best TC racers in the main, even on some of the largest tracks we have. Offering a slower class was necessary to bring in drivers who'd run Stock in other formats.