Thanks Jon. Yes you are quite correct re the wheel diameter limit for HMC. NZ has suffered for many years with a general lack of enforcement in historic racing, which is partially a result of economics, meaning competitors will elect to run a diameter size based on tyre cost and availability, which for many years was 16". Unlike the US where cross-ply tires are the norm, in NZ, its a DOT radial that is most popular, as it is much cheaper, but in 15" is only available in narrow sizes that don't suit a ponycar sedan. In fact, its not uncommon to see cars in NZ on 17" diameter wheels, which, I'm sure you'll agree, looks completely wrong.
Because HMC was created with the knowledge that many cars that would race in the class already exist, and are currently fitted with 16" wheels at DOT tires, and because we're enforcing other changes, such as cast iron heads etc, we've allowed those with existing cars that are currently fitted with 16" diameter wheels to continue to do so for the first 18 months, at which point they'll be required to then change to 15" diameter wheels. This is just an economic decision, to help soften the blow for these car owners in a difficult economic climate, but they will in time be required to fit their cars with 15" wheels, and we'll require that they also use a cross-ply tire, be that a Goodyear or Hoosier.
For those building cars to HMC rules, they'll be required to fit their cars with 15" diameter wheels immediately. As you say, 15" diameter was what these cars raced on in period, and its interesting to note that in most cases, a diameter was never set in period by the various rule makers, as 15" was really all that was available in both wheel and tire choice. Of course, in Trans-Am, 8" width was the limit, and I assume it could have even been 7" in 1966 and '67? In Australia, the limit was 8" until 1970, when it became 10". Here in NZ, it was 8", but by the early '70s became 14", which was extremely wide, and Britain appeared to be the same. 14" width is effectively an F5000 wheel.
For HMC, we've opted to allow up to 15" x 10", which still captures the look and feel of the cars that raced here in period, but we feel 14", which would be historically accurate, is really too wide.
So what we've really done in bringing the HMC rules together is to take a little from the Trans-Am, Australian Improved Production, and NZ Saloon Car Championship regulations and work them all together. Essentially, that is how the NZ sedan landscape looked in the late '60s and early '70s, with race promoters bringing Australian and US cars and drivers here to race, as you can see from the Bay Park pictures I posted above. So it just seems right that we merge elements of all three countries sedan regulations into what we've created. In fact, our front spoiler rule is almost word for word what the SCCA wrote for the 1970 Trans-Am. It was thanks to your posting of the 1970 Trans-Am regulations on this forum that we were able to study the wording, and incorporate it into our own rules.