Amateur racing culture

[Starter]

It is not the same thing..
Both SCCA derived "Otter-cross" and "Grass-o-cross" are slower and flatter and "more restrained" the the Pommie things with similar names.
Both emphasise the ability to do them in a bog standard road car or SUV or Mini-van with road tires. That also explains the classing structure with over 30 classes so, like anything in America, everybody is a "winner" just for showing up even if there is only 1 car in the class that day..

I guess you're not familiar with all of the US types. While there are plenty of Autocross (Solo) classes, most are street driven and street legal cars. There are a number of those classes that allow special Solo or racing tires. There are also several classes for racing cars and "modifieds" It's a fairly flat course because its a parking lot. This is the most popular participant form of motor sport in the us (with the possible exception of drag racing) with the SCCA national championships in Nebraska last year having over 500 entrants from all around the country.

.RallyCross, on the other hand, consists of just six classes, three two wheel drive and three four wheel drive. A majority of the RallyCross cars are also street legal and many are driven to the events. These courses are off pavement and can have a good amount of elevation change.

It's true that top speeds in both would rarely exceed sixty to sixty five MPH.