difference between 1.9 and 2.2
 

what is the difference between 1.9 and 2.2 crawlers?.
is it just tires?
wheelbase?

RE: difference between 1.9 and 2.2
 

The difference is the diameter of the wheel and the corresponding hole in the tire. 1.9 is smaller, so a 1.9 tire has a smaller opening in the middle. 1.9 and 2.2 wheels/tires do not interchange well because the tire with the wrong wheel will be too loose or too tight. 1.9 wheels/tires tend to be narrower, as well.

1.9 wheels and tires look more 'scale' in size and will tend to roll under the body while 2.2 wheels/tires usually stick out and generally do not fit the wheel well of the vehicle. 2.2 is more like a modified truck with a lift kit. :D

RE: difference between 1.9 and 2.2
 

As far as comp crawlers go, the differences are: wheel dia, wheelbase, gate width, req'd body dimensions, overall tire dia, and a few others.
The 1.9 class doesn't allow dig during comps either.

The rules section on RCCrawler.com has the complete 2010 USRCCA rulebook.