I run my F104 Sybrand Scythe with 17.5T Team powers with 93/30 ratio asphalt outdoors with foam tyres an itīs just ok, much better than using rubbers. Maybe rubbers is the option for small "parking lots" with 21.5 motors as you do in USA, but for "medium" and "big" nitro tracks 17.5 +foams is much better for me.
