Hi,
you can reach much more rpm also with outrunners:
I had a Cyclon brushless one capable of 5500 kv/volt and I ran it with lipo 2s.
Everything depends by the motor design. They probably shown you some motor
to run propellers directly to the shaft of the motor without reductions.
I think you should understand how many rpm you need looking for the car gearing.
If you suppose you need to reach 50 mph, you simply will find the right rpm
using your speed formula "inverted" (difficult to say in english for me, I hope this
stay understandable).
When you will have understood the right rpm, you will choose the right motor design.
BTW have a look to the Cyclon ones, I think they are really good motors.
www.cyclonbrushless.com
Ciao,
Daniele