I did notice that crawler motors have bigger numbers (45T, 55T, 75T) which means these ones have more turns. Does this mean they deliver less RPM out of the box?
If so, would they be good if I use old rc cars on a small space (in a garage) or they just can't stand being on the throttle much of the time?

from what i understand is yes, the higher the number the less top rpm there is. and with it not having a high rpm, as long as you don't gear it too high, you should beable to stay on the throttle the entire time. but then again, it would be a good idea to check how hot it is getting, so if you do have it geared high, you don't cook your motor....

Atleast thats how i understand it, so i could be way off...

any motor will work ..... it just needs to be geared correctly.

you can stay in the throttle as long as you want, like Heffer said though if its geared to high your gonna kook it.

to be honest, your probably gonna have to gear them up because your not gonna get much RPM out of something like a 55 or even 75T motor.