Relation between RPM and speed and power

[pavanbabut]

Hi,
I would like to know how rpm of an engine is related to the speed of the vehicle and its power. Nowadays since I am carpooling to work, I am observing the cars of my carpool mates. Everyone is having different cars, Lexus, Audi, Toyota, Benz, Nissan. Among those I noticed some differences in Audi, Lexus and Benz. Particularly Audi, even when she is doing freeway speeds its rpms stay between 2k and 2.2k while for benz it stays around 2.5k and 3k while for Lexus its same as Benz. Also while I am watching some previous episodes of TopGear, they tested the Vauxhall -Monaro and at 65mph it stays at 1.5k rpm. I know they all have different engine capacities, powers etc. But how can we realte between those parameters??

-Pavan