I have read the S will have a single speed transmission, but how does that work?
How does it deliver low end power but not overheat when going 65mph for hours at at time?
It'll be the same basic design as the roadster. Electric motors are very efficient, so they heat up very little for the amount of power they are putting out. At freeway speeds, the airflow is ample to cool them. The only times I've seen the motor overheat is climbing mountains on tight twisty roads.
So there is a single, fixed, reduction gear to take the motor RPMs down to axle RPMs. As for torque, electric motors have full torque from zero RPM, up to some point, then it drops off linearly until the motor won't go any faster (about 14,000 RPM on the Roadster).
There are graphs on one of the Tesla web pages. Believe them.
If I'm not mistaken the Model S is going to be one step improved with a liquid cooled electric motor instead of the air cooled electric motor on the Roadster.
An electric motor, in the application Tesla is using it, has it's speed changed mainly through varying the frequency and provided the cooling can handle it (at low speeds) and the bearings can handle it (at high speeds) an electric motor can have a very wide range in rpm.