Actually turning a car takes almost no energy whatsoever.
If you ever get a car on a single-post center lift (like the next time you get a set of new tires), you will notice that you can rotate a car with your pinky finger.
At highway speeds you could remove the belt from your P/S pump, and you wouldn't feel any different. (Please, nobody try to remove their belt while driving at highway speeds!) The only time you need P/S is in parking lots.
Even if you take a hard corner, there is really no energy required to do that- the only energy required is to change the rotational inertia of the vehicle. Once you have started to take a corner, no further energy input is required. (To visualize this, think about a zig-zagging car instead of a car doing a single circle. The zig-zagging car will lose its momentum faster, since you are rapidly accelerating the rotation in opposite directions.)
Holding a car in a circle does load up the suspension, but those are static loads and do not require energy "input" once the car has started to rotate.
-Bob C.
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