[stefanv]

Remember, from high school physics, that turning == acceleration.

If you make a 90 degree turn, say from west to north, all the while maintaining 30km/h, you have to accelerate from 0km/h in a northerly direction to 30km/h in a northerly direction. You can get most of the energy to do that from the deceleration (technically, acceleration from 30 to 0km/h) in the westerly direction, but since nothing is 100% efficient in the real world (especially in government work), you'll need to add some energy from the engine.

The sharper you turn, the more energy you lose due to friction (mostly in the back wheels, where the tires aren't pointing in exactly the right direction during the turn), so the more you have to add back in from the engine.