Going downhill with the clutch depressed doesn’t need petrol.

Not correct! It will be true if you step on the clutch, i.e. in the Neutral drives, consumes very little fuel, but you can save even more: you don’t use any fuel at all if you drives with the gear engaged without accelerating. This is primarily useful for downhill stretches. Because of the so-called deceleration fuel cut-off, the vehicle does not need any fuel at all. The momentum of the vehicle then drives the engine, as well as any consumers via the alternator. In any case, you should avoid idling and braking downhill! This could overheat the brakes and, in the worst case, cause them to perform less. Instead, you should coast downhill with the gear engaged and brake as needed.

The following example: You are driving on a stretch of motorway that leads (slightly) downhill for several kilometers. Speed ​​limit is 100km/h. When idling, the vehicle would become relatively fast, and you would have to “catch” this speed with the brakes. But if a gear is engaged in this example, for example, the fourth, fifth or sixth, the vehicle cannot be that fast at all: the so-called engine brake.

Never idle and brake permanently on longer downhill stretches! Always drive in gear. This prevents the brakes from overheating, saves a lot of wear and tear and, moreover, requires no fuel at all!