The 15km rule is certainly not valid at all time, as at current warmer climate, mine will start switching of the engine (even after being parked overnight) already after a few minutes and less than 2kms in urban driving. depending on SOC, it will even set off straight in EV mode Even at cold winter temperatures, it never took me 15kms for the engine to shut off.
That said, very short drive average varies a lot depending on battery SOC before parking it, topography (especially as with PD52 it seems to charge more when it recognizes hills) and of course time. 'But in general I'd say that without any extreme scenario, 5l should be well doable in 4kms (I have already managed setting of with a full battery after a previous downhill drive, and driving a little over 3kms in the flat without the engine restarting, having an average of 0.0 for that trip, though that I'd rate as an extreme, a positive one nevertheless)
