Response:

Definitely by car

Even if it’s only 50 meters away, the whole point of a car wash is to wash the car, so taking it there on foot would be… complicated (and suspicious to the neighbors).

Now, seriously:

Distance: 50 meters is basically next door.

Time: by car it will literally take 10–15 seconds.

Effort: minimal.

Logic: the car needs to be there to be washed, so you have to drive it anyway.

The only reason to walk would be if you first want to check whether there’s a line or availability, and then come back for the car.

Pro tip: If you see it’s empty, take advantage immediately. Car washes tend to fill up quickly around midday.

Is it just an exterior wash, or interior as well?