Sizing a surface cleaner to your machine
The most common equipment mismatch in this trade, and it is entirely avoidable.
The pairing
4 GPM runs a 19 or 20 inch deck. 5.5 GPM runs a 24. Eight GPM and above earns the 28 and 30 inch decks. Between sizes, go smaller.
What going too big looks like
The deck starves. Each nozzle in the bar gets less water than it needs, so you slow your walking pace to compensate, and you still leave stripes where the passes overlap unevenly.
You end up with a heavy, expensive tool that is slower than the smaller one would have been. We see this constantly and it is always the same story: someone bought the biggest deck they could afford.
What going too small looks like
Nothing dramatic — you just cover less ground per pass. It is the safer error by a wide margin.
Castors are not optional if you do this daily
A deck without castors drags. On a residential driveway that is fine. On a commercial lot it will wear you out by mid-afternoon and it wears the deck skirt as well.