I'm looking at a few gasoline-powered pressure washers.

Naturally, the typical specs a

a) the hp rating of the engine
b) the gpm
c) the psi

So my main question is - are these numbers fudged, or have a wide
accuracy range?

What is the minumum rating (in terms of gpm and /or psi) that make it
worth while to actually buy one of these things?

The range I see are from 2 to 4 GPM, and roughly 2000 to 4000 psi.
Engine HP from 6 to 13.

Your answer should not include "it depends on what you want to wash".

I want to know if a washer that does 2 gpm @ 2000 psi has only half the
"cleaning power" as one that can do 4 gpm at 4000 psi (ie does the
cleaning power scale linearly with these numbers, or geometrically or
exponentially).