Water pressure is a matter of height/elevation. From my fire department
days, you will get static pressure of 1/2" pound PSI per foot of
elevation.. so you need 10 feet of water to get that 20 psi.

You've got approx 1.5 psi there of head pressure.

Considering you will be flowing very little in the way of gallons per
minute, I wont address the flow related pressure loss in that 5/8" hose.

Chances are you will actually do just fine.. If not, then you need to
"make" a tank that can handle pressure (such as a large air compressor's
tank.. coat it for rust proofing, and put an air nipple on it and charge
it to 20 psi.. which is prolly overkill in my opinion.. people have
pressure washers in the back of trailers all the time with a feed tank
right next to it, running atmospheric pressure feeds.

Dave

Newps wrote:

Got a new pressure washer today and I want to bring it to the airport to
wash the plane. It needs 20 psi to work properly. If I put a 100
gallon water tank in my truck and fed the pressure washer with a
standard 5/8" garden hose and a 3 foot drop to the pressure washer on
the ground would that give me 20 psi?