Pressure washers made me a living before I retired. The technology hasn't changed much over time, but in general:

1) Cleaning efficiency is a function of gpm (gallons per minute) and pressure at the tip of the wand. Generally speaking, avoid washers with a low ( 1 to 1.5) gpm delivery as they don't provide enough striking force regardless of the the advertised pressure.

2) To establish a method of comparison between different brands, multiply gpm X pressure. ( 3gpm X 800psi = 2400 "units") A good operating pressure for car washing/floor cleaning is between 1800 and 2500 "units", IMO.

4) For car washing, avoid excessive gpm/pressure combinations because of, a) Fatigue: handling the wand will wear you out, and b) potential trim damage.

If at all possible, try one out before you buy.

These are personal preferences, FWIW. Some may feel otherwise, but I've washed a lot of cars in my day, demonstrating and selling.

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62 220sb
67 250S
72 280SE 4.5
74 280C
77 300D
82 240D
85 190E 2.3
86 300E RIP 12/28/09
85 300SD
92 300D 2.5
00 E320 Current
Over 1,000,000 miles in Benzes, Since66

....and a whole passel of BMW 2002 and Tii