Here is how this works. Flow, rated in Gallons Per Minute (GPM) is the amount of water you spraying on your car. Pressure, rated in Pounds per Square Inch (PSI) is essentially the speed the water is flowing out. These two measurements are largely directly disproportionate, meaning when you increase pressure on the same pressure washer, you are restricting flow. When you decrease pressure, your flow would increase.
This is where reading specs can get dicey. Many cheap pressure washers give you the maximum PSI rating and maximum GPM rating. The problem is that you get your maximum of each when the other spec is at its lowest. For instance, it may read 2000 PSI | 1.8 GPM. Well many would actually be getting 2000 PSI at 0.9 GPM and get 1.8 GPM at 1000 PSI. Get my drift here... You almost have to punt the specs in the trash and give different sized nozzles like these a try and see how things work out in the real world.
This is why the quality and power of the pump matters. A higher powered pump can provide higher pressures at higher flow, but that's going to cost you. It's going to cost you a lot if it's an electric pump.
Now, since we are washing cars, we want more flow and reasonable pressure. Flow gets work done, pressure blows holes in paint. I think we need somewhere in the 1000-1500 PSI range at the maximum flow possible. If you have a really nice pressure washer it will have an unloader valve on it that will allow you to adjust flow and pressure simultaneously. It is my understanding that you'd much rather leave that unloader valve alone and use nozzles like these to adjust your output. You will have much better results and less loss of flow.
Here is how it works. There is a hole on the tip that mounts on your pressure washer gun or wand that essentially squeezes pressurized water out. The smaller the hole, the more pressure you get and less flow you'll have. Get it?
So how do you know what sized tip to buy? That's the tricky part. I don't know. The only thing you can do is use the published bogus spec your pressure washer is giving you and use the chart below to pick a size.
To make sure you get the correct nozzle for your machine, check out our Pressure Washer Spreadsheet and find your model. If you have something not listed, see the below chart from MTM.
To use the MTM chart, you want to find your rated GPM on the left. Then select your DESIRED PSI on the top. We like something between 1000-1500. Where the numbers intersect is what nozzle you should consider. Do note that this is simply an approximation and your results may vary. In general, if you have a gas pressure washer, something in the 5.0 range will be good. Same thing for most European Kranzle machines.
MTM offers a 60 day warranty for any possible defects with their products. Info about their warranty can be found Here.