And it all depends on whether you have fan jets or needle jets, not forgetting the type of brush you are using.
A 2mm hole on a needle jet, used at the same pressure as a 3mmwill of course use less water, the trade of is just like pinching the tip of your garden hose, pressure is the same but instead of running out it blasts out.
If you are using a fan jet, you NEED that blast to get a good spread on the fan of water.
I use the brush that Peter Fogwill supplies - the salmon - two rows of bristles removed, fan jets in the brush, and the water is contained nicely inside the bristles.
Rinsing is done mostly on the glass, so the water is yet again contained in the bristles and strikes the glass directly, so if you want to clean just the glass only then you can do so.
If there were needle jets in the brush I think I would struggle with 2mm because I would want the greater volume for the rinsing aspect.
If you have that greater volume, it should enable you to work faster, but if you are a relative newbie to WFP it isn't easy.
When I first started I had to work with the flow rate turned down, I just wasn't skilled enough to work at the rate I needed to if the flow was flat out.
Now I'm experienced I can do so.
If you are using 3mm needle jets and you are worried about the water you are using then you may not be working fast enough!!!
That isn't meant to sound like an insult...honest!
But if you get everything in balance, using the greater volume of water that the 3mm will give you, should enable you to get more work done out of the 650l of water.
As has been noted, using the 2mm needle jets means turning down the pressure because the water will be splashing all over the shop.
So you turn it down to gain more control, and this also means you are going to work a little slower as you need to spend more time to get the glass clean and rinsed.
But then again, working at the slightly slower pace may suit you better...horses for courses eh?
