Well, I tried out my new free low flow 2.0 gpm shower head this evening. Here are the two heads.
Original head. It says 2.5 gpm max on it.
New low flow head. It says 2.0 gpm max on it.
Just to make sure it in fact did flow less water, I did some testing. To do this, I took an ice cream bucket I had laying around (1.125 gallons) and turned the shower on. I timed how long it took to fill it up with each shower head. Here were the results.
original shower head = 40 seconds to fill = ~1.69 gpm
new 'low flow' shower head = 35 seconds to fill = ~1.93 gpm
Aha! My original shower head which says 2.5 gpm max on it actually flows LESS than the one rated at 2.0 gpm max.
Then, I decided to play around just a little bit. On the original shower head there is an adjustment in the middle (see pic). This kinda lets you adjust the stream size that comes out of the head. So, I tightened it up and retested.
original tweaked shower head = 53 seconds to fill = ~1.27 gpm
I think we have a CLEAR winner here. I guess the lesson is make sure you test your new 'low flow' fixture before you trust it, and you might be able to tweak your existing shower head. I was able to get a solid 25% reduction in flow just by adjusting my current head.