Your shower has a certain maximum "volumetric flow rate".Image via WikipediaYour shower has a certain maximum "volumetric flow rate". This is the speed at which the water comes out of the shower head in Gallons per Minute (GPM). Usually this is between 3-5 GPM. If you use a hot/cold ratio of 90/10 and your shower head has a max of 3 GPM then you're using 2.7 GPM of hot water.

It is easy to turn on the water and max out the flow rate, however, if you dial it back so that the shower head is not running to it's full flow rate, say you're at 2/3 maximum pressure, and if you still use the same ratio (90/10) then you're now only using 1.8 GPM of hot water. This will allow you to use as little cold water as possible which is good because in effect you don't really want any cold water, and by adding any cold at all, you are merely cooling down the water you have already paid to heat up.

1.5 GPM shower heads also exist. Here is a link to the "Jet-Stream"