Heater off too..............
Turning off the A/C or heater has NO effect on mileage at all! The climate control system is ALL electric. There is NO DRAG on the gas engine at all since both the A/C and heat are driven by ELECTRIC motors, not the gas engine. In cold weather, the heat comes from an auxiliary tank of coolant that is heated by electric heaters until the gas engine warms up. Really, people, read your owner's manual, that thick book that comes with your car and explains everything. If you can't/won't read the manual, get to know your service people very well. Buy them a soda and ask a lot of questions. They are very knowledgeable and very helpful people. By the way, the power steering pump is also driven by ELECTRICITY, not the gas engine. Educate before you speculate!:)
Electrical Drain WILL hurt MPG
About 90% of my drive is off-highway. I notice no difference in performance. As another user stated, this is because things like the heater, A/C, and headlights draw power off the battery. Not directly from the engine. However, I do notice a difference in mileage when using anything that draws additional electricity over regular driving. For those of us that drive city or roads with non-highway speeds, the decrease in mpg will feel greater. Since speeds are lower, the electricity gained from regenerative braking or the recharge from pulse will be less.
There is a section of my drive where I travel a 3 mile stretch of level, low traffic road. Once I'm on that stretch, I put the cruise control to 35 mph and can go a little over 2 miles on electric mode before the engine kicks in to assist and recharge the battery. If I'm using the A/C, I might max 1 1/2 miles before the engine kicks in.
For anyone that questions this, try this test. Coast down a big hill to max your battery. Let your Prius sit without using the A/C, heater, or headlights. Time how long it takes until the engine kicks in to recharge. Max your battery again, but this time let it sit with the A/C running. Your time until recharge will be considerably less. In my drives, I've notice that the heater hurts MPG very little, headlights a little more, and A/C the hurts the most. Figure this into your drive. Those of us with less of an opportunity in our drive to recharge the battery will be hit harder in the MPG.