Interior temp and mpg?

Hi Guy, welcome to PriusChat! The Prius main design goal is low pollution. Great milage results from not burning more gasoline than necessary and doing it as cleanly as possible. To have low emissions, the Internal Combustion Engine and exhaust system must be hot. That's why hot coolant is pumped into the ICE first thing, and why the ICE comes on a few seconds after putting the car in READY mode. Warming up the cabin air using the air conditioning system takes heat from the ICE, so it has to run more to get to a low-emissions stage. When you pre-heat the cabin air with your space heater, you eliminate that draw on the ICE heat. So your observation is perfectly valid. People who use a plug-in Engine Block Heater warm the ICE using electric power instead of gasoline, so they, too, experience an initially higher fuel efficiency. Most people in North America see a 25 mpg efficiency in the first 5 minutes. In other parts of the world where Prius don't have the hot-water bottle, 12-15 mpg equivalent is seen.

 

It would help MPG vs. running the heat during warmup. Running the heater during warmup will cause a longer warmup and keep the engine running at a stoplight. By not running the heater you are shortening the warmup time. I have also thought of doing what you do, but I dont like the risk of fire with a space heater. If you are parked outside while doing this, how does it do with keeping the windshield ice/frost free?