I would also assume that the reduced MPG is due to the car idling and not driving thus resulting in a 0.0mpg time?
No its not harmful to the car, just to your wallet. The Prius is designed to warm up fast so it can run really really rich (read: use lots of gas) warming up. Its best to just press start and drive. Otherwise, you are going to have bad average mpg's (read: possibly in the 30mpg range!) The block heater really sounds like a good idea for you. And its not expensive. About 40 mins to install. PriusChat Shop : Toyota Prius Engine Block Heater [Prius EBH] - $59.00
Priusguy32, I really do appreciate all of the help!! Did you install the Block heater or have a mechanic do it? In the mean time I will hit reset after I idle so I don't cry~!
I did it myself and its easy. Errr.... generally easy, you need to have a skinny arm to snake the block heater up between the firewall and engine and snap it into the block. I had a helper with a skinny arm snake it up to the spot in the engine block, then I finished the job (snapping it in place and routing the plug out the front grille). Dont hit reset! lol. I mean, dont hit reset unless you fill up the tank, if you choose to track your mpg's between tanks. If you reset it every time you idle, youre going to be even more disappointed with the numbers and it wont jive up to your tank.