I asked someone I know with a prius about that just out of curiosity once. I said: "What if its cold out, does the engine shut off?" Him: no. I said "What about when its hot out and you have the A/C on, does the engine shut off?" Him: no.

So, if the temperature outside is just right 60-75ish degs....then it will be most efficient, otherwise, the engine keeps running.

I can't stand the Prius. I share the same opinion as Jeremy Clarkson. "Any diesel will do better than this! A lot better!" (on mileage)

Diesel Lupo: 75mpg
Technology, battery, tons of things waiting to break prius? 45 on a good day

Another thing people never consider with the hybrids is the amount of energy that goes into making the car, and batteries, especially the batteries. They are big, heavy, and full of acid. Producing, transporting, and recycling them takes a LOT of energy. So in the end of a hybrid's lifespan, they don't really help the environment at all...they just produced the emissions and consumed the fuel someplace else. (power plant)

__________________
-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)