Horrible MPG in stop and go but getting 50MPG when not having to stop.

any trouble lights?

how many miles on her?

 

- When the engine is really cold (usually only about the first minute after fully cold start) then engine won't cut out. It needs to get the cat-conv warm.

- After about a minute then the engine should cut in and out, as required (or not required) by power demand. You can pretty much control this with the accelerator pedal.

- After another 10 minutes or so (more or less depending on outside temperature and how hard you're driving) then it's fully warmed up, and at that point it will no longer cut the engine on and off (as required by power) if you are driving slowly (under 35 MPH).

- At this point if you come to a complete stop, the transmission in "D" and foot on the brake, then it will idle for about 10 seconds after which the engine will cut out. After that little idle calibration test then it will properly cut the engine on and off as required, even at low speed.

If you are crawling along in slow traffic, then it is really important to get the Prius into that final mode where the engine cuts in and out as required. Otherwise the engine will often just keep idling even when it's not needed, and this really increases fuel consumption in a low speed situation.

 

I'll have to double check this on the Prius, but I'm pretty sure it does this... Not only when the engine and cat are cold do these cars never stop idling but they also idle inefficiently with the timing very retarded. This is to warm up the catalytic converter(s).