High mpg( km per liter) at idle speed

You reading that correctly?

It is either miles per gallon (mpg, US models), or litres per 100km (l/km, everywhere else)

When you are idling with the engine going (has not reached S4 yet), you are burning something (so litres > 0) and you are going nowhere, km = 0.

Math tells us when you divide by 0, you get infinity. Well, that is the generally accepted theory anyways. Since the meter can only display XX.X, the closest number to infinity it can show is 99.9 litres/100km. In US models, it shows 0 since miles = 0, and gallons > 0, you get 0.

 

No, the OP is correct. -Some- Prius do read in km/litre. For example, the Japanese ones.

If you are moving and the engine goes to "idle" (I usually see about 940 RPM) and the fuel use drops to zero, you are in "stealth mode", and yes, you will see infinite mileage. Of course the Prius instrumentation doesn't display that, it tops out at 99.9 km/l, in the US 99.9 MPG, and elsewhere 0.0 l/100 km. The engine is being spun by electricity to avoid overspinning MG1, no fuel is being injected, and the car -may- be using electric propulsion. Or it may be coasting.

 

99.9 km/L is very good right (very low fuel usage). So why is it something to worry about?

Unless of course you mean 99.9 L per 100km, in which case it is very poor fuel economy (high fuel usage), but never the less is normal if the engine is running but you aren't moving (or are moving very slowly).

Please verify whether you really mean "L/km" or if you mean "L per 100km". There's a good reason either way, but they are completely different reasons.

 

Ok that's an easy one then. When you're not moving, if the engine is using any fuel at all then you'll need an infinite amount of petrol to make it 100km. Actually you'll never make it 100km (remaining stationary), but you'll use an infinite amount of fuel trying.

Luckily though the engine on the Prius should cut out after a fairly short period of time (typically when it warms just a little) if you're stationary.

BTW. If you think the engine is sometimes idling at low speed when it shouldn't then that could be the case. Do a thread search here for "hybrid stage 4" to learn more.