mpg when stopped

Please read Car and Driver: The Truth About EPA City / Highway MPG Estimates | PriusChat to learn about the EPA tests.

 

The multi-display is tracking fuel consumed vs. distance traveled, as do most on-board computers. In my BMW while idling at the light I can see the mpg reading drop accordingly. Elapsed time doesn't play into it so no, that wouldn't explain the discrepancy.
The car company is fully motivated to rig it that way. If it's pessimistic there will be no end of customers bringing their cars in for "poor mileage" without doing the calculations.

 

The mpg reading you get is really a "miles travelled" per "gallons of gas burned". If you travel 0 miles when stopped and burn any amount of gas (idling) you get 0mpg. If you are in the Prius and creeping along in the parking lot, even if you only go 0.1miles, you have burned 0 gas or infinite mpg. On the Prius this gets you 99.9mpg in MPG displays and 0l/100km in metric-lands.

The Prius engine really only gets mpg in the high 30's. When your traction battery is almost failed and the average "tanks" to 38mpg or something, that is because you are running the gas engine all the time. The ability to shut it off at lights, and when coasting, going downhill, or times of low power demand make the miles per gallon higher because you get "normal" MPGs from the engine averaged with the Prius' infinity reading which it pegs at 99.9.

 

The EPA highway test spends 0% of the time at idle, the EPA city test spends about 5% of the time at idle. (The European test spends 20% of the time at idle)

 

If you are going zero MPH, then whatever amount of fuel you're burning, you're still getting zero MPG.

 

FWIW, if you are not moving and the ICE is burning fuel, you are actually getting a "negative" MPG rather than zero as every drop of fuel burned is calculated in the equation.

Ron

 

You can't really get an instantaneous negative miles per gallon, because you can't travel "negative miles." But I get where you're coming from. The Prius is wasting fuel when the gasoline engine is on at a stop and it can lower your average MPG.

 

It is a fact that if you are not moving and the ICE is burning fuel, your overall MPG will drop which is in reality "negative" MPG. The display will show 0 (zero), however it is not designed to show a negative amount. The only way you could be getting a TRUE zero MPG is to be "not moving AND not burning fuel at the same time". IMO, it is a flaw in the design of the readout's for the Prius but the engineers probably decided that most would not be that critical. However mathematicians can be very critical at times, my self included. But at least the system does adjust the overall readout as the MPG fluctuates through the various driving cycles and routines of daily driving.

Ron