[White Lightning]

I drive an ICE (gasoline) smart car. In stop-and-go city driving under 45 miles per hour, the vehicle's display is hitting 100+ mpg at times throughout the city drive. How does the smart calculate the 100+ mpg? Does the spike occur when I remove my foot from the gas pedal upon approaching a red light? I put the 5-speed manual transmission into neutral for only about the last 20 feet or so before the stop.

 

[InjuredAgain]

For probably the last 25 years or so, many/most fuel injected cars will entirely shut off fuel delivery when coasting in gear at higher speeds. At those moments, your fuel economy is practically infinite as no fuel is being used, and your gauge should show the maximum economy it can.

 

[Mister_smart_LA]

It's coaching the driver on the fly, so that you are aware of what driving habits need to be sustained in order to achieve high mpg figures.



At the end of the day it's an average of driving conditions and habits. You can hit 100 mpg for a few seconds and 15 mpg's for another few seconds. You average it out and you're sustaining 57.5 mpg for those sample size seconds of driving.

 

[Chopper]

At 30mph in 4th with a very light throttle my 450 petrol genuinely does in excess of 100mpg imp for brief periods. As aforementioned above when costing, even slightly, the FI shuts down and indicator displays tend to default to their highest displayable figure to indicate no fuel being used.