display max 99.9 mpg. does car know the real mileage?

From my experience it uses the actual, not simply the 99.9 that may be displayed. You may test this by resetting the mpg display just before you begin a descent where your engine will be off and then judge by the amount of time it takes the reset mpg to drop below 99.9.

 

when it shows 99.9mpg, the engine is off, thus the number is irrelevant...
however, the distance u traveled at 99.9mpg will be spread out through the rest resulting in higher mpg...
at least that's how i understand it...

 

MJ50, correct except that the engine is not necessarily off when it is showing 99.9 mpg, but it is getting 99.9 or more mpg. The engine does not shut off at speeds over 41 mph.

 

At speeds above 42MPH or 67km/h engine has to spin.
But it can be spinning by:
-Force from wheels (engine braking). With no fuel consumption.
-Electric motor spinning it. With no fuel consumption.
-Running or idling. Using fuel. But (if) it's still getting more than 99.9MPG it just shows 99.9MPG. Unless you have Prius with L/100km display. It will show about 2L/100km in this situation (depending on speed and other stuff). And 0L/100km if engine isn’t using fuel at all.

 

All the computer in the car "knows" is the distance travelled and the amount of fuel used.
The 99.99 just indicates that, at the moment, the engine is using little or no fuel.
The miles are still counting up and the gas usage is still counting up for the real average.

 

The others covered it well but I'll add that with a Scanguage hooked up or using the Torque app you can see exactly how much fuel is being used at any given time. The displayed iMPG is simply for your viewing pleasure and maxes at 99.9mpg. The actual calculations are going on behind the scenes. FWIW even the Scanguage maxes out at 9999mpg. lol

 

My metric set Scangauge shows best fuel economy as zero (gas consumed). Much better.

 

I have a Dash Dyno SPD scanner hooked up to my 2004 Prius. It has AVG mpg, Instant mpg and RPM and other handy measurements. Not as good as a Scangage, but I already had this one.

When I coast and the engine shuts down the mpg shows the calculated mpg. The MFD shows 99.9 mpg, if I press on the throttle to get into EVmode and I get numbers that flash wildly and the highest is over 7000 mpg! It is fun to watch.

 

now that's crazy!