So... Why 99.9?

My two cents... and you'll likely hear from some folks who have the Scanguage tool (which can display more digits for MPG than just xx.x) that in the cases of times when the ICE is not running (according to the MFD) Scanguage will show finite MPG values in the various hundreds, or at gasoline-free times (I think) a 999.9 or 9999.9 display (I don't have a Scanguage.)
As the Prius display is limited to just 2 digits and a decimal, any division-by-0 (otherwise suggesting an infinite # of miles per gallon spent; true golf-cart mode with no gas being burned) will show as 99.9 and any finite MPG calculated by the computer, but greater than 99.85, will show as 99.9.
Can't say why Toyota didn't opt for xxx.x display instead, or a neat little infinity symbol, for those gas-free times

 

<div class='quotetop'>QUOTE(ChristoB @ Jul 16 2007, 01:23 PM) [snapback]479781[/snapback]</div>

Toyota probably chose 99.9 is the max display range in their trade offs to distribute computer resources and display real estate. It seems like it wouldn't have taken much room or resources to add an extra half digit (that is to say an extra leading 1), but they may also have felt that the accuracy above 99.9 was less meaningful than the more static diplays people can get in the 42.0 to 70.0 range while cruzing.

When getting 150 or 250 or more MPG, it is a very short transient time interval, and there is a lag in the display between updates to allow people to actually read them. As the mileage went from 105 to 375 to 125 over a very quick time, one or two seconds, the display would only give a few snapshots in time like maybe 150,205,135. The exact time the reading is displayed becomes sort of random, and you would never even know that your peak instantainious mileage hit 375. The instantanious metering of the feul flow may not be that acurate either. Fuel flow may be a better representation of avearage feul per 5 seconds, than exact feul flow per milisecond.

You can see this easily in your digital spedometer. As you accellerate from 0 to 35, You will not see the numbers counting up 1, 2, 3, 4, 5, etc up to 23. Instead you see something more like 3, 7, 14, 25, 32, 35. In this case, when you see the 14 on the display, you may be going anywhere between 14 and 25.

 

With more digits displayed, the readout would just be in continuous flux. Too many digits, the instantaneous MPG would be useless, because you couldn't read it.

 

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I suspect the 99.9 is shown as a consequence of the built-in divide by 0 avoidance programming in every current microprocessor. Otherwise they tend to "lock up". So the software sees divide by 0, and displays the max. number it can, avoiding problems. The Scanguage II does the same. I don't have it set to display mileage any more, I display fuel consumption (l/hr - it shows 0), as the MFD displays l/100 km (also 0).

 

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At it's maximum economy, during brief periods, the ICE is only capable of just over 100 MPG. When gliding and stealth, it's infinite MPG, but for even briefer periods of time. I guess they figured the point is made with 99.9MPG, and eliminated all the unnecessary extra digit flux.

 

<div class='quotetop'>QUOTE(catgic @ Jul 17 2007, 06:57 PM) [snapback]480547[/snapback]</div>

Question for you then: do you lease or finance or buy straight up?

 

Well, I'm not a hypermiler, just a technology geek. The Scangauge II can be put on any Prius with no modifications. I used velcro-tape and stick it above the on/off switch. the cable runs down by the steering column and is zip-tied into a bundle under the dash, where it is wedged just above the accel. pedal. It can be removed in less than a minute, and leave no indication it was ever there. The velcro tape pulls off the dash leaving no residue. I like to use the hook side on the dash and the loop side on the device. That way, if you carry the device in your pocket, you don't get "hooked".

The Scangauge has no effect on the car (unless you intentionally clear codes with it). It doesn't need any attention. I just put it in "Gauge" mode and leave it alone. It takes care of itself.

-I- like to know what my car is doing. So I display:
fuel consumption (l/hr)-----------------------------12V battery voltage
Engine RPM------------------------------------------Coolant temperature

Engine RPM, so I know when it's running (sometimes it's hard to tell).
12V battery voltage so I know if/when it's in trouble and don't get stranded.
Coolant temperature so when I block the front openings this winter I will know when it's time to unblock them.
Fuel consumption was my choice of all the rest of the available readouts. None was needed, but it was the most "interesting".

Try to convince her it's a good thing to have (for you) reliability wise. She can just ignore it. Or not. I find it interesting so far, but not really necessary.

 

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