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RE: [OM] [OT] No more big SUV (going green, sort of)

Subject: RE: [OM] [OT] No more big SUV (going green, sort of)
From: julian_davies@xxxxxxxxxxxxxx
Date: Fri, 7 Feb 2003 08:13:43 +0000 (GMT)
This is the *ONLY* accurate way to measure average fuel consumption. Trip 
computers don't. They have no reference to absolutes (total fuel volume 
springing instantly to mind) only rate of flow and time.  They also DO NOT work 
by dividing the mileage travelled by the fuel consumed, or vice versa. They 
work by integrating a very dodgy instantaneous fuel flow measurement by time, 
and doing the same with speed. I guess some of the better ones do a calculation 
involving total mileage, but the earlier ones only integrated the instantaneous 
readings, which lead to hilarious numbers.
Why is the fuel flow measurement dodgy? because AFAIK, all modern cars have 
fuel recirculation. You have to measure the fuel flowing to the engine, and 
subtract the fuel flowing back to the tank. These flows are subject to a phase 
distortion which makes an instantaneous measurement invalid.
Of course, the whole subject is relative. You may well BUY your fuel in liquid 
measures, but your engine consumes it by weight. This is the primary reason for 
diesel engines appearing to be more "efficient". My diesel does minimum 45MPG 
(imperial) average, BTW, but without you knowing exactly where and how I drive, 
that is a completely meaningless number.

Julian

>  from:    Marc Lawrence <mlawrence@xxxxxxxxxx>
>  date:    Fri, 07 Feb 2003 05:51:53
>  to:      olympus@xxxxxxxxxxxxxxx
>  subject: RE: [OM] [OT] No more big SUV (going green, sort of)
> 
> It's undoubtedly nowhere near as accurate as those nifty trip
> computers

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