Date: Sat, 1 May 2004 15:46:47 GMT
From: hohnospamlid.invalid (Goran Larsson)
Subject: Re: Fuel consumption figures computer wrong

In article <c708s6$gfqrk$>, Dave Hinz <> wrote: > Tire diameter maybe? If the tires are significantly different than > what the car shipped with, that'll throw the distance part of the > equation off as well. Assuming that the car calculates fuel consumption from * how long the fuel injectors are open * the fuel pressure * how much fuel will flow through the fuel injectors per time unit * the distance driven and that the driver calculates fuel consumption from * how much fuel he/she bought * distance read from odometer then any error caused by tire/tyre diameter will change the calculations done by the car and the calculations done by the driver with the same amount, i.e. the tires/tyres doesn't matter when comparing the fuel consumption numbers for this car only. The tires/tyres sure do matter if comparing fuel consumption numbers between different cars, but then the driver introduces much more variation in the numbers. Error sources that affect calculations done by the driver: * Station pump shut off point. * Station pump volume measurement accuracy. * Fuel temperature during fill up (volume changes by temperature). Error sources that affect calculations done by the car: * Fuel rail pressure. * Fuel injector (volume per time unit accuracy). * Fuel injector (the difference between electrical signal and the actual mechanical open/close times are not what ECU expects). * Fuel temperature. Error sources that affect both calculations done by the driver and by the car: * Tire/tyre dimensions (different brands or size). * Tire/tyre dimensions (wear) -- G–ran Larsson

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