Author Topic: Puzzled about fuel consumption  (Read 2980 times)

guest904

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Puzzled about fuel consumption
« on: May 19, 2009, 11:39:55 AM »
I just bought a 2007 Jazz SE CVT. My previous car was a Yaris Auto. I've just used my first tank of fuel in the Jazz and at the end of the tank the fuel consumption display said 51.1 mpg. When I calculated the mpg it came to 39.3. I'm having difficulty accepting that the fuel consumption display has an error of 25%! I always calculate my mpg as I enter fuel bought into a database on my PDA which calculates the mpg at each fill-up. The Yaris displayed mpg was usually within 1.5 to 2 mpg in 40mpg of my calculated value. I assume modern engine management systems know quite accurately how much fuel is injected into the engine to be able maintain efficiency and low emissions. How can the display be 25% in error?

Thank you for any comments.

guest806

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Re: Puzzled about fuel consumption
« Reply #1 on: May 19, 2009, 03:21:29 PM »
If you see the general topic on fuel consumption you will see the first calculation i did also produced a big discrepancy.  Give it another try and reset the trip meter every fill up.  You probably are doing this and i had done it when i got my first reading.
It would be interesting to see.

culzean

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Re: Puzzled about fuel consumption
« Reply #2 on: May 19, 2009, 04:06:29 PM »
Depending on driving style I have seen about a 10% error on Jazz mpg readout vs real life fuel used.  Agressive driving delivers the worst error, steady driving the lowest error. (display only updates every 10 seconds and takes a 'snapshot' of the rate fuel is being used at that time - so if driving is erratic it will sometimes miss some of the peaks during rapid acceleration)

If you are going to work out the mpg yourself it is best to zero the trip meter every time you fill up, otherwise the previous mileage and fuel kinda muddies the equation. You may notice that after the 'trip' is reset the 'mpg' display reacts very quickly to (for example) going up and down hills, but as the trip mileage rises the rate of reaction slows down, this can only be because the computer is taking all previous fuel and mileage into its calcualtions and giving an average spread over a much longer period.

Everyone buys petrol in litres now, so to work out mpg (mixing your metric and imperial units).

Taking 1 imp gallon = 4.55 litres ( 1 US gallon = 3.8 litres)

Say you did 300 miles and then top up with 28 litres ( 28 / 4.55 = 6.15 imp galls)

divide 300 miles by 6.15 gals = 48.8 miles per gallon.
Some people will only consider you an expert if they agree with your point of view or advice,  when you give them advice they don't like they consider you an idiot

guest765

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Re: Puzzled about fuel consumption
« Reply #3 on: May 29, 2009, 06:43:05 AM »
On consuption checks i do it without the electronic bits........i take the mileage at start
fill up to the top so you can see the fuel........then do your running and fill it back so you can see the fuel take the mileage and divide the amount of fuel into the mileage.
Gives an accurate reading of MPG..........then see what the electronic bits do at the next fill up.  ;)...........i average 51-52 mpg.......

JazzyB

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Re: Puzzled about fuel consumption
« Reply #4 on: June 01, 2009, 09:08:10 PM »
Just filled up and worked out the fuel consumption. It worked out at 47.5mpg!

Finally mpg nearer the official figures

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