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HowardL

Mpg Confusion

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I've been on holiday and kept a log of fuel and milage

Done the calculation

569 miles, about 10.5 gallons

It works out at about 54mpg

But my computer says 44,3 mpg...why the difference?

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Can be many things here is just one example (figures are not what the car would use but have been made simple for the sums)

You drive 10 miles and use 0.5 gallons, your MPG equals 20

You drive nowhere with the engine running and use another 0.5 gallons your MPG is now equal to 10

So even though your total milage didn't change your MPG was halved

Not sure how the trip computer handles idling but using zero (as in travelling zero miles) either gives you zero or infinity as a result depending if you are dividing or multiplying.

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Also remember the on board computer (OBC) doesn't actually measure the amount of fuel flowing to the engine - it only extrapolates figures based on engine settings.

I wouldn't say OBC's are liars but they can be economical with the truth.:)

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Have my own question guys, how on EARTH do you use the MPG calc on the dash? Likewise with the average speed! I've never worked it out and for some reason I just haven't bothered reading the manual :P

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Have my own question guys, how on EARTH do you use the MPG calc on the dash? Likewise with the average speed! I've never worked it out and for some reason I just haven't bothered reading the manual :P

:lol: I just keep mine on average MPG setting. If you have it on the setting for actual MPG you are doing, you will find that coasting down a hill without any throttle reads as 99 MPG. Driving the car hard will make it plummet, sitting for ages with it idling will see it drop to 0 as you are not going anywhere.

The average speed one, i don't really use.

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Have my own question guys, how on EARTH do you use the MPG calc on the dash? Likewise with the average speed! I've never worked it out and for some reason I just haven't bothered reading the manual :P

:lol: I just keep mine on average MPG setting. If you have it on the setting for actual MPG you are doing, you will find that coasting down a hill without any throttle reads as 99 MPG. Driving the car hard will make it plummet, sitting for ages with it idling will see it drop to 0 as you are not going anywhere.

The average speed one, i don't really use.

Yeah, that's what I keep mine on as I don't know how to use the average/ trip function! Playing 'keep above 40MPG' is a fun game :D Though having since filled up with premium its become 'keep above 4,000 RPM' :P

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I have two yaris both built in year 2000. the verso does a trip mpg that seams more influenced by the last 10 miles than the whole trip,

the yaris gs does an average of the fuel consumption since the last time the Battery was removed. weird!

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...I get the opposite effect with premium and get under 40mpg with it and over 42mpg with regular.

I also find the Yaris trip computer accurate to one decimal place! Though I do re-set it after every re-fill.

However a 569 mile trip means motor ways, country roads and not much of a cold engine so I presume 54mpg is more accurate in a 1.3CDX so your trip computer is indeed wrong and your calculation spot on.

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