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post #1 of 19 (permalink) Old 8th March 2007 Thread Starter
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Laguna II mpg calculator

Hi guys, don't know if this has been talked about, can't find it if it has.

My 53 Extreme is reading 41.5mpg, this has averaged out over some 2000 miles now and is pretty constant. I used to reset it every week but it changes too quickly and doesn't give a true inpression of average mpg over time, bouncing between 37 and 44mpg all the time, so I've started just resetting my trip computer on fills and nothing else.

The thing is the mpg estimate of 41.5 just doesn't compute. I put 30 worth of diesel in a week, alternating between the BP normal (so I get 33ltrs) or the Ultimate (where I get 30ltrs).

On the 33ltr fill I average 340-360miles, depending how I drive, and with the 30ltr fill I get around 330-340miles, but I've never had less than 330 miles for 30 litres.

Well, at 4.54 litres per gallon this works out at 30l/4.54l = 6.6 Gallons, 330m/6.6g = 50mpg.

or: 33l/4.54l = 7.2 gallons, 360m/7.2g = 50pmg

So the figures are relative, but how the hell can the computer be so far wrong? It's a simple calculation - what's more, it even estimates how much mileage is left in terms of fuel when I fill up correctly (usually within about 10miles either way), so if it knows how far I'm going to get on the fuel I've put in, how can it work out the mpg and be so wrong?

Anyone shed any light on this?
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post #2 of 19 (permalink) Old 9th March 2007
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I believe the comp is set to measure in KMH not MPH and there is either a conversion to MPH. Also I believe they are more accurate in KMH
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post #3 of 19 (permalink) Old 19th March 2007
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Do you also reset the mpg trip counter? it may be you're still getting a total avg.

On our Lag II there is a seperate trip counter for the MPG (next screen after MGP)

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post #4 of 19 (permalink) Old 19th March 2007
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Fuel computers ain't worth the money - I my opinion they are at best only an estimate - with an accuracy of approx plus or minus 10%. In all honesty they're about estimating and without accurate measuring devices how can they calculate the amount of fuel used. They just seem to another bell or whistle added on to impress the buyer. In all honesty the only accurate method of measuring fuel consumption would be to use a specialised flow meter and I have'nt heard of one yet being fitted to a car produced for the public. In all honesty I wouldn't rely on them. Fuel consumption needs to be measured over a broad range of circumstances as there are just too many variables such as driving style, weather, road conditons, traffic, type of road surface, tyre pressure, etc., (the list goes on) to take into consideration. The best way is to fill her up to the throat on each fill and then calculate the average over about 3 tank loads. Also I'm of the opinion that the computers are set up to give the best results as otherwise it would affect sales. Think about it.
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post #5 of 19 (permalink) Old 19th March 2007
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I was shown over 50Mpg in my 1.8 Lag 11

This was going down a long hill off the throttle in 5th gear. LOL

I never trust my fuel computer.
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