My trip computer is always accurate to within 0.5 mpg. A variation that could easily be explained by three facts: the volume of liquid in the tank changes with air temperature, one brim to the top is not always the same as another and some fuel pumps are not calibrated correctly.
The worst I've ever seen has been 2% off (over several thousand miles) from the several cars owned with trip computers.
I'm not sure many people know how to correctly calculate their mpg accurately either. I've seen people moaning about over-optimistic computers, and they were using anything from 3.8 litres to 1 gallon (US gallons!), to 4.4 litres to 1 gallon, rather than the correct 4.546 (or 4.54609188, but the fuel pump probably isn't that accurate...) . Even just using 4.5 vs 4.546 will introduce a small margin of error.
To make matters clearer it would help of people could share the results of at least one brimmed tank run, including miles driven and litres dispensed at the pump - then we can cut through the haze and see the real figures! :)
I'm not suggesting that everyone here is making that mistake, just that I've seen it very commonly in some forums where there's much grumbling about poor economy.
For my part: Vectra 1.9 CDTI, last tankful I covered 587 miles and brimmed with 53.2 litres. 50.16 mpg, computer was claiming 50.3 mpg. Close enough for me!
Official combined is about 48mpg IIRC. I do lots of motorway driving and have a very light right foot.
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Passat PD130, my computer is consistently 5% optimistic which isn't too bad.
Last tankful was 530.5 miles, brimmed with exactly 55 litres = 43.85mpg. Computer said 45.9mpg.
Only a third of those miles were motorway, the rest was town / commuting.
One thing the fuel computer taught me about my car, is that it's more economic to accelerate firmly & quickly to the target speed, than to accelerate gently.
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That's interesting, some over-reading 5%, same saying 10%, all on the Passat. All other things being equal, I would have thought the computer would measure in exactly the same way.
The one factor none of us can take into account is how well calibrated the fuel pumps are. Maybe people aren't always getting as many litres as they think they are? That would explain a lower mpg measurement compared to the computer reading.
If I was that sad (and I might be :), I'd be tempted to compare the readings when filling up at different petrol stations.
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The trip computer on my V70 2.4D 163 bhp Euro IV geartronic reads consistently 11% optimistic over about 10,000miles. I think this is deliberate on Volvo's part to obscure the terrible fuel consumption - less than 34 mpg overall driving with a very light right foot.
The service manager at my local main dealer, who I have known for many years, confided that he did not think it was a coincidence that the trip computer had become much more optimistic with the intoduction of the Euro IV engine with the dreaded particulate filter! Sjb's much more accurate trip computer on his older V70 would seem to confirm this.
HectorG
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My VAG computer was ~10% out for a while, but the dealer re-set it (you can adjust a fiddle factor to get it more accurate). It is now within 1 or 2 mpg of actual consumption.
Joe
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My Volvo S60 T5 mpg indicator is spot on. Problem is that it reads 27mpg!
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My V70 D5 manual is a Euro III car and never gives more than 34mpg, probably 30mpg average. The only thing to do with the fuel computer is to turn it off. It is always wildly optimistic, to the extent that I have wondered if it is reporting km per gallon (and km left in tank) rather than miles.
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Another Passat PD130, also reads between 2 and 4 MPG optimistically, I always fill to the same point (first click) and only use two or three different pumps. In over 24k miles the actual and computer MPG have shown a average of 2.5MPG difference.
Best yet on a round trip to the lakes 610 miles on a single tank, 54MPG computer, 52MPG actual.
StarGazer
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Over the last 40K, based on brim to brim fill ups, my Omega trip computer has pretty much always been within 1 mpg of calculated figure, either way.
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I always reset my computer's memory '2' when I fill up to pump cut out (Ist time) and the readings here are always within 2% of the figure calculated from petrol used versus mileage covered for that period. On all occasions the computer has over read the mpg but within limits as stated - I'm quite happy with that.
The car, by the way, is a 1.6 petrol Golf MKV and has averaged 41.5 mpg since purchased new 2 years ago.
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A few years back, making an unexpected late night rural journey, the petrol station I had planned to call at was locked up tight, even knocking on the house door failed to summon anyone, so i had no alternative but to try and press-on. Needless to say, I ran dry, luckily a passing gent stopped and gave me a gallon out of his boot, (kind man!). I still had about 20 ish miles to go, so i nursed the old Senator along as gingerly as i could, the trip computer very rarely registered less thal 27mpg, but I still ran out again about 2miles from home!
billy
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Does a mpg indicator computer actually measure the rate of flow of petrol , or does it just calculate it from some other paramer assumed to be related to fuel consumption?
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On the basis of one brim-to-brim check, my Legacy shows 0.5mpg pessimistic.
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As my car doesn't have an onboard computer I use one that plugs into the diagnostic port ( scanguage II). It gives me a consistant 33/34 mpg which is very close to the pen and paper calculations. It provides a lot of other info too. Some of which is useful, some just interesting and some that I just plain don't understand!
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My 2005 Mondeo TDCi trip is consistently 5% over optimistic, reading around 60mpg at a constant 70mph on the motorway (very light traffic), calculated to 57mpg.
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My ST TDCi overreads by about 8-10%. I have noticed if you fill the neck of the tank up that increases the inaccuracy, presumably it doesn't account for the extra 3 or 4 litres you can get in if you try hard enough in its calculations. I'm not entirely sure how it calculates, but it's not fuel flow because common rail diesels pass fuel back to the tank.
For example, last tank I covered 565 miles and brimmed the tank (as I always do) with 54L of diesel. By my reckoning thats 47.6MPG, trip comp was showing 50.4MPG.
I usually reset every tank, I'm going to leave it for a few tanks this time and see if the accuracy increases.
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My VW Bora's mileage figures are an indication rather than an actual, definitive mpg return.
It's useful to have but I certainly don't rely on it. On the overrun I can get up to 199mpg, which is the maximum figure...:-)
However, I do know that if I do a lot of short local trips then the mileage per gallon can be up to half of those on a much longer run, which is hardly surprising.
That's more informative than taking it all too literally.
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What\'s for you won\'t pass you by
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Does a mpg indicator computer actually measure the rate of flow of petrol , or does it just calculate it from some other paramer assumed to be related to fuel consumption?
It's my understanding that the injector open duration is the parameter used for estimating fuel flow.
Injectors are open or closed, so if you know the specified/expected delivery for any given open duration you've got some numbers to play with.
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Golf IV 110TDi - 1% optomistic
Passat 110TDi - 5%
Laguna II diesel, Golf V 105TDi PD - both 10% out
Indicated fuel consumption over the same 67 mile journey varies widely day to day, even when at seemingly same speeds but the trip computer averages for a brim to brim were consistantly as above.
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Interesting to note that almost everyone who is grumbling about the mpg calculator in their cars has a diesel.
Is mpg really that important to you that you get all excited about 10% here and 5% there? Really you should be thinking more of the steady stream of pound notes dripping off your (mostly new) cars in depreciation! That figure will absolutely dwarf 10% of the value of the fuel you put in the car.
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jase1
Got it in one..:-)
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What\'s for you won\'t pass you by
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Since mine only ever shows something point 2 or something point 7 (never any other tenth) I can't but doubt whether it has any value at all. I rely on my calculations ~ I record every litre I buy and calculate the mpg at the end of each month .Easy peasy.
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L\'escargot.
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. Is mpg really that important to you that you get all excited about 10% here and 5% there? Really you should be thinking more of the steady stream of pound notes dripping off your (mostly new) cars in depreciation! That figure will absolutely dwarf 10% of the value of the fuel you put in the car.
But some of us have cars with zero depriciation and fuel bills of £300 - £350 per month.
I have a company car (therefore zero depriciation) but have to pay for fuel to cover my 40,000 miles per year.
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Sorry, it's early, depriciation should of course read depreciation!!!
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I have a company car (therefore zero depriciation) but have to pay for fuel to cover my 40,000 miles per year.
What, you do 40K/yr of private mileage?
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>> I have a company car (therefore zero depriciation) but have to pay for fuel to cover my 40,000 miles per year. >> What, you do 40K/yr of private mileage?
Unfortunatlely, yes!!!
150 mile round trip to work every day.
Having said that, I live in a beautiful area of Gloucestershire and work in an area I don't consider so attractive, Slough.
The quality of life at home is well worth the 2 1/2 - 3 hours in the car a day plus the fuel costs.
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