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Remember to correct mileage readings before calculating mpg figures as odometers typically overread by up to 10% as regards trip computers they are notouriously inaccurate.Brim to brim on corrected mileage is the only accurate way to do it.Anyone that relies on fuel gauge readings or trip computers is fooling themselves.
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errrrr you cant correct on mileage if your odometer overreads because you dont know if they are 10% out. Typically they aint that far out, but speedo's are because they are set that way.
Brim to brim on odometer is correct, and my mpg read out is typically about about 5-8% optomistic.
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Ive compared my trip computer and odometer with brim to brim on the pump and also checked distance against my previous omega and map data. Ive had excellent agreement.
S.
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You can easily check your speedometer's mileage reading accuracy by using the mileage posts on a motorway - even better is to stick to 60mph and check that it moves on exactly at the same point once a minute over several miles' travelling.
No two people are likely to get the same mpg figure even if they drive the same car in turn over a set route - everyone has a different driving style.
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>by using the mileage posts on a motorway
If you're relying on distance signs, as in x miles to junction x or y miles to place z, then you're in for a disappointment. They are not accurate.
If you mean the little posts by the side of the road, then how many of them to you have to pass to travel a mile ?
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"If you're relying on distance signs, as in x miles to junction x or y miles to place z, then you're in for a disappointment. They are not accurate. If you mean the little posts by the side of the road, then how many of them to you have to pass to travel a mile ?"
16.09.. , Mark, as they are (intended to be) at 100-metre intervals, measured from some origin darn sarf (Charing Cross?). Previous threads have told us that they don't always get put back in the right place after roadworks, but if you measure over 40 or 50-km (500 posts) these errors should be negligible.
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>>If you're relying on distance signs, as in x miles to junction x or y miles to place z, then you're in for a disappointment. They are not accurate.
I wrote quite clearly "mileage posts" and not signs.
Two different things...:-)
Marker posts every 100 metres on the hard shoulder indicate the direction of the nearest emergency telephone; these are generally situated every 1500 metres, approximately a mile, depending on intervening junctions etc.
It would seem the curse of mixing miles and kilometres and yards and metres has afflicted the DoT as well these days...:-)
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Two - the first one and the second one. That's why they are called mileage posts...:-)
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But they are not based on miles but kilometres.
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>No two people are likely to get the same mpg figure even if they >drive the same car in turn over a set route - everyone has a >different driving style.
Indeed.
>You can easily check your speedometer's mileage reading accuracy by >using the mileage posts on a motorway
Or (easier and very accurate) passenger using hand-held GPS?
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Aren't the little red mileage posts actually 100 metres apart?
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Espada III - well if you have a family and need a Lamborghini, what else do you drive?
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and when did they paint them red?
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Yeah.. arent they white with red rectangles?
Also, I've see some different ones on the M6 - can't remember what colour (orange?) but they seem to count up (or down depending on direction) the total number of kilometres in 500m sections.
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Espada III - well if you have a family and need a Lamborghini, what else do you drive?
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Yes new ones are rectangular signs in 1/2k readings counting up or down with an A or B to show which way. Seen on M25 and M6
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Question is.. are they accurate enough to calculate the error in speedos and odometers?
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Espada III - well if you have a family and need a Lamborghini, what else do you drive?
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I have checked my odometer against a hand held GPS. Over 100 miles it was less than a mile out. Thats ...errrrrrrr (counts fingers) 99% accurate. Good enough i feel. The speedo over reads by about 3mph at 30mph. (27 real 30 indicated) and 5mph at 70 (65 real 70 mph indicated)
My fuel computer is typically 5 - 10% optomistic when checked brim to brim against my odometer. And no wonder really, if you knew how prehistoric fuel gauge senders are. Its really 1930's technology that bit. Given that this 1930's part provides one of the main parameters for 2000 technology (specially on the miles to empty reading) its a wonder it gets any accuracy at all.
However given that it is feasible that a well designed computer can provide accurate point to point consumption readings, given accurate fuel flow (from the injector computer not from a silly impellor in the fuel line) and accurate odometer.
Whatever, sometimes my neandethal urgings take over my right foot and disconect the brain/wallet interface.
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"And no wonder really, if you knew how prehistoric fuel gauge senders are"
RF - I thought computed mpg was based on integrating the volumes put through the injectors? Don't see how you could get an instantaneous reading any other way. When you reset to zero, new values appear after about half a mile.
Of course that too depends on how accurate the metering is. I presume the only calculation requiring data from the fuel gauge sender is the distance left in the tank. It would be interesting to know if the program notices when you fill up, and guesstimates how much you have put in.
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>>Aren't the little red mileage posts actually 100 metres apart?
Why on earth should mileage posts be 100 metres apart?
They'd be called 100 metre posts in that case.
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If we are talking about the small white posts with numbers on Motorway verges they are NOT called mileage posts because they are 100 metres apart.
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But not so easy and certainly not very accurate if you don't have a passenger who owns a hands-held GPS...:-)
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My father informs me that in fact the little red and white markers are approximately 90metres apart, as apparently they are still set on imperial measurements and so are actually 100*YARDS* apart.
Whilst i have spent my life practicing disbelieving most things he tells me, as he is a Civil Engineer who was a Planning Manager on the M25 Northern stretch when it was first built i'm inclined to believe him.
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H - I'm afraid I think you should continue with your established practice, if for no better reason that I don't believe my car's odometer is 10% inaccurate. And if they WERE imperial, surely they would be in furlongs or some other multiple of chains? (they may well be 110 yards apart, which is near enough 100 metres)
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From the UK highways agency web site, Marker Post definition (2001)
Marker post (MP)
A numbered post on the edge of the carriageway providing a reference point and indicating the chainage. Marker posts are usually placed at 100 metre intervals and the number on the post indicates the chainage in kilometres with the number of 100 metre sections within that kilometre, recorded underneath, eg 146 1.
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RF, Andrew-T - Thanks for that, nice to know that the assumption i have spent 21 years building still holds true!
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M25 civil Engineer?
Probably why it was a couple of miles short when they finished it?
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Ah, RF. That weasel word.
usually
That's the sort of disclaimer I am wont to use in correspondence. Essentially therefore those posts could be any distance apart whatsoever!
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"chainage in kilometres"
That would be chainage as in the unit of 66 feet (a tenth of a furlong) I take it?
You'd think by now our masters could decide whether we were metric (as in bananas) or not (as in roadsigns, although not marker posts, apparently)...
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An edition earlier this year of the IAM's quarterly magazine, Advanced Driving, includes a letter asking whether a claim that speedometers always read 10 per cent fast is true?
The reply states that this is more or less correct and that a speedometer could legally over-read by 10 per cent; however, it is not permissible for the reading to be lower than the true speed.
Actual legislation is governed by EC directive 75/443/EEC) stating that between 25 and 75mph the tolerance is 10 per cent plus 2.5mph above the true speed (which is what I have always understood); therefore an indicated 79.5mph would, in fact, be a true 70mph if all the tolerances were at maximum.
To get a rough idea of your actual speed, it continues, do a measured mile (for example past 17 100-metre motorway marker posts) at a constant 60mph and note the time taken; spot on would be a minute.
It adds that a Scottish IAM centre had recently tried this experiment with members' cars and even the most accurate speedometer (on a 13-year-old VW Scirocco) over-read by three per cent.
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Stuart, your latest post seems to have muddied the water a bit more. For MPG we aren't really discussing accuracy of speedo but odo. It's agreed that speedos usually overread, to give a margin of safety.
But "a measured mile (for example past 17 100-metre motorway marker posts)"? This does at least confirm the distance between the posts, but 1700 metres - what is that? Neither one thing nor the other. One genuine Imperial mile should be 1609. metres. Please explain.
My checks suggest that my odo reads about 2% over, the speedo about 7%. But I have to make assumptions about those marker posts, and tyre size comes into it too.
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It does state that it is an approximate mile rather than being absolutely accurate.
You are confusing "passing 17 100-metre motorway marker posts" and your 1609 metres - the first post represents zero and the 17th post you pass will reach a total of approximately 1609 metres .
Think of three posts; according to your logic it would be 300 metres whereas it is, in fact, 200 metres i.e. 0-100 metres between the first and second posts and 100-200 metres between the second and third posts.
Another thought. If the speedometer reads slightly over why should the odometer be any more accurate?
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"If the speedometer reads slightly over why should the odometer be any more accurate?". As I said earlier, to give a safety margin; and because indicated distances are more easily compared with known facts than any instantaneous speed measurement, they need to be more accurate to be credible.
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Back to topic.....1981 to 1985: Citroen Visa Super E, 4speed economy-minded gearbox, 1.1 litre engine, 12 miles each way to work and back, average 50mpg. Late 1985 changed to Honda Civic Shuttle, 1.5 litre 5-speed, over same commute for first 4000 miles of its life, 46mpg. So much for economy superminis! Its mpg then took a slight turn for the worse, but still averaged over 40mpg for many thousands of mixed road miles.
More recently, Rover 820 single-point injection, 36-37mpg on long runs. Moved house due to enforced change of jobs, mainly town stop-start driving brought this down to nearer 22mpg. Replaced it with Renault Scenic direct injection diesel with aircon switched on all the time - average about 46mpg irrespective of type of driving - urban stop-start, long motorway journeys always seemed to produce the same average from each tankful of diesel. Now with Galaxy 1.9TDI manual also with A/C - climate control left switched on, around 32mpg in stop-start town driving, more usually around 37mpg in "suburban" driving, and just under 50mpg on motorways/dual carriageways.
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