Speedo Accuracy - '94 E36

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I set the Cruise to 80 and timed two miles by the mile markers; it took 92 seconds to travel the distance. This works out to 78.26 mph, so the speedo
is fast by 1.74 mph at 80. The SPEED reported by the OBC was 78.2, so it seems that all is well in my universe.
The speedo is accurate to within 2 mph at 80, what more can a guy ask?
By the way, I run 225/45 x 17s on my 325i Convertable.
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J Strickland wrote:

I did a similar test just after buying my e32, although over five miles - it was very early in the morning - and checked against my GPS. The results were also similar with about a 2mph error at 60, 70 and 80, increasing to 3mph at 90. Didn't test above that.
My V8 Discovery gives almost identical results too. The GPS was +/- 1mph at all speeds.
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Steve G
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I'd ask for a lot more. A pulse counting device can be made accurate to within 1% for pennies. Check out any DVM. Ancient eddy current speedos could better this too - let alone chronometric types.
BMW - and others - deliberately make their speedos over-read. Which is disgraceful on a car of this cost. They should be accurate to within the width of the needle. Or so that with worn tyres so they don't under-read with new.
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On Wed, 31 Aug 2005 22:54:21 +0100, "Dave Plowman (News)"

The trouble is all tyres are slightly different. The diameter of a new tyre from manufacturer A will be slightly different to one from manufacturer B. During the life of a tyre you loose approx 2% of the diameter too. Then there's the tyre pressure and temperature. Your tyre expands when it get up to temperature, so there's yet another variation.
And how many of you out there obey the tyre pressure guide to the letter? Do you drive to the store and then put the extra 2psi in when you've dumped your shopping in the back? No, of course you don't. Oh, and you'd have to let the tyre cool down completely before checking it too, and when you get home, put the shopping away, let the tyres cool down, let the pressure back out... Yeah right! lol!
So I'd much rather the manufacturers read a little over, than read under. Mr policeman doesn't like people saying "Oh really officer, the speedo said I was doing less than that".
With all that, I think 2.5% is damn impressive.
Dodgy.
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wrote:

In the UK speedo's can over-read by up to 10% but is not allowed to under read (or you fail your MOT / type approval). Since a new car has new tyres, one has to make an allowance of a further ~2% for that compared with the reading when the tyres are worn (and so smaller). 5% is typical, 7% on older caars and more for Fords.
You can do better than that with doppler radar or GPS.
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On Thu, 1 Sep 2005 14:34:42 +0000 (UTC), "R. Mark Clayton"

And don't we just know about the doppler radar!
Dodgy
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Oh yes. TC's used to quote XX.123mph, although the last figure could vary depending on the officer's pulse!
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I've had many many MOTs, but never had the speedo tested.

Smaller tyres (wear) would make the speedo over-read, so is fine with the concept of never under-reading.

If you bought a one dollar digital watch that gained 5%, would you be happy? Mind you, the digital clock in my E39 is poor too.
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wrote:

My speedo, '94 E34 seems to be reasonably accurate. Within about 1-2 mph of the reading given by a sat nav at 70 mph. The analogue clock keeps excellent time. I've only had to reset it once in 18 months, and that was about 8 months ago after fitting a new battery. It's still correct within about half a minute. Mike.
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That means you don't reset it twice a year?
DAS
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in
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Ok. Yes, I did mean the seasonal changes spring & autumn.
You called it "analogue". Does that not mean a display with hours and minutes hands?
Whatever, your clock's time-keeping seems pretty good.
DAS
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news:43177a55$0$1291$ snipped-for-privacy@ptn-nntp-reader02.plus.net...
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It does. On my E34 it's fitted instead of computer. I'm still debating whether to change it for an E34 computer I've picked up. Apparently it would give all the std functions, apart from outside temperature.

Just luck I suppose. I can't imagine they're as accurate in general. Mike.
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It's not rocket science to design a clock or watch that keeps time to within 30 seconds every half year. I've got a 10 year old plus quartz analogue wrist watch that does just that. Of course a wrist watch tends to have the its temperature kept near constant.
But my E39 clock looses some 2 minutes per week. Strange that they didn't drive it off the RDS part of the radio since it's in the same panel.
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wrote:

Still relatively new then. :-) My regular wristwatch is a quartz Seiko analogue baught in 1984. Is only on it's 3rd or 4th battery, and keeps similarly good time. Expensive when I baught it, but cheap when it's still running perfectly after 21 years.

Even if they didn't, I'd have thaught it would be quartz regulated, therefore having similar accuracy as that of most quartz timekeepers. 2 minutes a week is disgraceful in a car purporting to be luxury drivers car etc. Come to think of it, why not radio controlled. Accurate to within a second in umpteen years. They're cheap enough these days. Mike.
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wrote:

Sure it can, but the variables are numerous. Tire size, timing of my pressing the start and stop buttons, to name but two.

I don't agree that it's disgraceful at all.
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Dave Plowman (News) wrote:

SG: Had to do it. Just looked up the specs on several of my DVM/DMM's. Depending on the quantity being measured (e.g. volts, amps, ohms, etc) a 1% error is quite typical. Then you have to go on to read the conditions under which this can be achieved - ambient temperature having a big influence as does wave shape for ac measurements - and the over-riding caveat to all displayed values +/-1 digit. The accumulative errors can easily be more than 2.5%

SG: You should actually thank BMW (and all other European manufacturers for that matter) for making their speedometers read high - they may well save you from a speeding ticket one day.
No matter how good the pulse counting system is, the final display that you, the driver, sees is electro-mechanical and there's probably more than a 1% error simply in the conversion.
If you really want needle width accuracy then go buy a car with a fat needle :-))
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Err, the likely error will be different depending on what it's measuring.

BMW uses an analogue reader. And anyone designing a pulse generator - and reader - would be well aware of the pitfalls.

Not on a custom designed system. In any case, BMW speedos don't have an 'accumulative error' of 2.5% - they all over-read. If it were a tollerance, some would be near accurate and some over-read by 2.5% above this. But this isn't the case.

Heh heh. Since I know my E39 speedo over-reads, I just put this into the equation. So happily pass speed cameras at 5 mph over what it says.

Don't be silly. The analogue display can be easily made to be accurate to within the width of the needle at any spot speed. In the UK, this would be 70 mph. And if BMW etc use such crappy meters that it isn't still within tolerance at 30 or whatever they need to get some decent engineers.

I've got a 20 year old car with a Smith's pulse counting speedo. Remember them? Similar to Lucas. 0.5 mph fast at 70, and spot on at 30. As checked by Autocar at the time using a fifth wheel driving a chronometric speedo - same as the police used to use for checking speed. And exactly the same on the five variates of this model they tested, from basic to top of the range. With 5 different axle ratios and four different tyre sizes.
BMW *deliberately* make their speedos read high. Draw your own conclusions.
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<top post>
Sorry to beat this dead horse to death, but my only point was that people that claim they have a speedo that is off by 5 ~ 7 mph are making an error.
Using the formula, <width> * <aspect ratio> / 25.4 * 2 + <rim> = <overall diameter>, I find that all available tire sizes are within 0.5 inches of having the same diameter. Surely if all of the tires are within a half inch of being the same overall diameter, then the speedo readings should be pretty close no matter which tire is installed, and how far it's worn down. Assuming my beat up old car is anywhere near approaching normal, then everybody should have similar error rates as I have, and my error rate is below 2%, which is pretty damn close considering the application.
My car is the E36, '94 325i Convertable (which replaced a '94 325i Sedan), carrying the 225/45 x17 tires. I captured the tires from the sedan and put them onto the convertable, and the sedan came with the Sports Package that included 225/55 x 15 tires that are only made by Michelin. The sedan always had the 225s on it while I owned it - except that it had 205/60 for a few weeks that the PO had installed, and were near end-life when he sold the car to me, and I did not check speedo accuracy with them -- but I digress.
Sure, it would be nice if the speedo was dead nuts accurate, but is that _really_ a requirement? Not to me.
Width Ratio Rim Diameter 225 55 15 24.744 205 60 15 24.685 225 45 17 24.972 235 40 17 24.402 245 35 18 24.752 185 65 15 24.469
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Dave Plowman (News) wrote:

SG: That's why I said the error was "typical". Do you have a problem with English comprehension?

SG: What makes you think BMW speedo's are custom designed. To my certain knowledge they use instruments designed and manufactured by Bosch and Siemens and probably others too. The speedo display tolerance is -0/+6% - as defined in the current version of construction and use regulations. There is, in any case, an accumulated error as with any system that goes from an electro-mechanical origin through digitisation, computation and back to electro-mechanical display.

SG: Yes, an analogue meter can be made to be spot-on accurate but at what cost? And we're not just talking about the meter are we? Errors are introduced at every point in the chain - right from the car tyres to the actual instrument. You have to compromise between absolute precision and lowest cost (as the two extremes) and come up with something that the masses will accept.

SG: I've got a 30+ year old motorbike (1972 Royal Enfield Interceptor) with one of the Smith chronographs on. I'd hate to think what it would cost to replace if it got broken :-(

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