E90 speedometer error - can calibrate?

This is normal. The speedo is not having an error because they can't make it accurate, it is having an error because they made it that way on purpose. You pointed out that it is a safety margin against speeding tickets.

Deja vu (search group).

The speedo (high for legal reasons - only -3 to + 10% allowed), speed limit check and true speed (by calibration against markers or by GPS) are all slightly out.

The speedo is affected by tyre size and wear.

IMX BMW speedos read about 5-7% high, speed warning about 2-3%.

Gents, thanks to both for the reply.

Glad to know it's not an actual instrument error.

Will keep that extra 4-5MPH in mind.

Regards,

Mike

It is, but a deliberate one. It's possible to have it calibrated. Dunno where in the US, though.

I have a spare speedo head from an e30 in my shop and I do notice that it has a trim potentiometer on the circuit board. There's no easy way to be sure that it's an adjustment for gain but I can't really think of anything else that would need to be adjustable on a speedometer. Access to this adjustment would require removal of the instrument cluster from the car and removal of the speedo head from the instrument cluster so it must have been set when the speedo head was manufactured. It wouldn't surprise me if someone had experimented with this to see what it does. You might try googling for it. I agree with you that it is wrong to make the speedo read high just so we can all feel a little naughty without having to pay for tickets. The engineer's rationale probably includes adding up all the adverse tolerances that could occur to relevant components over time; accounting for someone installing oversize tires; throw in another mph for parallax; and then one more mph to make sure it never reads low. Now that the odometers are also electronic I wonder if they read long because of the skewed speedometer readings. Does your gps agree with the odometer??

"Mike" wrote in message news:pdydnf5qiMZwQRbbnZ2dnUVZ snipped-for-privacy@adelphia.com...

Most are guaranteed to be accurate @ 30mph or within 10% Apparently if you have the know how you can re calibrate via the on board computer The E38 with the business radio system can be used but I have lost all the paperwork I used to have but I know it's available on the net somewhere - Unix Nerd might have some info here.

Deja vu are wrong (again ?) - In the UK speedometers may indeed read up to 10% high but by law they must not read under your actual speed.

That is when new IIRC, but yes that makes the problem worse.

Hi

I own a 1990 E30 convertible and like every car I have ever owned according to my gps it also reads high. But I am sure it is deliberate. The reason is this:- according to my owner's manual my auto transmission should "lock up" at 53mph. It does so at an indicated 58mph BUT my gps correctly puts this at 53mph. MY BMW IS FULLY AWARE OF WHAT SPEED IT IS DOING. It is just laziness on the part of motor manufacturers to display an accurate speed. Modern engineering can measure distances to millionths of an inch; time to millionths of a second weights and measures to unbelievable accuracy. Are you telling me that some of the best engineers in the world can't be sure of a car's speed to +/- 10% or so - nonesense. You wouldn't buy a beer glass that might/might not be a pint or a thermometer that tells you an approximation of the temperature or maybe more appropriately a tyre that fits the rim of your wheel to +/- 10%; so why produce a scientific instrument like a speedometer that indicates the speed that somebody else is doing???

I'll get off my soap box now.

Dave

And I'll get on mine about the EU regulators, whose fault these inaccurate reading speedos are. The EU - and I note that YOU, David, are in the UK are hence part of the problem set - in it's wisdom has said that speedos can never read high. So we're stuck with speedos that read low *BY DESIGN* of some random paper-pushers that you (indirectly) hired.

FloydR

way )does not mean it cannot be ACCURATE to ,say, 1% or better.

By the way if you want to look at restrictive car legislation look no furthur than California. I would rather have the EU any day of the week!!

Affectionately

Dave

What are you talking about? I never mentioned high/low at all.

FloydR

You said "in it's wisdom has said that speedos can never read 'high' " - surely you mean 'low'??

You said "so we're stuck with speedos that read 'low' *BY DESIGN* " - surely you mean 'high'

Yours pedantically

Dave.

Not so. The speedometer can be as accurate as you like. It just mustn't read under.

You can't have owned much of a selection. Rover of all people used to fit accurate speedometers as did Jaguar and Rolls. Others too, possibly. I have an SD1 Rover dating from the mid '80s and that is exactly accurate at 30 mph - or at least as accurate as you can read with an analogue instrument. At 70, it reads 71. Checked on a rolling road with new tyres. Contemporary road tests also bear this out.

Absolutely. A cheap DVM that measures frequency will be accurate to within

If it were manufacturing tolerance, some would be accurate, some over read a little and some up to the maximum. But the error is pretty consistent so must be deliberate.

High, low; depends upon if a person is in the north or south hemisphere. ;-> You're right, I did mean "cannot read low".

See elsewhere in this thread about legislation, wheel sizes, tyre profiles and wear.

Not only did you get it the wrong way round - speedos are not meant to read low, so we are stuck with speedos that read high by design*, and this is IIRC part of the MOT test, but of course this arises from UK Construction and Use regulations, getting on a for half a century old, not the relatively small number of Eurocrats.

Amusingly you post your anti European tirade in a BMW group. It may have escaped your notice (and judging on the basis of your post, not much gets past your prejudices) that apart from the Mini, that they took over and revamped**, BMW's are made in Bavaria, Germany, which is in continental Europe, so regulars in this group, most of whom own BMW's are already convinced of their superiority to UK cars and you are banging your head on a brick wall (some more).

• years ago Ford used to take this to extremes, so that you thought your car was going faster than it could,

"R. Mark Clayton" wrote

I've already acknowledged that I wrote it wrong. Too bad you replied before reading that.

I've owned BMWs (and Porsches) for 30 years; have you?

My rant was mostly an attempt at a humorous-counter-rant to David, not against the EU/UK members of this group. It's always fun to raise the ire of that group.

FloydR

That why I bought one -------- new 730d SE loaded.

However back to speedo's - back in the bygone days of the mid 60's - early 80's I used to tune, build fast cars and latterly design and fit turbo systems for a couple of manufactures of imported cars. When we did major mods like replacing rear axles to handle the power, tidy up 'hot-rods' so the went as fast as they looked, changing axle/final drive ratios to give realistic fuel/performance combinations we had to re calibrate the speedometer.

An independent company at that time was "Thomas Richfield" operating off Great Portland Street in central London. The equipment they used was not as scientific as one would imagine but it worked. All they wanted was the rolling radius of the driving wheel (remembering that all speed measuring was done from the gearbox at that time) and the distance covered by one revolution of the said wheel and the number of turns of the propshaft (not many cars were FWD).

Every one we had re calibrated was accurate @ 30 mph and no more than 3% @ 70 mph only one time checked a car at over 140 and the speedo was reading 142. Not bad at the time.

However, history has failed us.......... Thomas Richfield was bought out by Smith's Industries - Transferred to LUCAS (Ho Ho) and on its demise to TRW Thomas Richfield was sold off to Speedograph a one time accessory manufacturer and wholesaler.

Gone are the days of the backroom genius..............

Hugh