Can the speedo be recalibrated?

"Pete" wrote

No.

FloydR

This is a BMW "feature." It has something to do with German law requiring speedometers NEVER be pessimistic, so BMW insures that's not the case. Your error is just about standard for the E39.

R / John

You can put on larger tires to slow the speedo.

Having said that, setting the cruise at 80 and actually doing 76 could save you a ticket or two in your life. Your speedo is 95% accurate though, pretty good in the grand scheme of things.

If you are not the first owner, it is possible that the tires have been changed to a cheaper model that is smaller than what should be on the car. You can bring the speedo to about 98% accurate if you swap a 235/40x17 with a 235/45x17.

Post your tire size and I will tell you a new size that should solve your "problem" the next time you buy tires.

Well, it's a meter with a pointer, and the pointer is usually a taper fit on the pivot. Moving it slightly could make it correct at least at one spot speed.

There must be a way, though. UK police use BMWs and their speedos will be accurate.

"Jeff Strickland" wrote

The tires are 235/45/17.

Going up to 245/45/17 would increase dia. by 1.4%

The thing is, I don't want to increase the sidewall height because steering response will suffer. And I don't want to have to deal with rubbing issues either.

BTW, which axle does the car read the speed from - rear, front, or both?

Thanks, Pete

If you car is like my '94 3 Series, the speed is taken off of the rear diff.

I can't imagine how you would feel a few millimeters of extra sidewall. If your tires are 235/45s, then you can see if you can get 235/50s, and this should just about make your speedo dead-nuts-on. This would be 1" larger in diameter, or one-half inch in radius. You could also go with a 245/45 and get a half inch in diameter and about 3/8 inch in width.

Unless you live where it rains alot, I think I'd go with the 245/45s and take a bit more width and the added traction that it would provide. Your speedo would read about like mine (when it says 80, I'm doing about 77.5), which is pretty good.

"Dave Plowman (News)" wrote

Remember, they're not mechanical any more (other than the sender unit.) Probably a different capacitor/resistor or software.

FloydR

Speedohealer.com

while they started out for motorcycles; the same can apply to cars. I had ordered one for my car a while back -- worked like a charm.

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The actual speedometer still has a pointer which turns, though, unless I'm out of date with new models. And repositioning the pointer on its pivot will obviously work - although it's probably a job for an instrument maker.

That would only work if the correction is linear. If it is 1mph at 10mph and 5mph at 80mph, moving the needle won't work.

Tom

Within reason yes. Which is why I mentioned 'spot speed'. But although the return spring is designed to be fairly linear it will still have a slightly greater effect at some readings than others.

The car's computer uses a conversion number to convert the revs of the rear axle (or of the wheel on newer models) to mph/kmh for the speedo. I believe this can be reprogrammed. They did it this way to allow for different final drive ratios but I guess it might be fixed now if the speed is taken off the wheel sensors.

As for the police I'm pretty sure they'll use their own speedo equipment for the purposes of trackign cars rather than the BMW speedo.

I used to have a document that showed the way to re calibrate Speedo - odometer inaccurate's and change other factors in the ECU. Seems to have disappeared but I think it was on or came from

Sir Hugh of Bognor

The difference between men and boys is the price of their toys. Intelligence is not knowing the answer but knowing where and how to find it!

Hugh Gundersen snipped-for-privacy@h-gee.co.uk Bognor Regis, W.Sussex, England, UK

I have a spare speedometer from my e30 so I thought I'd have a close look at the circuit board to see if there was an adjustment on it. Sure enough there is a small 'trimmer' potentiometer on the circuit board. I can't say for certain that it's function is to calibrate the speed but I can't think of any other reason for it's presence. There is no access to the screw from outside of the cluster so making the adjustment would be somewhat onerous.

If it's the ODBC 'trick' to recalibrate -- I've not run into a single person that says "yes it worked"

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Wouldn't it be easier to find out what the error is at various speeds and then just make up a new number back-plate to sit behind the speedometer needle? Paste it on with glue if necessary. No messing around with the electronics then.

Steve

Feed the output of a variable oscillator to the head?

If you are in the UK there is a company called SPEEDOGRAPH that does just this. Smiths used to own a speedo recalibration company in a mews off Gt Portland Street called "Thomas Richfield & Sons" The company still exists and are now part of Speedograph Ltd (I think)

TR used to make speedos in the OLD days like 1912 ---- 1950s and still repair and recalibrate the antique units. I had several recalibrated when I used to do a bit of "Hot Rodding" -----------

Try them ----------- Speedograph Richfield Ltd

Sir Hugh of Bognor

The difference between men and boys is the price of their toys. Intelligence is not knowing the answer but knowing where and how to find it!

Hugh Gundersen snipped-for-privacy@h-gee.co.uk Bognor Regis, W.Sussex, England, UK