88 325 speedo is slow,..

I used to a HTO but now I drive legally. Since I bought this used car I've noticed more people are tailgating me. Today I passed one of those speed contol things the cops park by the side of the road. It
says, "your speed is 33mph but my speedo said 40. Is there a way to fix this? Thanks again.
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The likely thing is it's had an incorrect - probably secondhand - component fitted at some time. Could be many things depending on model etc. Even major parts like the final drive.
On another make I came across one model where the speedo read very slow. This model was available with a wide choice of engines - from diesel to V-8 through four and six cylinder - all of which used basically the same gearbox where the speedo drive was situated. But no less than four different rear axle ratios. The gearbox had been changed by a gearbox 'specialist' and the speedo drive was the wrong one for the rear axle. To change the drive involved stripping the box as the drive gear is obviously different too.
--
*Out of my mind. Back in five minutes.

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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Fit the correct size wheels and tyres!
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On Wed, 9 Jan 2008 11:00:01 -0000, "R. Mark Clayton"

~~~~~~~~~~~~~~~~~~~~~~
We have a winner... This is the most likely cause. Chances are, the tire circumference is considerably smaller than it should be. This means that the tires are turning faster to go the same speed which will run the speedometer on the high side. It's also possiblr that the instrument cluster has been changed and the wrong coding plug is installed in the new cluster but that is fairly unlikely. Another option is that the rear has been changed but this is fairly unlikely as well.
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Michael Yeager wrote:

You would have to make a dramatic change to your tire and wheel size to make the speedo read well over 15% higher than your actual speed. It's got to be something else.
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True, but I have seen some pretty dramatic tire changes out there.
It's also possible the differential was swapped out for a different ratio one. --scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

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Well maybe, but typically BMW speedos read ~5% high and we are not talking a huge difference here.
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R. Mark Clayton wrote:

If you have a lease, 15% is a lot. My 45,000 mile, three-year lease would start costing me $.20 per mile after 38,250 actual miles. So if I drove 45,000 actual miles I would have to pay $1350 due to the higher odometer reading.
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Fit the correct diameter wheels with the original profile tyres. The speedo is driven from a sensor on top of the diff and the speedo expects pulses from the hall effect transmitter based on the original rolling circumference.
Alternatively, just use GPS/Satnav for a more accurate reading.
Al.
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Or time off the marker posts at the side of a motorway - this is how I found out that my current 735i reads 405% high only two weeks ago.
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Wow, that's some error! So when you are going 50 kph, your speedo reads 202.5? What happens when you hit 60 mph/100 kph? Tom K.
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4 to 5 percent
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Was trying to figure out the typo but it should have been obvious as "-" is next to "0". While 4~5% is annoying, it seems to be pretty much normal these days.
Greatest error I ever saw was a '76 BMW R75 motorcycle which read 125 mph at it's top speed of about 103. IIRC, Road & Track magazine in the 60's & 70's normally published the speedo error at 30 and 60 as part of their road tests, with some of the worst offenders being the Ferraris - as much as 20% high.
Tom K.
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Could have been 4.5% (actually 4.4%, but the margin of error is about 0.3%.
Legal limit is 10% IIRC, and Fords were the ones most often nearest the maximum over.
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Wheels are original, tires are correct. The work receipts are extensive and make no mention of final drive work, which appears to be original. I had problems with dash reminder lights and when I went to replace the dash battery I found it had been replaced already at 60k miles according to the work records. The rechargeable battery had been replaced with a lithium battery. I guess the dealer replaces the whole board assembly at a whopping rate and the replacement is lithium instead of rechargeable. I just soldered in the same battery as was there so maybe it's loosing juice already after 4 months. I wonder if that could give things a false reading. Thanks as always.
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