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speedometer (reads slow)

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Old 06-13-2005, 03:43 PM
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Pedro Diogo
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Default speedometer (reads slow)

the speedometer in my 81 euro mt 928, indicates aproximately 20% less than the real speed. Any clues?

Last edited by Randy V; 06-13-2005 at 11:52 PM.
Old 06-13-2005, 11:54 PM
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Randy V
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Perhaps tires with a rolling diameter greater than the stock size have been installed?
Old 06-14-2005, 03:32 AM
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Benton
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Mine does the same thing. I've been wondering what is the issue. Tires were stock.
Old 06-14-2005, 07:25 AM
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Pedro Diogo
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Default absolutely stock

The car is absolutely stock. (It´s a concours winner). In top, 5th gear, the speedometer needle can't touch the 200 km/h mark. On the clock, he is a litle over 230 Km/h.There is nothing in the workshop manual, i think, how to solve this problem.
Old 06-14-2005, 08:25 AM
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Garth S
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What is the rpm:Kmph correlation at 'normal' speeds? eg. 2750 rpm @ 100 Kmph.
Compare this to the charts in the owners manual. I think your rear gear ratio is 2.75. A lot of speedos are not reliable on the top end - they just give an optimistic number to make you feel good
Old 06-14-2005, 09:06 AM
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WallyP

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If both the odo mileage readings and the speed readings are always off by a certain percentage, then a possible (but very unlikely) cause could be that one of the magnets is missing from the ring in the differential.

If the tire size is close to stock, a more likely cause is a problem in the instrument itself. Take it out and take it to a local VDO-approved speedometer shop.

You might want to get the famous odo gear replaced WYAIT.
Old 06-15-2005, 12:16 AM
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jpitman2
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Mine reads 10% high - have I picked up some magnets? A scope shows a nice clean waveform.
See another post re yellow box speedo corrector package here www.blackrobotics.com
And BTW, the charts in the owner manual are not worth much in this area - mine shows 100kph at nearly 3k rpm, when it should be ~2300rpm. and yes it has stock wheels and tyres, 2.75:1 rear end etc.

jp 83 Euro S AT 49k
Old 06-15-2005, 01:24 AM
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Mine reads high too, when it reads at all. I have been telling people that it operates like a teen ager. 'Only works when it wants to and when you need em, you can't find em'. I have a new sender on order from 928.gt and if that doesn't fix it I will pull it and send it off. I cannot stand not having a accurate speedometer!
Old 06-15-2005, 01:52 PM
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Don Carter
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This is a long shot, but after I sent my speedo off to have the odometer gear replaced, it read 10mph slow. After discussing it with the speedo shop it became appearant that the needle had been pushed back on the shaft in the wrong position. I haven't yet felt like removing it again to fix it.
Old 06-16-2005, 02:49 PM
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Pedro Diogo
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Thank you all, especially Don.
I think that I can solve the problem by puting the needle a little bit to the right.
I will tell you the result of that.



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