[timo944]

I seem to have an odd situation with my new 968. According to the documentation I have found, at 3000RPM in 6th gear, I should be doing about 75 mph. The speedo shows exactly 70. In 5th gear at the same RPM, the speedo shows almost 60 and in 4th gear speedo shows exactly 50.

 

I did the math - 245/40/17 tires, with the stock gear ratios including a 0.778 6th and 3.778 final drive the speed should be quite a bit higher. Testing with a GPS in both the iphone and Garmin the speedo is wrong.

 

So.... how does this happen? I have a feeling that the speedo is actually correct and there is some kind of different gearing. It would need to be a final drive of about 4.00 to make this happen. Has anybody seen this before. Is there anything different in the gearbox for the M030 option?

 

TIA

Tim

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[Waylander]

Nothing in the in the gearbox on M030 it's only the bouncy bits and brakes that are different


Possibly the frequency converter in the fuse box is playing up, I have seen that before.


The drivers side abs sensor provides the signal for the speedo which is multiplied by the frequency converter then sent to the speedo


Try changing that to see if it solves your problem, another thing to check is that the abs ring on the rear of the drivers side hub is free from crap

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[flash]

the stock size for the rear is 255-40-17 not 245-40.

 

the outside diameter should be just about 25"

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[Waylander]

I'm blind did not notice the size issue

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[timo944]

the stock size for the rear is 255-40-17 not 245-40.

 

the outside diameter should be just about 25"

 

I didn't realize that. However this only changes things by 1 mph. Waylander I will try your suggestion. Thanks!

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[flash]

not sure what outside diameter your tool is using, so i can't speak to how much it changes things.  also, every tire is different to a degree.  that's why the tool on tirerack is so cool.  it actually gives yout he outside diamter of each brand of tire in the size you are looking at.

 

also, keep in mind that worn tires are smaller than new tires.  that will change things a mile or two at that speed.

 

another thing to remember is that speedos are calibrated for the center.  that means that at low and high speeds, they can be off.  for example, in a magazine article from way back, the stock speedo had been shown to read 140, but only be doing about 130.  the last time i checked, they only have to be within 10% to be within tolerance.  that's something that makes me crazy about bmw.  EVERY one of them, regardless of model, reads 2-3mph high at freeway speeds.  that means that when you turn in a lease, or are dealing with warranty, they are nailing you for about 3% on every mile.  that can really add up, and i'm sure it's no accident on the part of bmw.

 

for reference, on a stock 968, at 80mph, in 6th gear, you should be at 3400rpm

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[Bulti]

Citation from Wikipedia:


"Vehicle manufacturers usually calibrate speedometers to read high by an amount equal to the average error, to ensure that their speedometers never indicate a lower speed than the actual speed of the vehicle, to ensure they are not liable for drivers violating speed limits."


The electronic speedo on my Suzuki reads exactly 10% high, at any speed.



Just a thought: maybe they fixed the speedo once with a wrong toothed gear? There are different onces available I believe.

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[Kim]

every Porsche that I have owned, wothout exception, has an indicated speed that is exactly 5 km/hour above the actual speed.  It does not matter if I am driving 30 km/h or 110 km/h - the 5 km/h variance is always there. I have checked against GPS units and against the various digital speed meters that are often placed in school areas, etc.  All other cars that we have had recently (all Fords) have been bang-on.

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[Bulti]

Every car over here is definitely reading a bit high. Maybe it's a legal thing or something...

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[Rap]

My speedometer never matches my data acquisition. It always seems to be lower.

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