Mechanical Speedometer Calibration

[Artfrombama]

Speedometer is fast, at 70 MPH, speedometer is reading 80. Would prefer the gauge to be a little more accurate, not necessarily for gauging my speed but for the extra miles that are accumulating.
Did a search, found loads of info on electronic units but nothing on the early mechanical gauges.

[blucille]

calibration of a mechanical speedometer typically is not something DIY-able.....it involves a magnetic flywheel type device and the calibration is to make the guts more or less magnetically charged in order to raise or lower the needle placement.....

Are you running the correct wheel and tire combo?

[Jeff Higgins]

Have you verified that it is actually clocking mileage incorrectly? It's easy to use the mileposts on the freeway to check this. Sometimes it's a simple matter of the needle being clocked wrong on its shaft. It's easy to just pop it off and re-clock it.

There are calibration marks on the speedometer face that allow you to do this. Take the bezel off and lift the needle up over its stop pin, and it should swing down and come to rest somewhere in these calibration marks. Pull the needle off its post and move it counter clockwise a few notches, corresponding to 10 mph.

[Artfrombama]

Problem may be that I have a gauge from a 74-78 (911 641 502 29) .

[Trackrash]

Jeff has the method. If your speedo IS mechanical, there is a clock spring that resists the movement of the needle in the gauge. I imagine an adjustment of the pre-load on the spring, should help. The hardest part is removing the bezel....

[Artfrombama]

Originally Posted by Jeff Higgins

Have you verified that it is actually clocking mileage incorrectly? It's easy to use the mileposts on the freeway to check this. Sometimes it's a simple matter of the needle being clocked wrong on its shaft. It's easy to just pop it off and re-clock it.

There are calibration marks on the speedometer face that allow you to do this. Take the bezel off and lift the needle up over its stop pin, and it should swing down and come to rest somewhere in these calibration marks. Pull the needle off its post and move it counter clockwise a few notches, corresponding to 10 mph.

Originally Posted by Trackrash

Jeff has the method. If your speedo IS mechanical, there is a clock spring that resists the movement of the needle in the gauge. I imagine an adjustment of the pre-load on the spring, should help. The hardest part is removing the bezel....

Yes, mechanical. Has cable.
Discovered previous owner swapped in a 74-78 speedometer
so will try to find (trade anyone?) the correct one for my car. Should be a 150mph 911 641 502 00

[Trackrash]

There should be a date stamped on the back of the case. I don't imagine there is any difference in gear ratios among the mechanical types. Do your other gauges have the silver bullet?

[Artfrombama]

Originally Posted by Trackrash

There should be a date stamped on the back of the case. I don't imagine there is any difference in gear ratios among the mechanical types. Do your other gauges have the silver bullet?

721
6.88

All the other gauges kosher

[Jeff Higgins]

I believe that the differences in transmission gearing (early 7:31 vs. later 8:31) are accounted for with different speedometer drive gears at the transmission. The gauge itself is mechanically the same, and interchangeable through the years in which they were mechanically driven. If anyone can confirm this, we can clear up or eliminate one possible cause of the error.

[Trackrash]

Didn't Porsche go to the electric speedo when they changed to the 8:31?

But then again, there was more than one 5th gear ratio with the 7:31 units, so you bring up a good point.

[Jeff Higgins]

Originally Posted by Trackrash

Didn't Porsche go to the electric speedo when they changed to the 8:31?

Good question. My son's SC is electronic. It has, of course, the 8:31 ring and pinion.

I'm not at all familiar with mid year cars. I believe they were all mechanical speedometer, but I could be wrong. Our OP has has a speedometer from one that certainly is. Did they switch to 8:31 prior to the SC?

Originally Posted by Trackrash

But then again, there was more than one 5th gear ratio with the 7:31 units, so you bring up a good point.

Hmm... you had me for a minute here, then more than my usual state of confusion set in... That would make sense, until you stop to think that the speedometer is accurate in every gear, not just fifth. It's been forever since I've had a tranny open, but I think the speedometer drive gear must be driven by the main shaft to which the pinion is mounted, thereby negating the affect of being in different gears. Am I thinking that through properly?

[Trackrash]

"'75 was the last year of the mechanical speedos" according to Pete Z.

OK, taken from Pete Z's Wiki. http://porsche.wikidot.com/how-to:915-repair-tutorial-part-ii

" 1972-75 models used a mechanical drive for the car’s speedometer, replaced with an electronic unit in 1976.

1972-74 versions were the only 915s fitted with a 7:31 ring and pinion ratio. "

So it looks like the "75 would have a mechanical speedo with the 8:31 fd.

Yes, after my cup of coffee, true the gears don't make any difference, except of course the final drive.

[Flat Six]

Originally Posted by Jeff Higgins

Sometimes it's a simple matter of the needle being clocked wrong on its shaft. It's easy to just pop it off and re-clock it.

+1
Did this when I replaced odo gear