[spuggy]

Originally Posted by toddetch

Also, does this drive the odometer as well as the speedometer?

Both the speedo and the odo are driven from the same signal input. If you correct the speedo signal, you'll also correct inaccuracy for the odo.

I fitted a Yellow Dog in 2012 or thereabouts; built-in period-correct legally-mandated optimism[*] with the extra inaccuracy added by 17" tires was really annoying...

It was especially jarring/noticeable because the PO had adjusted the trim pot on the original speedo so it was dead-on accurate - but I replaced that when it broke the odo gear with 237K on the clock; every other factory speedo read much, much, higher and it was driving me crazy...

Measured the rolling diameter, calculated the conversion factor/set the dip switches, made up a little spade terminal pigtail harness to provide power/intercept the speedo feed and tucked it behind the gauges. Checked it out against GPS, called it good...

I'd actually completely forgotten it was still nestling in the dash just doing it's job, hadn't given it a thought in years/many 1000's of miles...

Question that seems to come up is whether the electronic speedos/senders interchange; I've had everything from the '77, to an SC, 3.2 and a 930 speedo fitted - and went from a 915 to a G50. No difference that I could see.

Also worth mentioning that if your speedo needle is erratic and you've checked everything else, the 100uF cap in the speedo board is known marginal - rated for 16V. If your voltage regulator has ever gone over-voltage.. The speedo will likely eventually fail entirely (mine did), but can behave oddly (like steady reading then drops down & back up a few seconds later) for years first...

The cap can be easily replaced with a 50V part - old thread with bonus Early S Man content: http://forums.pelicanparts.com/porsche-911-technical-forum/247776-carrera-vdo-speedo-sucks.html

[*] The Wikipedia article on speedometer accuracy https://en.wikipedia.org/wiki/Speedometer indicates requirements all over the map - so the factory likely just went with the Euro ones, as they were tightest...

Current EU Type Approval regs state:

The indicated speed must not be more than 110 percent of the true speed plus 4 km/h at specified test speeds. For example, at 80 km/h, the indicated speed must be no more than 92 km/h.

The UK is different:

for all actual speeds between 25 mph and 70 mph (or the vehicles' maximum speed if it is lower than this), the indicated speed must not exceed 110% of the actual speed, plus 6.25 mph.

For example, if the vehicle is actually travelling at 50 mph, the speedometer must not show more than 61.25 mph or less than 50 mph.

The 1997 US Federal standard states (emphasis mine):

maximum 5 mph error at a speed of 50 mph on speedometer readings for commercial vehicles.

But no idea what the laws actually said when these vehicles were made - e.g. Australia apparently had no accuracy requirements at all until they introduced speed cameras in 1988...