Adjusting speedo on mech drive after tire change?

[lateapex911]

I've searched, consulted PET, maybe I've missed it...
How can I change the speedo reading?

Here's what I know:
1973 915, 5 speed, stock, should be 7:31.
Tires are currently 225-60-16. Diameter should be 26.6
Speedo drive comes from a mechanical gear in the transmission and speedo reading/ calibration is done there via gear change.

Issue: my engine is out after a fire. The trans speedo cable removal broke the tailcone threaded boss way back when it was removed.
Got a new tail cone, (thanks Mike).

But, I counted the gear teeth to be sure it all matched.. My stock is 18 teeth, the replacement is 19.

I don't know how "off" my speedo was with the new wheel/tire. (My bad, been a decade, don't ask lol)


So, what effect would the 18 to 19 tooth change have?
IS there a chance it's serendipitous and actually matches the tires? (pffft)
Or makes the situation worse?

I imagine there are ways to convert a mech drive to an electronic drive and just install a electronic speedo from a 75.5 or up.
BUT I'd prefer to not do that because of the mileage difference on the odo. Maybe I could adjust an old odo?

If I can determine the right teeth number to have with a 26.6 tire diameter, (and it's not 19), how does the old one get swapped? Looks like it's press fit?


It would be nice to have a unit that could convert the speedo at will to match different tire sizes, but it's certainly nothing vital.

I can't seem to find the actual part (gears) other than a plastic 17 tooth gear* in the Pelican or Stoddard catalog, nor in the PET.

*Plastic? Pelican says it fits my car, I've got doubts, the stock one sits in the case, looks like steel.

I can just put it back together and see what happens, but since it's open and a[art, now might be the wise time to get it done.

Lots of questions, sorry. I thought this would be one of those things that's been done a hundred times.

[gled49]

Switch the both gears from your old shift cover, as new cover was for 75 8:31 R&P. You won’t get accurate speedo reading until you go back to 185/70/15

[lateapex911]

OK, good info, so thats helpful, thanks. By "Both", not sure what you mean?

[Matt Monson]

Originally Posted by lateapex911

OK, good info, so thats helpful, thanks. By "Both", not sure what you mean?

You don’t need to change both, you just need to keep them matching. Take the gear out of the tailcone and replace with your original. They are matched pairs.

[gled49]

The pair of gears I refer to, are the pinion that the cable hooks up to and the cross shaft with two gears. They have different tooth count between 7:31 & 8:31.

[Matt Monson]

Both 27 and 31 each have two options in the PET. I see what you are saying now. I was previously thinking of the gear on the end of his pinion, and the first gear on 27. I agree with the comment that you have to change them both, 27 as a complete assembly and 31.

[lateapex911]

OK, that makes sense!
It seems like the best number would be a 17, but finding that seems impossible.
The swap seems the smart play barring finding a 17. Who am I kidding , just getting this thing back on the road is the goal, I can fuss with the speedo later. This aint no Singer haha.

Thanks guys!

[Walt Fricke]

Jake
You can't install the speed sensor for a Porsche electronic speedometer in your transmission case. The case casting doesn't have the recess for it (I'm pretty sure), and for sure the differential doesn't have the 8 magnet disk.

You've got to disassemble the nose piece to get at those gears, don't you? Not a big deal if you have the tranny out at this point.

You could concoct (perhaps with some help) a way of getting a suitable signal to a later electronic speedometer. I'd start thinking of Hall or other sensors reading the bolts on a CV joint, and some circuitry to convert six per rev to eight to send to the speedo.

To drive around, maybe just get a nice GPS speed display? Maybe a system which will read miles traveled also?

But you want your stock mechanical speedo with its original ODO reading? Why? Resale value?

Of course, if you can get the mechanical system working, neither the speedo nor the odo will be accurate unless you run the old stock rear tire diameter. I suppose owners who drive the cars of this era just mentally adjust the speed and miles they see to account for the (usually shorter?) tires?

With yet more effort, maybe using an Arduino or the like, you might able to rig up a system where a small motor turns a cable which turns the speedo/odo? Then you could adjust with software to tire diameter.

[blucille]

In the past, I’ve been able to use a device that converted GPS speed sensing to be compatible with either a mechanical or electronic speedo. This would fix your problem without opening up your gearbox. I believe I got mine thru summit racing back in the day but it looks like even Amazon has a variety to choose from

[PeteKz]

This sounds like smashing a fly with a sledgehammer. Either go GPS speedo or just apply the correction factor between your ears. John Bell offers a GPS speedo that looks like the regular speedo, I think.

[lateapex911]

All good info. Glad to see there is no easy button!
I’ve swapped the speedo gears in the tailcone. As Walt points out, there’s a lot of work to go into the conversion inside the transmission. So I’m just gonna deal with getting it close for now and maybe we can examine it later but this won’t keep me from installing the transmission on the engine and the engine in the car which is the number one goal right now. Anything down the road is gonna be done outside the transmission anyway so I’m not hurting myself. With the way, we as 9/11 owners switch tires from the originals to whatever we have now it’s interesting how often this problem must come up.

[Walt Fricke]

I remember trying to time between Interstate mile markers holding a steady 60 mph speed, and also seeing how the odometer correlated with 10 or so mile markers so I had an idea of how much slower/fewer miles the speedo/odo was reading. Much easier now with GPS etc. Pete is right - not too hard to get by fine guestimating.

I still do that with speed in my daily driver Tiguan - it reads a couple of mph faster than actual.

[PeteKz]

Walt, you do that too huh? I measure and time between interstate mile posts often, especially if I'm bored.

[eastbay]

Just another data point.

I am running 255/40/17s on 17x9 wheels on a stock 1974 transmission with a stock cable driven speedometer and the speedo is dead nuts on, verified by GPS

I did not expect the speedo to be too close, but I'll take the win.

[PeteKz]

Lucky you. I'm about 6.5% fast on my 1973 with the tires and wheels listed on my signature line. The original tires and wheels were about 24.7" diameter. What diameter are yours? I'll bet around 26". That would just about correct the error.

Interestingly, my odo error is about half the speedo error, about 3% high. Go figure...

[Walt Fricke]

As far as I know, the odo is just a geared counter. Nothing to tweak or adjust where it is all just gears which update the ODO reading.

VW beetle cable driven speedo/odos have a spinning magnet drag a ferrous cylindrical cup against a spiral spring. The faster the magnet rotates, the more magnetic drag, the higher the pointer goes. Something like that. The spring is adjustable, at least in theory, though perhaps only the 0 mph point?
I don't know if the Porsche cable speedos are fancier than that or not. The electronic ones sure are fancier.

[PeteKz]

The Porsche cable operated speedos work the same way. They are called Hall effect meters, because that magnet spinning around inside the cup creates an electromagnetic coupling that twists the cup (and attached needle) against the spring. The cup is aluminum, IIRC. John Bell would know for sure.

[Walt Fricke]

All I recall from 1990 is that the cup part was a silver color. Aluminum is not magnetic.

I don't think the Hall effect is involved, though.

[PeteKz]

Walt, it is aluminum, and it is an electromagnetic coupling, not a "magnetic" coupling. it's similar to the electromagnetic coupling between copper wires in a motor. Copper isn't magnetic either. These are also called "eddy current" speedometers.

For a full description see this:

https://www.youtube.com/watch?v=DssASTqXqK8

If you really want to geek out, see this:

https://en.wikipedia.org/wiki/Hall_effect

[Walt Fricke]

Eddy current sounds intuitively more like drag on the needle's cup, which is all a guy like me needs to know about how that kind of speedometer works. Perhaps eddy current operation uses the Hall effect, even though dedicated Hall effect sensors were not in common use until much later than these 20s or earlier speedometer technologies? Like the SC distributor uses a Hall effect sensor rather than points?

Anyway, short of using a different hair spring, looks like you can't adjust this kind of speedometer for tire (or differential) size changes. Different size little magnet? A clever mechanical interface between the cable and the speedometer with its own gears to decrease (tires usually smaller diameter) the rotation speed?

For rallies you could get an interface for a mechanical speedometer (like the one on my VW bug for the La Carrera) which sent a signal to an electronic speedo/odo. You set a bunch of little switches to get the conversion ratio you needed for the add on device (mounted separately so the navigator could use it). A lot of work for accurate mileage. Much of a market for such devices these days?