Technical Using a Speedometer as a Tachometer?

Sure; lots of ways to skin a cat (though don't tell @alanp561 that!) — even if the idea is to have something unique and appropriate to the build, and have some fun at the same time. Another way is to take the movement out of a little 3¼" or even 2" tach and mount it to a larger face, behind a glass and bezel from a clockmaker's supplier, e.g.:

Flat faces are easy. Laser-cut out of thin metal plate, and either have a decal printed for the entire face, or paint the face and find a place with a flat-bed printer to print directly onto the face. A longer pointer could also be laser-cut, but you'd need to take care that it's balanced. If you really want to be fancy you could rig a conical rim reflector for internal rim lighting.

Another source of bits and pieces is stainless steel kitchenware. There's a Ø11" x 4½" stainless steel mixing bowl here on my right, which came out of some stuff of mine that was at my parents' house. I'd bought it years ago to repurpose as something, but I can't for the life of me remember what.

 

Falls under the header of finding difficult ways to perform simple tasks ,usually best left as a mental exercise .

 

5" pet-food bowl, about US$2:

Cut about ¼" from the top, then paint the top bit the same colour as the face on the inside, and bright white on the outside. Find a way to suspend the face so there's about a ⅛" gap between the ring and the face, and a somewhat bigger gap between the edge of the face and the sides of whatever you're using as a housing. Mount a few light bulbs behind; wire to come on with the side/parking lights.

 

It's been two years, and I'm only now beginning to realise how tired I'd been. My mum died in October '21; my mother-in-law died in March '22; and my dad died in August '22. That, and all it entailed, exhausted me so that I'm only now beginning to feel like I'm coming alive again.

And I've been feeling guilty because, being a true gentleman, @Budget36 had an optical tachometer delivered to me when he heard that I was strapped for cash. Thanks again, Mike! Hopefully I'll get to doing something with it soon, only two years late ...

I scored a '60s/70s VW Bug speedometer on Saturday at one of our annual swap meets, for about US$13: a good candidate for the experiment. It proved upon inspection back home to be encrusted with overspray, so thick in places I could chip it off with a fingernail. First job was seeing if I could clean up the bezel enough to see what I've got. I was hoping that I'd been looking at polishable metal, but it turned out to be silver paint. I'll see if I can get it plated; otherwise it'll have to be painted black. Next, subvert the OEMs' penchant for sealing things permanently by prying the bezel off.

Not an ideal detail. The trick seems to be to be patient and pry up a little at a time, rather going round the circumference a few times than once. Pry up square, and dont twist the screwdriver, or this happens:

Minor damage: it'll be hidden behind the panel once the bezel gets tapped home eventually.

The bezel came off without too much deformation:

The glass appeared to be held in a complex gasket which seemed to be splitting and coming apart ...

but it turned out to be three separate pieces that I'll probably be able to clean up and re-use:

The lighting reflector ring needs a coat of paint:

The back of the ring could probably benefit from a coat of white paint:

The glass was sound and cleaned up readily:

Here's what we're looking at:

The turn indicator repeater lens was completely perished. Note the little white mark on the edge of the face, directly below the generator light:

That is a calibration mark for the needle. Gently lift the needle over the stop, and it should point at the mark:

Here's the back of the case. Someone had done a rough-and-ready respray with the speedometer in-situ:

Extracting the fuel gauge:

It only needs a bit of a clean:
Now the rest of the innards can be pulled out of the case:


The intention with this stripdown isn't to be at all VW-specific, but rather to see what kind of unforeseen details can turn up when tearing into a speedometer with which you don't have prior experience. It's all experimental; it might be destructive, and it has to be cheap, which is why I didn't start with a '50s American truck speedometer. Here is a very VW peculiarity, in that the light lenses sit in soft plastic caps which simply slide over the bulb holders. Not all speedometers will have that:


The lenses aren't in very good shape, but it should be easy enough to replace them with bits of plastic in whatever colours you want. I was able to pry one lens out and, though it seemed quite brittle, to get it back into the holder without breaking it:

I was ready to try the spoons trick next. Speedometer needles are a push fit onto their spindles, and the way to get them off is to wedge two spoons on opposite sides under the central hub, and gently pry the needle off that way. It wasn't necessary — I could pull the needle off by hand:


Right. I've always reckoned the jobs I'm better at are the fine ones, preferably ones which require use of a loup. This isn't quite down to that scale, but at last I am at a point where I can break out the precision screwdriver set and that stuff:
The dial pan will need a light clean. The pan is surprisingly flat, apart from the rim, useful for any creative patchwork and/or printing that might be needed. I have no idea what the figure "0002" on the rim means. VDO do not appear to condescend to reveal turns-per-mile. I'll probably be reconfiguring the lights, changing the shape of the turn-indicator light window, and closing up the odometer windows. I have yet to figure out how.
At last, the magnet bearing assembly is revealed. The angled ring magnet is a surprise: I didn't know what to expect behind the angled cable attachment on the back of the case.

The magnet and its shaft weren't hard to extract. I tested the magnet's polarity with a fridge magnet. It has four poles arranged at 90° to each other, obviously N-S-N-S. I must confess that I was expecting a simple bar magnet. I wonder how common ring magnets are in speedometers.
Everything pertaining to the odometer was removed, as it would be surplus to requirements. I could simply have left the odometer in, but I'm not sure that I won't want to do something with the space it occupies, and the whole point of the exercise is to see what can be done. The odometer came out with a bit more violence than I'd have liked, but no harm done:
And now I shall place it one one side for the moment. I hope I don't take another two years to get to the next stage.

 

The odometer reading is purely mechanical, so I can calculate turns per mile back from gearing. The odometer drum is driven from the magnet shaft via two nylon intermediate shafts, each worm-driven to the next.

1 start on the magnet shaft : 10 teeth on the first intermediate
1 start on the first intermediate : 10 teeth on the second intermediate
1 start on the second intermediate : 16 teeth on the small brass gear on the odometer drum

The odometer reads 1/10 miles, so one rotation of the white drum = 1 mile. So:

10 x 10 x 16 = 1600 turns per mile.

If I want to use the magnet that's in there, assuming it to be good, I'd need an overall reduction drive of 3.75:1. I tested the magnet on a scale, and it seems to be good for lifting ±6½lbs on its flat face. Of course I have no idea what it ought to lift.

Taking drive from a tach-drive distributor would require a 1.875: reduction drive. Taking it from the back of the alternator, assuming there is an opportunity, might require over a 9:1 reduction drive.

 

Hey, Dawie;
This'll be interesting, to see what you come up with for a drive-conversion. .
Have long wondered about the feasibility of using a speedo as a tach.
Marcus...

 

It will be interesting to see what I come up with! Thus far I'm going around in circles.

One thing I've thought of, and that's why the ring magnet was a surprise, was to use a small electromagnet instead of a permanent magnet, wired via a voltage stabiliser, a potentiometer, and a pair of slip rings. I'm having trouble identifying a suitable electromagnet, though.

Another question is whether how a speedometer cable will hold up to spurts approaching 20000rpm.

 

This is interesting: https://hackaday.com/2023/07/01/a-cvt-for-every-application/


As we're dealing with tiny torque at high speeds, a miniature CVT might be worth considering.

 

@Ned Ludd This should interest you. [instead of the friction style CVT's]

 

That is extremely interesting! Not sure how it'd miniaturise, though. I'm all for clockworkishery, but I don't really need continuous variability, just the facility for calibrating to a non-integer ratio. And friction CVTs can be stupidly simple, especially if the required range of adjustment is small.

 

Easier to just buy a GPS speedometer! Just have to hide it.

 

Sure. But deciding not to solve a problem isn't a solution to the problem! It misses the point.

 

Well, with speedometers, if for instance you have a quick change…you will always have problems.
The only real reason to have a “working” speedometer on a Hot Rod is to avoid speeding tickets…might as well have one that is spot on.

 

My Dad's 1941 ford 2 ton straight truck had a graph on the speedometer that showed the maximum speed in each gear

 

Hey, Dawie;
"Thus far I'm going around in circles."
. As do a lot of speedometers & tachs --- .
See, you're already well into the program. .

My concern, initially, was how to reverse the operation, so's to mirror the speedo.(For either the long thin rectangular types, or the half-circle versions - so's they'd make a full circle, either as top/bottom halves, or a side-by-side version. Was always planning on re-facing the #-plate anyways, since a 120mph -> a 6000rpm would give a nice long sweep.). Then how will it stand up to relatively rapid acceleration(s) & de-accelerations, (if it even can be somewhat accurate???)continuously? I figured trans-speedo-gears-combos, speedo-change-gears, & maybe the right-angle versions might make a useable, but complicated, drive-speed-alteration. Driving it could be done from the crank, the cam, or a few accessories, but none are easy, but then neither is altering the distributor, when you don't have a cable-drive version already. I suppose a small thin belt(V or flat/toothed) would work also, since you could make any ratio you desire. There used to be a small thin belt used on the old catalyst AIR-pumps that never wore out, & also I suppose, thin belts from sewing machines & the like. Or rubber-drive from one wheel to another via o-ring or somesuch, idk - maybe even H.O. drive gears(longevity ?), since there isn't much force on a speedo. Possibly similar to a snowblower-drive wheel-n-platten, but miniaturized. Or even the old 60's-version of a bicycle speedo driven from the tire sidewall. Ideas got outta-hand, fast, so they stopped. I'm guessing the cable is the biggest source of friction & drag. There was a small geared mechanical/electrical sender available on a Camaro, for ~ 1 yr, that converted trans-speed into an electrical signal, but that doesn't help those of us wanting to use the older speedos/etc. ;( . The early acvw speedo-drive from the lf wheel isn't a bad idea, & could be adapted - or reconfigured - into an engine-mounted rpm-drive, I suppose. Amusingly enough, the early teens+ Studebakers(& others, I assume), used a front-spindle geared open-drive for a speedo cable. But was a kinda monstrosity of a thing. Cool & hairy looking, but don't get your body-parts or clothing near it if it's moving... . Not converting that to a rpm-drive, unless I was going full SteamPunk. Shades of Wild, Wild West... .

But I never got too far in the disassembly of components, much less empirical testing(read: learning how to build one from scratch). Way too many other things in life keep requiring my attention, energy & time. I also considered that maybe the cabledrive wasn't actually needed, but never contemplated what would reasonably replace it, - since I wanted to stay w/the oem-type-guts.
As usual, you are soooo far ahead of me, I'm not seeing your dust... , not even your footprints in the dirt road anymore... . So, I'm following where & as, I can... .
Marcus...

 

I am into old Mallory "flattop" distributors, and usually buy them at rock bottom prices when they come up. SBC distributors are particularly desirable as they can easily be converted for use in 8BA flatheads. The last one I got had a mechanical tach drive on it (I guess so they could be used on Corvettes), which got me thinking. A quick trip to eBay yielded a guy who had 9 NOS Stewart Warner mechanical tachs for $60 each. On a whim, I bought one and it turned out to be a very nice unit. The only drawback is that it is a 5000 tach, but that suits my uses just fine.

Just shows what's out there if you look. I'll probably have to have a cable made, but that's never been a problem.

 

I just showed you that out of a matter of curiosity.
The speedo to tach project in my opinion is an exercise in futility. [or creating a problem for the sake of it]

A simple solution would be a 0-8000 rpm mechanical tach from a Yamaha DS7 [250 twin] pull the face off and place a strip of black vinyl over the Yamaha logo.

A dirt cheap solution [I saw one at a swap meet last weekend for NZD $50 and he was negotiable]
That is < USD $30




It has a 90° drive off the back



Some tractor engines have a mechanical tach drive off the alternator/ generator. You can adjust the pulley ratios

 

Marcus, you've just described all my conjectures of the past day or two!

Another factor: the engine and everything belt-driven on it turns counterclockwise when seen from the firewall side. We want a clockwise input for a clockwise tachometer. So whatever the drive does will need to incorporate some kind of reverser. In my case, driving from the crank or anything that turns at crank speed will need a reduction gear of 15:4. A worm drive would have the advantage of turning the drive 90° in a very small space, but a 4-start worm starts to get very specific. I suppose it could be cut on a lathe. And the 15-tooth wheel could be tiny. The nice thing about a worm drive is that the diameter of the worm isn't an issue: it can be big enough to act as a spacer under the cooling fan, say, without requiring a wheel the size of a manhole cover. Hmmm, let me see.

The electromagnet option is proving recalcitrant. It'd have been a lot simpler if the speedometer cable had been attached to the aluminium cup and the magnet to the watch spring and needle. Eddy currents work on relative motion: it doesn't matter if it's the magnet or the cup that moves. Then the magnet could be replaced with a bar electromagnet with two longish, flexible wires because it only rotates over the sweep of the needle. The angled ring magnet in the VW speedometer messed that up a bit.

In any event, finding somewhere on the engine to take drive from — barring happening to find a tach-drive distributor for the weird engine you (might) choose to use — is already proving tricky, the drive ratio notwithstanding, whether the electromagnet thing is done or not. Best confine the trickiness to one place.

 

It's a bit of fun exploration but, if a broadly applicable solution to this could be found, it could open up lots of non-futile possibilities. Once the nut is cracked, once this particular wheel is invented, doing other applications would be simple.

What is the diameter of that? I'd leave the Yamaha text: they make musical instruments, so why not gauges?

 

I like the endeavor. Not everything done needs to be practical, but sometimes “just to work it out” -as long as you don’t go broke trying- is 100% of the exercise.

 

I've seen, but sadly don't own, tach drives from the crankshaft pulley-bolt; for both a Ford 4-banger n v8 flattie, an acvw flat4, & one other I've long forgotten. I was thinking iffen a fuel pump could be driven off a cam, why not a cable-drive hex? Either straight out or a tight 90* w/a decent seal on the cam timing cover. I've seen dist drives off of dohc, & drives off of generator-backs. I thought maybe some of the gearing could be done there, & another portion done at the back of, or near the back of the speedo-now-tach. Idea was to do most speed-change right at x-speedo, instead of running cable fast(er). None of this was "needed-like-right-yesterday", so's it just fermented in the gray matter over the decades past. Still a passing interest, although I always suspected it was tilting at windmills, & an exercise in futility. But an interesting one, at least for me. . One of the problems/mistakes(?) I originally made, was I ass-umed that the speedo could be used as-is, except for re-numbering, & probably a (small) gearing change. I'm finding out I was wildly wrong. Still... .

I'm sure you will beat me to the conclusion "= not reasonably possible", but I also don't know what'd it'd take to redo the speedo input(just after the cable-drive-connection) to make it a tach. Which is what you're chasing - yes??? I'd now like to see the guts of a cable-drive tach. I actually didn't think there was that much difference... ;( .
Marcus...

 

The obvious solution to all this is surely to gut the case and install a new electronic tach movement instead. I have no real problem with an electronic tach, despite my obsession with doing the things we've grown accustomed to seeing done electrically by absolutely any other means. The only reason I shied away from this was the likelihood of a mismatch between the sweep of our chosen speedometer and that of available tachs, especially if the idea is to reuse as much of the original dial graphics as possible.

It shouldn't be that hard to rig a miniature belt drive between the new tach movement and the old speedometer spindle to correct the mismatch, I thought early this morning while unable to sleep. That is why I am more than almost disappointed to find that the VW speedometer very nearly matches the 270° sweep of this ugly little 52mm (2 1/16") tach:

Probably entirely close enough to be practical if we wiggle it a bit anticlockwise and split the difference:


This 52mm tach is about a third of the price of the other one, but its 8000rpm sweep is 240°. A 1.14:1 belt drive would be needed: Ø27.4mm and Ø24.0mm pulleys, which could be 3D-printed?

The more expensive one seems worth it.

Of course I've got no idea what the tach movement would look like once removed from its case. What are the chances of it clearing that flat shelf below the fuel gauge?

Otherwise the entire tach case would have to go behind the speedometer case, with an extended spindle through that little hole in the middle. I'd like to retain the speedometer's lighting, indicator lights, and fuel gauge.

Oil pressure senders are available which match the resistance range of the VW fuel gauge sender, i.e. 73Ω - 10Ω, which would allow me to use the fuel gauge as an oil pressure gauge. It'd even read the right way round, low to left and high to right. I'd need to determine the scale and print a sticker for the face.

 

I'm not sure a meter movement is strong enough to handle a belt drive. It's built to handle a needle moving with almost no resistance.

 

Hey, Nedd;
True, that is the proper way to do it, at least somewhat cheaply.
My motivation was generated by wanting to use oem-type speedos that were/are not round. I get that the sweep is what it is, & could/should be mod'ed as needed. Maybe the needle length doesn't matter for a conversion swap. ???(Seems a lot of these are 90* or 180* sweeps). What started this off for me, was the ~'50 Stude Champ speedo, the ~57 Stude Champ(sedan) vertical-barrel speedo, & a few others(maybe even the ~'67 Olds Tornado horizontal barrel speedo), that weren't tach-like.(Say, could you imagine a ~'62-ish Chrys Electroluminescence tach??? ). Of course, Stude used a lot of the usual round 270*+ tach-like faces too, & even real aftermarket gauges in the '55 Speedster. Part of the exercise for me was, in theory(& it seems to be staying in that category... ), a 100mph speedo would get a nice wide 5k sweep, a 120mph could = a wide 6k, & a 140moh could be a wide 7k sweep.(Don't really need/want more than 7k rpm) & could mirror the speedo. I'm thinking a whole lot more thinnin' needs doin' yet. .
Marcus...

 

It is not that difficult these days to re-print the dial with a computer onto self adhesive vinyl.

20 years ago I wanted an 0-8000 rpm tach for my Lotus Cortina [factory dash pod]
The "Smiths" Cortina GT tach went to 0-7000 [red line at 6000]

Even the rare "Smiths" Lotus Cortina 0-8000rpm Tach redlined at 6500 which is no good for a racer.

Eventually I used a "Smiths" Tach from a V6 Zodiac Mk4 which had the same sweep but went to 0-6000 rpm.
I printed off a new face that read 0-9000 rpm for exactly the same sweep [this new face had exactly the same style numerals that were spaced closer]

A 4cyl at 9000 has the same amount of trigger pulses as a V6 at 6000 rpm.
When installed it looked OEM

I still have a few spares laying around in my ceiling somewhere

 

I've gone down that rabbit-hole too!

My favourite was the barrel seen through a slit, so that a bit of the triangularish colour field you see looks like a ribbon with an angled tip, which gets longer as the barrel rotates. They were found in Oldsmobiles, Buicks, Volvos, Saabs, iirc. and some others.

Volvo Amazon:

 

Hmmm. Two 3D-printed sheathes, fishing line, minimal tension?

 

Maybe. But I wouldn't get my hopes up too high.
If you are going to make more or less complicated solutions perhaps it's worth looking at something based on a stepper motor instead? They can rotate freely in a number of steps per revolution, so the controller decides how big the sweep will be.

 

I'm trying to design-out code, so a controller is out. As far as I know electronic tachs are simple analogue electronics, so they get a grudging pass.

@Kerrynzl's suggestion of reprinting the face — not that I hadn't thought of it before he mentioned it — is better. Then all I'd be using of the speedometer would be the case, bezel, glass, lighting, indicator lights, needle?, fuel gauge (as an oil pressure gauge), and the metal of the dial pan.

 

Fair enough. Yes, older tachos will indeed be more or less basic analog electronics. For a modern one I'm not sure, micro computers have become so cheap and universally used to solve just about any problem I wouldn't be surprised if they have started taking over most simple electronic jobs.

Reprinting a meter face would be a great solution when you can find a suitable meter with the wrong look. You could even take the guts out of that meter and put it in an old one if you have to to get the right case, glass etc.