Technical Using a Speedometer as a Tachometer?

I don't know if it takes a particular kind of weird brain, but I've discovered a fascination for instrumentation. I've spent pretty much all of 2023 thus far down a gauges rabbit-hole, and one thing which came out of a side tunnel down there was the similarity between magnetic/eddy-current speedometers and tachometers. And that got me to thinking that a mechanical speedometer can be used as a mechanical tach. It's just a matter of drive ratios.

Why? Aftermarket electronic tachs are freely available, easy to hook up, and relatively cheap. The problem is the choice between an affordable 3¼" gauge which isn't too ugly and an affordable 5" instrument right out of the Fast and the Furious, with zippy graphics and disco lights all over it. Big + pretty costs money. At the same time there is a robust supply of old mechanical speedometers which are either not working because we've been told that they are something which can't be fixed without lots of money or a doctorate in clockmaking, or unsuited to the purpose at hand because they only read up to 90mph or less.

Tach-drive distributors generally turn a tachometer cable at half the engine speed. At an input of 3000rpm, the tach will read 6000rpm, i.e. "60 RPM x 100" — or it should, if it's calibrated properly, and that can apparently be done at home if you know how. With some exceptions, mechanical speedometers are calibrated to an input of 1000 turns per mile, or 1000 turns per kilometre. At an input of 1000rpm, the speedometer will read 1 mile (or kilometre) per minute, or 60mph. Adding a reduction gear of 3:1 would allow a speedometer to read "60" at an input of 3000rpm, and so function as a tachometer. All you need to do is change "MPH" on the dial to "RPM x 100".

So, we'd want a speedometer from a slower old car, because unless we're running a motorbike engine we wouldn't need a 15000rpm tach. Trucks and small-capacity motorbikes are other possible sources, but motorbike speedometers of that kind might not be any bigger than the nice 80mm chrome-bezel tach you can perfectly well afford. We'd want to delete the odometer, which may or may not be simple. We might gain all kinds of handy inset gauges and warning lights, which we may or may not want to put to other purposes.

I had a go in Gimp:

I used a pic of a 5" Smiths speedometer of the kind found in early Minis and mid-'60s Morris Minors. Like a lot of Smiths speedometers it takes an odd TPM input, in this case 1408TPM rather than 1000TPM. That means that we'd need a 2.13:1 reduction gear rather than a 3:1. The indicator repeater lights in the face have been retained, as they'd be handy in a prominent tach, but the ignition and high-beam lights have been repurposed as a shift light and a knock-sensor light, respectively, driven by little homebuilt stand-alone circuits.

Fun fact: Smiths fuel and temperature gauges are internally identical, so the one can be used as the other just by changing the faces. Both are effectively voltmeters. Here, the original fuel gauge becomes a temperature gauge. I wonder if this is true of fuel and temperature gauges from other manufacturers.

Now: how about an easy way to do the reduction gear?

 

Or perhaps use the odometer as an hour meter?

 

And if you hook it up backwards, can you go back in time???

.bjb

 

Nascar drivers use tachometers as speedometers as not to break the pit road speed limit.

 

I believe you are correct because that is how mechanical tachometers work. However, won't a tach drive distributor cost more than an electric tach? I was able to get a new, still in the box, Sun Super Tach off EBAY for around $50.00. I think it looks pretty good.

 

When logic and proportion
Have fallen sloppy dead
And the White Knight is talking backwards
And the Red Queen's off with her head
Remember what the dormouse said…

 

I saw a machinists' tachometer aboard ship in the early '80's; it was archaic looking (even had a sweet oak case it was kept in) and had to read with a stopwatch. It was, however, the only accurate way to read feed speed on the lathes and mills. Yeah, I know. Totally off topic. Cool nonetheless.

 

I can think of worse earworms to have: thanks!

 

Here is a 3:1 planetary reduction box for US$17:

https://www.injora.com/products/met...tary-gearbox-transmission-box-for-axial-scx10
I reckoned RC model stuff might be a useful source.

 

If you used a belt driven cable you could tailor the drive ratio to whatever you desired.

This would negate the need for a tach drive distributer.

I know early MG's and Austin Healy's drove the tach of the rear of the generator.

 

I can't quite decide if it's a surprisingly simple solution, or if it's an unnecessarily complicated one. Either way I like it.

 

It's limiting gratuitous complication to only that which I like

 

Not just NASCAR drivers. I have had more cars with tachs and no speedo that speedo and no tach in my life.

 

The idea appeals. To get the rotation right you'd want to drive the cable off the outside/back of the fan belt. I'm seeing a sort of rough-surfaced wheel pressing against the belt. But then, to get the requisite reduction the wheel would have to be six times the diameter of the crank pulley! If the crank pulley is Ø6½", the wheel would need to be Ø39" — about a metre!

So a secondary reduction would need to be part of any arrangement using the fan belt, which would certainly be an option if the engine you stumble upon doesn't happen to have a tach-drive distributor on it already.

 

I like Ned’s posts.
I’m in a box, Ned is 3 miles away.

 

American speedometers use standard, 1000rpm input will read 60mph. Could probably use speedometer reduction gear boxs [I think I seen one that was adjustable but don't know the ratio spread] in combination to suit needed reduction.

 

Back in the 60's, Thunderbirds had a washer pump that was driven by a small rubber tire that rode on one of the fan belts.

 

@Ned Ludd I don't recall exactly where I saw it but I know I've read about a speedometer being made into a tachometer in an old car magazine.

 

Cons:
Alot of work changing faces etc.
Finding a reducer
Just use an electronic tach

Pros:
Making a set (tach & speedo) that match
Alot of cool old speedos out there
Alot of room for individualism

So yeah I like it !!

.

 

Con: Not knowing which is which will never hold up in court.

 

That test on the Sun machine was worth the price of this thread, for sure!

 

This is from a bandsaw, I hope to use it on my flathead someday
At least the cable drive

 

Shop I worked at had a kit that you could make up speedometer gear reduction adapters. There was a wide variety of gears to choose from to make just about any ratio you might want.

However looking these up now they are a bit more expensive than I thought.
https://www.texasindustrialelectric.com/speedo.asp

.

 

We do have boxes in Africa, but cats keep getting into them.

The cats:

 

Those reduction boxes that Hutkikz showed can be found at most semi truck parts houses or shops and they are real common on 4x4 trucks if you happen on one in the wrecking yard that still has the trans in it. It's basically a little quick change inside.
I bought a pretty small gps speedometer for my 77 1 ton because the speedometer is bad and I don't want to put a lot of money in that truck. about 30 bucks and I tested it in my daily driver and it is accurate. It's smaller than my cell phone and can be hidden away when you don't want the world to see it. There a lot of them on Amazon and Ebay.

For a around 50 bucks there are some on Ebay that look like regular round hot rod speedometers eBay item number:234143723752 That sounds pretty cheap though.

 

When it comes to some of the stuff that occupies Ned's mind, most of us are in the box with you.
I think I get where he is going.
Just let me read it through a couple more times........

 

Almost half the size of a chrome Sun Tachometer...
When the 58 Impala was now my car, at the insistence of my friends with their shiny tachometers on their dashboards, the small black tach that fit nicely under the lip of the dash, over to the right side of the speedometer. It covered the 120 mph mark, but at the drags, the 120 needle was pegged at the start and was useless. On the street, no one goes that fast...

The Green arrow placement, was an out of normal driving, front road vision, location. Nicely tucked under the dashboard lip. No harmful additional holes were added for the installation.

Hello,

If one does not want to add on a tach of huge proportions, take a reading of the tach at various rpms and look at the speedometer. If it shows 2500 rpms and you are going 50 (or whatever speed) then remember the speed. Do several readings at various car speeds and you have a nice list.

Now, take the tach off and you can use the rpm log book for the speeds. One of the readings for the 4:56 was going 25 mph. There were several school zones with the police radar units being hidden between cars approaching the zones in our normal driving areas.

There were plenty of times when we had to do this technique as we changed the rear gears from 3:78 to 4:11 to 4:56 ratios. Each gives a different reading per rpm vs speedometer. We had a card made to show which rpms were noted with which speedometer reading and gears used.

Each week we were racing, called for 4:56 Positraction gears. But, from Monday to Thursday, it was all daily driving to high school and work. So, the 4:11 or sometimes the 3:78s were installed. If there was a long distance road trip on the itinerary, the 3:78s made the day a lot easier on all of us. But, it still gave us plenty of power. The 4:56 gears were used at the drags, just to be competitive with all of the other guys/girls that had the same motor and gearing.

So, it was my job to change the complete third member on Thursday nights after school. Then if it was planned to race again the following Saturday, then the race gears were left in place. If the longer road trip was coming up, then the 4:11 gears usually got the call. The 3:78s had to be exchanged with my friend who ran them in his white 57 Chevy Bel Air Hardtop for his own use during the week.

Jnaki

Some may think that was a lot of work. Well, it was long hours on Thursday after school. But, the rewards for Friday and Saturday nights paid off in bravado results at the drags and the local teenage drive-in restaurant parking lot hangouts. YRMV

@themoose alternative photo

The Skylark Wire Wheel time period had the 4:11 gears for most of the time. Only during Spring Break or Easter Vacation, or hot summer cruising were the 4:56 gears put back in place. So one hot summer night, the 58 Impala was stolen from a wedged-in place parking spot. Many hours later, a friend and I found it in a dark part of a hidden parking spot in a community park.

The Impala was jacked up and put on 4 sturdy milk crates. The Buick Skylark Wire Wheels were gone, and so was the small black tachometer. The remaining 5th Skylark Wire Wheel was still in the trunk and was sold later for the same original cost of the whole 5 rim set. That helped offset the loss. The small black tachometer was neatly unscrewed and no marks were on the dash or faceplate. Picky thieves...

 

To keep things simple on my T Modified, I use the tachometer as the speedometer. Besides, speedometers don’t play well with quick change rear ends!

 

Seat cover material!!