View Full Version : TR6 TR6 speedo to electronic conversion anyone?

11-21-2012, 12:17 AM
I'm doing an electronic fuel injection conversion and will convert the ignition system to fully controlled by the ECU (eventually.) So (eventually) I will do away with the distributor and need an electronic tach. In the mean time I have found my Tach is almost 1000 rpm off at 4000rpm. The ECU kept saying I was never going over 4000rpm when my tach was saying 5000. I bought a cheapo digital gauge and tye wrapped it to the steering column and it agreed with the computer. I've been quoted $250 from the Gauge Guys for conversion which is a little steep for me right now. Not that I'm cheap, I'm dirt cheap. So the question is has anyone converted their tachs to Electronic (cheaply?)

The TR6 gauge is a bit of an odd size (about 4 3/4 inches or 120mm) so finding something to replace it is hard. I like the white on black style. Reminds me of the classic '40s aircraft cockpit look.

I've searched around and most after market 120mm gauges are kinda goofy for what I want.

I've found some Smiths gauges but they are as expensive as conversion and they don't have the indicators in them.

11-21-2012, 04:13 PM
How about a servo motor attached to the tach. They make those for speedometers, but I don't know about tachs.

11-21-2012, 07:35 PM
I have done what you are asking.

If you are looking for someone to do it for you, the only other place I can think of is Palo Alto speedometer in CA. However, the price APT quoted you is actually quite reasonable.

Almost anything is possible if you want to do this yourself. What is involved is finding a donor tachometer with a similar range of needle movement (X RPM at Y degrees of needle swing) and mounting it behind the gauge face. That involves some machining of spacers and requires a donor tach that can be calibrated (a trim pot somewhere inside). Even with those things taken care of, you are likely to find errors between the donor movement and the scale of the TR gauge face. If you look carefully at the Smiths gauge you are likely to notice that the scale is not uniform and some numbers will be closer together than others. This is due to the way the mechanical speedometer works with its return hair spring and counterbalanced needle. In short, expect some errors with 100 RPM accuracy at any given point being your goal.

I think a copy of one of my old tach conversion documents was in the FAQ section here at one time. If the link below works, it will be to a PDF copy of that document.

With all that said, consider that you will spend anywhere from $50 to $100 for a donor tach to cannibalize and then you have all the conversion work to do. Calibration will require you to have access to a signal generator or you will have to send the converted gauge to a speedometer shop for calibration. You can see when you add this up that $250 is a reasonable price.

11-21-2012, 09:28 PM
Rather than edit my post above I figured I would make additional comments in a separate message.

I went to Paul Rego's web site and copied a close up photograph of his TR6 tach. I printed that out and made some measurements. Assuming Paul's tach is typical (0-7k RPM), the gauge is calibrated for about 39 degrees of sweep between the 1000 RPM increments on the left side of the face and 41 degrees per 1000 on the right side. Obviously this excludes the first 1000 RPM which are not linear... except 500 to 1000 RPM is about 19 degrees.

With that observation I can at least make a suggestion for you to ponder. Look at the gauge in the eBay auction linked below.
https://www.ebay.com/itm/230852705567?ru=http%3A%2F%2Fwww.ebay.com%2Fsch%2F i.html%3F_sacat%3D0%26_from%3DR40%26_nkw%3D2308527 05567%26_rdc%3D1
(eBay item 230852705567)
That is a 10k RPM stepper motor driven tach. Full scale is right at about 270 degrees of needle sweep. Obviously you don't need 10k RPM. However, the gauge can be set for 4-, 6-, or 8-cylinder operation. If you set it for 4-cylinders and connect it to a 6-cylinder engine it will display 3/2 (1.5 x) the correct value. So... set the tach for 4-cylinder, connect it to your 6 and with your engine idling at 1k, the tach will display 1500 RPM.

When you make that intentional switch setting, the tach will display 10k RPM when your engine is really going 6,667 RPM. That is 270 degrees of sweep per 6,667 RPM (270 / 6,667 = 0.0405 degree/RPM). So for each 1000 RPM, the needle will move 1000 x .0405 or 40.5 degrees which is VERY close to the average increment needed for the 7k Smiths TR6 tach.

So... you could use a tach like the one I linked above, mount it behind your gauge face, set the tach for 4-cylinder, REMOVE THE NEEDLE STOP pin on the Smiths face and ignore speed indications below 500 RPM. (At rest the needle will hang below zero on the Smiths face). The remaining issue is to confirm that this tach movement is NOT too long/deep to fit inside your TR6 gauge case. If it is, you can search eBay for stepper tachs by Depo Racing. They will have a similar range but are MUCH shallower. However they cost twice as much.

Oh... and the reason you may want to look at a stepper tach instead of a traditional electronic tach is that the stepper tach is not likely to be affected by the mass of the Smiths needle. Traditional electronic tachs may require additional tweaking/balancing to get the Smiths needle to behave properly after a movement transplant.

Good luck with your project.

EDIT: I forgot to mention that the tach linked to above is a small 52mm gauge (the same size as fuel and temperature gauges). I linked such a small tach because with your TR6 tach you still have indicator lamps (oil and ignition warning) that need to pass through the gauge. This small movement should easily avoid interference with those indicators.

11-22-2012, 12:15 AM
Final post for now.

I noticed when reading the subject line that you were asking about speedometer conversions. In the body of your post you spent more time talking about the tach. I hope the tach is what you were really interested in and that I have not wasted your time.

Speedometer conversions are also possible but they are more involved. I have done those using motorcycle and VDO electronic speedometer movements behind the original gauge face. However, this requires modifications to put the LCD odometer in the right place and it also typically requires creating and applying artwork to make a new scale for the original gauge face. Speedometer conversions therefore cost a lot more to complete.

11-22-2012, 03:35 PM
Most of the guys in 6-Pack have been using Morris at West Valley (https://westvalleyinstruments.com/) for all their gauge work. But.......quality work doesn't come cheap. I just asked him about doing my gauges in a white face with black lettering ($450), R & R (refurbish & re-calibrate) on the 4 small gauges ($50/gauge), Convert the tach to electronic which includes R&R ($200)... he's also looking into a GPS conversion for the speedo but hasn't quoted a price yet.

Picture below is his conversion of a friend's TR6 gauges.


11-22-2012, 06:45 PM
Thank you! That was what I was wanting and it's pretty much what I had worked out. What I was missing was donor electronics. After cruising Ebay and seeing all the hundreds of tachs there I realized the 7000rpm/270 degree and geometry, I searched for "7000 tachometer."
I came up with late three good chices. 70s 6 cylinder chevy Camaro Tachometers as shown in this ebay https://www.ebay.com/itm/181027451710?item=181027451710&viewitem=&vxp=mtr
or these Boat tachs. https://www.ebay.com/itm/TELEFLEX-58255-AMEGA-7000-RPM-INBOARD-I-O-OUBOARD-BOAT-TACHOMETER-/190607527988?pt=Boat_Parts_Accessories_Gear&hash=item2c61181034&vxp=mtr and https://www.ebay.com/itm/FARIA-TC9094A-PROFESSIONAL-GRAY-SERIES-7000-RPM-OUTBOARD-BOAT-TACHOMETER-GAUGE-/190607458090?pt=Boat_Parts_Accessories_Gear&hash=item2c6116ff2a&vxp=mtr
These are 3 inch gauges so a little closer to needle size and not too expensive. Comparing my donor tach to these pictures I can't see any major differences in the range or geometry. 3500 is almost straight up and 4800 is almost opposite 0 and if it is off a 100 rpm or so it will still be more accurate than what I have now. I figured I would have to build and adapter plate or maybe glue it to the inside of the face plate. I was worried about the needle shaft. I have wire gauge drills if over size and I really like the wire shim idea. I kinda like the boat tach idea, they may be more moisture tolerant when I forget to put the top up. ;)
I did mistakenly say Speedo and meant Tach however, I may go electronic on that also. My ECU has the capability to give MPG if there is a Vehicle Speed Signal. So if I'm going to do a VSS somehow, I might just as well do an electronic speedo AND since my speedo quit a couple of weeks ago (probably 90 degree adapter again) I might just as well investigate that too. In for a penny, in for a bloody pound.

11-22-2012, 09:32 PM
Here's a couple of places that have all sorts of custom gauges including GPS speedos......... the problem is the stock TR6 gauges are a funky diameter so unless you do a custom dash with custom hole sizes you're pretty much stuck with converting the stock gauge.



11-22-2012, 09:46 PM
Before discussing tachs, the GPS speedos mentioned above are an interesting new product. Google for "Speedhut" and take a look at their gauges. They start with a generic product and you can specify no-cost customization features like the font, face color, etc. They are reasonably priced in the mid $200s.

The marine tachs are a bit different than car tachs. You certainly can use them in a car (if they can connect to the coil... some are made strictly for alternator connection) but you will find their needle response is heavily dampened. You won't blip the throttle in neutral and see the needle immediately jump. On the other hand they won't jitter either.

When you fit the spacer, you are not likely to fit it behind the face, you are likely to find it better to fit it to the back of the case and mount the Smiths face to the front of the donor tach. Remember the needle is on a spindle of a given length so putting a spacer behind the face will make it harder to mount the needle. You mentioned you had access to small number drills. If you find you need mini hardware, you should find most of what you need (micro taps and fasteners) at Micro-Mark (Google for their online store).

For the needle, you may find you need to make a plastic coupler to join the movement spindle to the needle. I have used nylon rod about 1/8" in diameter. Typically the through hole needs to be a press fit onto the movement's spindle. The "top" of the coupler needs to be drilled to be a press fit AROUND the stem on the back of the Smiths needle.

Needle size isn't so important as balance. Just because one of these donor gauges has a needle close to the Smiths does not mean the needles will be balanced the same. When you remove the Smiths needle from your tach you will typically find a brass counterweight staked to the back to compensate and work with the hair spring on the mechanical movement.

Do keep us updated on what you do. These are fun projects for me and I for one like to see what paths others take.

EDIT: Sorry Bobby D... it looks like we were typing at about the same time. I wasn't intentionally trying to duplicate your information on Speedhut products.

11-24-2012, 07:30 PM
I was tempted by the Speedhut gauges but I don't think any of them will fit a TR6 dash. Anyone know the exact diameter of our big and little gauges?

11-25-2012, 10:35 PM
The "little" gauges are an industry standard 52mm (2-1/16"). I have no idea about the large TR6 Speedo and Tach. For other British cars the standard tach/speedo sizes are 80mm (3-1/8") or 100mm (4") depending on the vintage of the car.

11-26-2012, 01:20 PM
The TR6 large spedo and tach are an odd size that's very hard to find at 120mm. I wanted to go with the new Smiths magnolia line of guages, and the spedo and tach in that line of gauges is only 100mm, so I had to make an adapter bezel to go from 120mm to 100mm.

11-26-2012, 05:38 PM
I am not sure how moderators would cross link threads on the forum so consider the link below my simple approach.

Please read/see this thread:

11-27-2012, 12:52 AM
I settled on a Comaro Tach as seen on Ebay. I haven't received it yet. It was for a 6 cyl and fairly early and hopefully to be simple enough to apply to our needs. Those Speed Hut instruments sound interesting. They are Can bus controlled. "Works directly from the (2007+) vehicles ECU (CAN bus system). No sensor or harnesses needed." My ECU can control a can bus but figuring it out and using it is another chapter. I'm just getting the fuel tables sorted out. Maybe next year when the remodel and addition are done and I get the TR back on the road. I just don't have the time right now to work, help work on the remodel, clean up for the winter around the property and work on the car. I'm taking the TR off the road next month (if I can have my garage back) to replace the frame.

11-27-2012, 09:06 AM
From what I understand, while the Speedhut instruments can be connected to the bus, they don't have to be. The first installation I discussed with an acquaintance was for a classic Mini like mine. There certainly is no bus in our cars.

If you have trouble with the Camaro tach remember the others I mentioned on the first page of this thread (10k $27 eBay stepper tach set for 4-cylinder operation) and the similar "Depo Racing" tachs (also on eBay for about $50). The stepper tachs are likely going to be smaller than the GM tach and less susceptible to errors from the mass/balance of the needle.

From the Speedhut web site
(Page https://www.speedhut.com/gauge/GR4-GPS-01T/1/GPS-Speedometer-Gauge-120mph--(w--turn-signal-and-high-beam)
"Easy hook up- requires only power, ground and lighting and GPS accurately displays speed. included 15' long antenna cord to route antenna to convenient location in corner of dash. Speedhut Life-time warranty on all GPS speedos."