Old gauges playing nice with new engine and transmission?

I'm thinking about what to do for gauges in my TVR project.  It had all it's original Smiths gauges, and there's nothing there that makes me think they're broken.

To be honest, the only one I know works is the gas gauge, because I tested it when I rebuilt the sending unit.

I did twirl the speedo and tach cables with my cordless drill and the needles moved accordingly.

So what to do with the new engine and trans?  For reference, the engine is a 2.0 EcoBoost, and the trans is a 6 spd NC Miata unit.

I know there are GPS speedo units that will spin a mechanical cable at the right rpms.  They're not exactly cheap, but they're not terribly expensive either.  Anyone have experience to share?

Not at all sure if I could somehow pull a speedo signal off the transmission.  I *think* the earlier Miatas had a cable-drive, but I don't see one here.

For the tach, I might be able to add a pulley to the accessory belt and drive the cable with that.  Somehow getting the mechanical tach to work on such a new engine sorta sounds fun.  Haven't really done the math on whether or not it would be feasible.

What would my other options be?  I guess there might be something similar to the GPS speedo, but that would read the OBDII signal and output an rpm that way.

I could probably get the guts rebuilt or something, but I'm just looking for ideas at this point.

I'm mentally saving coolant and oil temp ponderings for later, but chime in if you have ideas there.

If I understand correctly, the NC used the ABS wheel sensors for vehicle speed.

Dakota digital had at one time an output speed converter.

Actually to all the rest, just run the original senders to the respective outputs and inputs and it'll be fine.

TVR Scott said:

I know there are GPS speedo units that will spin a mechanical cable at the right rpms.  They're not exactly cheap, but they're not terribly expensive either.  Anyone have experience to share?

Not at all sure if I could somehow pull a speedo signal off the transmission.  I *think* the earlier Miatas had a cable-drive, but I don't see one here.

NA had a cable drive, NB had a little cable-to-VSS-electronic adapter that plugged in where the cable drive went (same transmission).  I vaguely recall that NCs were available in ABS and non-ABS versions, and that the non-ABS NCs had a VSS on the transmission while the ABS ones used the wheel sensors instead.  Whether the transmissions are actually different or if they just put in a blanking plate instead of the sensor to save a few bucks on the ABS cars I have no idea.

Theoretically you could install a generic trigger wheel where the driveshaft meets the diff and rig up a bracket to install a sensor on that to make your own VSS.

There are apparently companies that will retrofit Smiths mechanical gauges with electronic internals, although I doubt that's cheaper than a cable-twirling thingey.

I have used the Jegs GPS to speedometer cable device a handful of times. They do the job pretty well and are easy to calibrate in the shop. Tap the button the correct amount of times and the needle will jump. Press or tap to make it point to 60mph and you are done.

The cables don't like a tight bend, but then.... no speedometer cable does. 

They are a definite easy button.

As modding circuit boards goes, it wouldn't be terribly difficult to tap into the electrical pulse inputs that come from the cable-driven mechanisms and actually cause the needles to move. You'd start by driving the gauges with a drill and measuring the pulses, then you just need to emulate those. If you're lucky you may be able to feed them a 5V or 12V square wave.

My AE92 took a high-voltage spike signal from the stock ignition system to drive the tach, since the engine had a CoP conversion I got a circuit made from a modded relay that turns PWM square waves into spikes and then fed that with an appropriately multiplied output from the Megasquirt.

I have taken aftermarket gauges with the same sweep and range and transplanted the guts into the vintage gauges multiple times. Keeps the looks and feel, but updates to correct movements for my application. Used autometers and vdo stuff are cheap on Facebook and ebay, amd not hard to modify. Same with the old smiths stuff. Matter of fact, had a smiths/sun hybrid in my 64 el camino. 

Reading the title of this thread gave me a great idea, which I will reveal at some future time. Thanks TVR Scott!

EDIT: LOL, just read the thread and see that Duster had this idea 20 years ago.

Revington TR does make an electronic speedo and tach.  They aren't cheap.  The problem I found with gutting the Smiths tach is that the markings weren't linear.  It wouldn't be off by too much.

Getting a signal to drive them is the fun part.  The Revington speedo can use a magnet on the driveshaft or a VSS sensor on the speedo cable output.  Check whether your transmission still has the cable drive.  I wouldn't be surprised to find that your ECU has a tach signal pin already, but they may be driving gauges the new fashioned way (CAN bus).

If it does use CAN bus, there's this.

CAN bus interface