Having some experience with modifying VDO gauges for various purposes (including converting 914/4 cores to 10K racing tachs, replicating GT 914/6 fuel gauges), I have in the past wondered whether one of the increasingly common GPS-based speedos can be adapted to the look/feel of the classic VDO gauges we know and love.

If you’re not familiar, these units attach to a small GPS antenna placed somewhere in the car where it can get the satellite signals (usually on top of the dash) and calculate ground speed in much the same way as a Sat Nav works out your velocity. Why do this at all? Cable driven speedos have a finite life; needles bounce, gears break, cables wear out (I’m on my 3rd in the 914/6, 6K miles driven), plus that cable run from the rear of the 914 gearbox has to be the worst: under the engine, past the exhaust, through the center tunnel then a 180 degree bend to finish!

Downsides are that you lose the trip meter function - I can live with that. My phone can work out travel distances with sufficient accuracy. I find the colored LED backlighting on the GPS unit somewhat garish; but that should be fixable.

First order of business is to identify a suitable candidate and break it apart to see if it can be adapted to a VDO casing. The unit shown in the photos seems to have multiple suppliers, costing around $60. It has a 160mph maximum scale, along with an LCD display for miles driven. On the rear, an RF connector allows for the connection of the supplied GPS antenna plus cabling, and there are three wired connections to the car: switched 12V, GND and illumination. It has an 85mm overall diameter:

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First step is to disassemble the unit, so I started (as you would with a VDO gauge) by prizing off the bezel. This proved to be somewhat of a pain, but with some effort it came away, giving access to the needle (pulled to remove) and the fascia (thick vinyl, glued on). Once removed, this exposes an upper (mezzanine) bezel, made from clear plastic; the LCD (mileage) display sits flush with this, and below this is a PCB supporting the mechanism and associated circuitry:

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The assembly seemed to be attached to the rear of the plastic casing - but no obvious screw heads were visible, so I took a rotary cutting tool on the Dremel to the rear of the casing, using my best guess on the cut line to avoid destroying anything important, and preserving the power connector. The photos show the results:

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The PCB in fact was supported from the rear on pillars, but the screw heads had been sealed / gooped over. The power connector was preserved, for possible reuse - lucky guess there. The GPS circuitry is on a separate module hot glued to the rear of the casing. This has a connector to the main PCB, as well as a short coax cable out to the RF socket on the rear. With the casing sliced apart, this was easily preserved and set aside for later.

(continues..)

The main PCB is seen below, the diameter and depth mean this should easily fit in a standard 914 VDO speedo casing, as the side by side photos show:

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Clearly, still lots to do to get a decent result: The mechanism will need some adaption to fit the donor VDO needle, and some non-conductive mounting of the GPS module will need designing and adding, as well as a fascia overlay to match the dimensions of the GPS mechanism. As a bonus, I reckon the the mezzanine plastic can be modified to house the usual hi-beam indicator (between the bezel and PCB - probably will have to be a LED). But it all seems pretty feasible. The shallow depth of the GPS assembly means regular, rear mounted VDO illumination can be used.

Clearly just looking at feasibility thus far: More on this will be posted as a I progress, but - as a bit of a diversion: this first prototype will end up in a different target housing - not unrelated to a certain mid-engined vehicle of Italian origin - but, the next one will be heading into a Porsche VDO case.

- Tony

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I have been in a number of situations where the GPS in my phone didn't have enough signal to give me a decent speed estimate. I wonder if the separate antenna and dedicated GPS function resolves that?

--DD

You are making this harder than you need to.



Just get an Autometer GPS module and drive a stock 911 Electronic speedometer.It works great, I am running it in my 914. Here is the thread on how to do it.

http://www.914world.com/bbs2/index.php?showtopic=230296


Clay

We installed a 6-digit 911 speedo in my Six that I had Palo Alto Speedo configure to look like my original silver-dot gauge. I then used a Dakota Instruments GPS sender and after getting it sorted and calibrated it works perfectly. The only time I lose a signal is in long tunnels or occasionally on heavily wooded windy roads. Then it is just long enough for the needle to drop to 0 and pick right back up.

I did this modification to add the extra speedo digit so I keep the mileage on my car documented. We restored it at around 86K and I am currently at around 92K and we all know the stock odometer returns to 00000 after 99999. My tripmeter and odometer work perfectly.

I suspect Tony is a fellow graduate of the "Built Not Bought" school of doityourselferism.

@Dave_Darling - unknown, never used a GPS speedo before.

@Cairo94507 - I didn't think about the extra digit issue. Good point!

@ClayPerrine @GregAmy - am aware that there are simpler solutions out there for this. Not claiming this is either the best or easiest way to get a result.

I'm doing this out of - for want of a better term - "technical curiosity", coupled with the fact I have a little more time on my hands than usual. Also, I have a junk mechanical speedo or two laying around...

I am simply posting this to share my "craft project" with folks who might have interest.

- Tony

And my solution will be to go to the Speedhut GPS version:

https://speedhut.com/gauge-applications/rev...-and-high-beam/

I am looking at all of these speedometer changes with great interest...

I would like to change out my MPH to km/h for use here in Canada with as little fuss as possible and without draining the bank account. Finding a 914 mechanical metric speedo is close to impossible (even searching Europe). If there is one somewhere on a possibly shady site, the price is sky high, over 1000 Euros.

I'm not interested in using an overlay since it doesn't change the odometer from miles.

I love @ClayPerrine 's use of the 911's electronic VDO gauge as it really keeps the stock look with a drop in fit. Again though, finding a Canadian km/h 911 speedo is is pushing $1000 (that may be a new price not used) plus the $250 GPS module. My biggest worry going this route is investing a lot of money for the metric 911 speedo that is from a later model year and that it won't work the same as Clay's 1976 unit. So many 911 metric speedo part numbers that I can't tell what to look for and what will work... 911-641-516-06, 911-641-543-00 or 911-641-516-FX.

@GregAmy
This is a great idea from a cost/simplicity perspective and Speedhut has metric versions too. Does this 4-1/2 inch fit the dash opening without mods? I thought the case diameter on the VDO was 100mm (3.94")? or does the 4-1/2" refer to the outside bezel dimension?

Just find a US version Electronic VDO speedo, and send it to North Hollywood or Palo Alto to have it converted to KPH.

You should be able to get it done for way under a grand.

Clay

Try http://www.specialtygauge.com/

They do all kinds of Porsche speedometer repairs and rebuilds, plus they make a thing of supplying replacement odometer gears to suit both mph and kph units.