GPS Speedometer

[stugilmour]

Greg, thanks for posting the alternative. I had also noticed the original product was not available. I am giving the cable method one more shot. Good to know there is a new supplier.

[gjz30075]

Mark, you're most likely right and I don't think my statement is totally correct. This unit is advertised
as for US Domestic speedos and it works correctly on my Federal Sprint speedo. A bit of a caveat here:
my speedo is about 5 mph slower than the GPS app but once I reached 30 mph it became accurate. The
speedo needle is always wavering a bit so I'm chaulking this slow speed inaccuracy to needing a good
cleanup inside.

I mounted the unit in the glove box so the cable is a straight shot behind the dash, then makes a gentle
90 degree bend to the speedo. Maybe its the cable rubbing causing this, or not, but overall, I'm pretty
pleased with this.

Two benefits: I can plug the hole in the gearbox tailshaft, ie, no leaks, and it's more accurate than the
previous gear driven, oem, arrangement. The speedo has never been opened in the 42 years I've owned
the car; probably time now.

If the cleaning of the speedo doesn't stop the wavering, then plan 'b'. Probably mount the unit somewhere
else to give the cable a straighter shot.

[My72Sprint]

I've been considering a Speedbox unit.
Lotus 2000e speedo cable drive = 3/4 x 26 Speed Box = 7/8 x 18
Custom cable needed for speedometer

Differences with Lotus Smiths speedometer are as follows:
The front of my Lotus Speedometer has a serial number followed “980” = 980 TPM

Speedbox follows SAE J678 specifies 1000 or 1001 revolutions of the speedometer cable core
shall cause an 1 mile indicator on the odometer. 1000 per minute = 60 miles per hour.

980 Vs. 1000 is a 2% error, Faster than actual.

Fixing the speed error:
Speed Offset (Optional):
The Speedbox™ is factory set to work with most USDM Mechanical Speedometers.
If the speed indicated on the speedometer is consistently off (+/- 10MPH) you can apply an optional Speed Offset. The Speed Offset feature can be used to dial in the accuracy.
IMPORTANT NOTE: The will NOT be accurate when a Speed Offset is applied.

[Foxie]

Have a look here, last post bottom page 1: viewtopic.php?f=31&t=43530

[mark030358]

I believe a GPS speedo is not legal in the UK, I stand to be corrected though.

cheers

[billwill]

What does a GPS speedo show when you are driving in a long tunnel?

[gjz30075]

Good question. I'm guessing it drops away and picks up again when exiting the tunnel, much like
satellite radio.

[1owner69Elan]

What does a GPS speedo show when you are driving in a long tunnel?

The Classic Speed GPS unit does the following:

"As for places where you can’t locate a satellite, e.g. tunnels, there’s no need to worry. Classic Speed works to accommodate! If satellite triangulation is lost, we have an algorithm to continue to drive the speedometer based on an average speed of the last few minutes. This is more to keep the inertia of the system going than to let you know how fast you're going. Exit the tunnel, automatically re-acquire sats, and you are back in business!"

Sadly, the Classic Speed unit is no longer in production and looking for an investor to buy the business.

Just hope mine keeps functioning.

[patrics]

Hi,
GPS might not be good in a tunnel but how accurate is it going up or down hill?

Regards
Steve

[1owner69Elan]

The GPS speedometers are certainly not absolutely perfect. Nor are mechanical and electronic ones as they are dependent on tire sizes, transmission, and diff changes. And even if calibrated for a given configuration then changes in tire pressure/temperature/traction can induce error.

GPS speedos, while not affected by these factors have other issues:

1. Sampling rate. Under serious acceleration there may be a delay in updating the true speed (latency). However, the Classic Speed has a sampling rate of about 5x that of your phone GPS (5Hz vs 1Hz). Thus, using your phone GPS, the speed seems to lag when under hard acceleration.

2. Going around turns or going up or downhill can theoretically also induce error. But the error is small. Interstates in the US are are generally constructed with no more than a 6% slope. A back of the envelope calculation shows that this might result in an under reading of speed by ~0.2% if going up or down a slope. Similarly, going around a curve at 1G will give an under reading of ~0.05%. So in practice, not really an issue.

3. As noted, any poor GPS reception situation will interfere or completely remove the speedo reading. Averaging can smooth this out but obviously instantaneous accuracy is sacrificed.

I think there are probably a lot more complex issues regarding GPS speedo accuracy (satellite broadcast accuracy, multi vs single satellite receivers, satellite positions, signal strength, algorithms used to calculate speed from multiple data samples, ...).

In the end, the GPS speedometer is probably not going to cause me to get a speeding ticket because of an inaccurate reading except for the fact that the GPS speedo does not have the typical over-reading built into OEM speedos by the manufacturers in order to comply with regulations. So, no margin of error with the GPS speedo unless you want to calibrate it to also over-read - which you could do.

[Foxie]

The GPS speedometers are certainly not absolutely perfect. Nor are mechanical and electronic ones as they are dependent on tire sizes, transmission, and diff changes. And even if calibrated for a given configuration then changes in tire pressure/temperature/traction can induce error.

That's an interesting post on GPS speedometers.

However, GPS apart, I would not put mechanical and electronic speedometers in the same boat.

As you say, when calibrating a mechanical speedometer, one would work forward from the diff ratio, and wheel and fitted tyre size. You will need to select a specific tyre pressure for your calculation. (As tyre pressure is a function of temperature you do not need to input for temperature). The only way to allow for any changes from the manufacturer specs in tyre, wheel or diff ratios is to change the angle drive gearing, by selecting from a discrete range of ratios.

So although reasonable or close calibration is possible, there is usually a detectable error. Over a life time of different cars, I always would establish the speedo error and be able to do a quick mental calculation if fine results were required for speeds and distances.

The other big disadvantage of mechanical speedos, as I do not need to tell anyone here, is how awkward it can be to replace a faulty cable or angle drive.

With an electronic speedo you do not even need to know the diff ratio, wheel and tyre size, or tyre pressure.

You just drive an accurately measure distance, whether a specific number of wheel revolutions, or a measure mile on the motorway, and input the resulting read-out back into the unit.

If tyres, wheels or the diff is changed, simply repeat the procedure to re-calibrate.

Finding a suitable fixing of the pick-up, normally to the diff input or output is all that is required.

Then fit and forget !

Incidently, out of curiosity I did a sample calculation of the effect of varying tyre pressure on a mechanical speedo read-out.

For a 185/14/70 tyre, dropping a tyre pressure of 2 Bar (29.4 psi ) by 5 psi would mean the speedo reads 1.2% faster, or 0.24% /1psi. Does anyone here drive with their tyres off by anything like 5 psi ?

[alan.barker]

If you stay with original Speedo when you get stopped at least you can say the Needle was moving around a lot
They might feel sorry for you when they still give you the Ticket
Alan

[nmauduit]

Does anyone here drive with their tyres off by anything like 5 psi ?

I try to keep my tires within 1psi (0.05bar actually) of the target pressure ... but there is also tire wear.

My original speedo (or right angle most likely) has been on strike for a while on my street elan, and so far I've done a good job procrastinating about the required engine out procedure (5 speed gearbox), so I've installed a cheapo gps speedo on the radio blanking plate, and when on treacherous monitored roads I check the rpm level at the limit then go by the tacho (which works in tunnels and down the hill, too).

[h20hamelan]

anyone thought of the cheap ones
looks like the 5" would fit the instrument panel ashtray

https://www.amazon.com/VJOYCAR-Speedome ... B07HQCCFWB

[nmauduit]

anyone thought of the cheap ones
looks like the 5" would fit the instrument panel ashtray

https://www.amazon.com/VJOYCAR-Speedome ... B07HQCCFWB

looks like the one I have (though mine is smooth plastic case) 58x105mm 3 buttons, not high performance I suppose but good enough as a disposable quick fix.