The Triumph Dolomite Club - Discussion Forum

Has anyone had any experience with GPS electronic speedometers

I have been looking at a dial version rather than a digital readout one but can only find them that go up to 35mph!

I have been looking at a dial version rather than a digital readout one but can only find them that go up to 35mph!
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That is because they are used mainly in tractors and other farm vehicles as the standard wheels are often changed for different sizes for different jobs. I tend to use my sat nav in my Dolly just to keep away from plod

Tony.

They are available need one for the 13B Project.

https://www.demon-tweeks.com/uk/motorsp ... edometers/

Also on EBay cheaper.

I assume this is to be used as a backup or on the track? GPS speedos are not road legal by themselves and can't replace a standard speedo.

Yes it’s just a back up....

https://www.youtube.com/watch?v=W7hl6UnlHJ4

This may be useful to you. Great series of 'blogs' too.

Regards

MC

If it's only for a backup, why bother wih a big round clock? Most smartphones are capable of accepting a GPS speedo app, many of which can be had free!

If you actually want a legal round speedo, and have no cable option, you will have to do what I did for the Dolomega and get an electronically sensed one. I looked regularly on ebay for over 2 years and never found a suitable used one (needs 100mm dia face for starters) most of the cheaper ones available are only 80mm. Anyhow, late last year I bit the bullet and invested in a new Smiths Classic one. Got it from Merlin Motorsport via Ebay as that was the cheapest I could find and was STILL nearly £180 with carriage! Then another £60+ for a speed sensor from Vintage Classic. Gave me a right lump in the throat!

If you DO actually have a cable drive at both ends, but a mismatch as I had with the Carledo, Vauxhall gearbox and Triumph speedo, places like Speedy Cables in Swansea will make you a bespoke cable for less than £40, My only complaint is they took their sweet time doing it, but it works fine and they keep records, should I need another, it's just a phone call away!

Steve

Thank you Steve, I will have a look at those options...

The main reason I wanted a circular one is that in my track sprint, the time clock is not there so an ideal location for a secondary speedo, the main speedo works fine apart from at higher speeds it is not so accurate. I think it needs the cable drive cog changing so I though an electronic one would be an easier option than trying to work out the ratio required. Would have to get my very rusty maths head on to work it out....!!!!

My digital readout one works fine just in my opinion not in keeping with the other analogue dials ... but then I ask myself in my modified track car does that matter!.. maybe not...

Thanks for all your help guys... food for thought on my options...

Nigel

storeyn wrote: ↑Tue May 05, 2020 8:46 am Thank you Steve, I will have a look at those options...

The main reason I wanted a circular one is that in my track sprint, the time clock is not there so an ideal location for a secondary speedo, the main speedo works fine apart from at higher speeds it is not so accurate. I think it needs the cable drive cog changing so I though an electronic one would be an easier option than trying to work out the ratio required. Would have to get my very rusty maths head on to work it out....!!!!

My digital readout one works fine just in my opinion not in keeping with the other analogue dials ... but then I ask myself in my modified track car does that matter!.. maybe not...

Thanks for all your help guys... food for thought on my options...

Nigel

I have a similar error in the pre-owned Sprint speedo I have in the Carledo, up to around 60mph it's fairly accurate, above that it's increasingly optimistic until, by the time you reach a "true" 100mph, it's showing around 112. It flatters the ego, but it's not much help really! I've just put it down to old age in the instrument.

A gearing problem would give a consistent error. It did with first (Vauxhall) speedo I used. The problem was, I used a FWD speedo (driven by the diff as standard) with a RWD (gearbox driven) drive gear. The error was a massive but totally consistent 40% under read. Which was easier to live with than you might think, your brain learns to count in 6s in a matter of days! So 30mph was an indicated 18, 70mph was 42 indicated and 100mph was an indicated 60. Seemples!

The Vaux dash got dumped for a number of reasons, mainly that the PCB behind it was increasingly erratic and I couldn't get another and the final straw was the Vauxhall speedo cable breaking and I couldn't get one of those easily either, So i went back to a more traditional looking Triumph (ish) derived dash. I was lucky really that the Vauxhall gearing in the gearbox was close enough to be reasonable, there is only 1 diff choice on the gearbox donors car (3.7/1), so only 1 speedo drive gear ratio choice (no choice that is) With other introduced errors like wheel and tyre size cancelling out the 3.7 versus 3.45 diff difference, i've got away with it more or less. Speedy cables built the mix-n-match cable supplied with 2 stub ends as patterns and an overall length required.

What I SHOULD do (and maybe what you should consider) is getting the speedo overhauled and recalibrated to match the CAR, rather than the faff of trying to match gearing up when it may not even be possible, the drive gearing choice to suit your car may not be available. And as I noted above, the speedo is probably old and worn too.

You will need to know a) the final drive ratio, b) the speedo drive gear ratio you have (these are colour coded on Triumphs so you can find out easily) and c) the rolling radius of the rear wheels, this is the distance between the hub centre and the ground when the weight of the car is on the wheels, with the tyres inflated to the correct pressure. Given this data, there are a number of companies who can do the job. It's just I haven't got around to it yet, it's not high on my list, the speedo is accurate enough at most legal speeds and the error is known. If I had to put the car in for a BIVA test or something like that (I don't) speedo accuracy would be more important.

On track, I don't really look at the speedo at all, I pay more attention to the rev counter, temp and oil pressure guage and lap times.

Steve

Here's another thought, no speedo that is reliant at any point on mechanical parts (so this includes electronic pickups on the prop or similar) can be 100% accurate all the time.

This is down to that critical "rolling radius" figure I mentioned in the post above. tyre wear alone accounts for up to 6mm of variation, tyre and ambient temperature, another couple of mm and weight bearing on the wheel, ie the difference between an empty car and one containing 4 adults a full fuel tank and a boot full of tools and spares, can probably run as high as 10mm. (assuming you don't adjust the tyre pressures to compensate)

So you are looking at a slight compromise figure to start with! This applies just as much to a completely standard car as it does to a highly modified track monster. It's one reason why legal speed limits are not largely enforced exactly but a certain amount of latitude (usually 10% + 2mph) is built in.

Steve

I have to agree with Steve, plenty of GPS app options but if it must be analog, a Ducati driving mate has one that measures silly speeds (definitely no 35mph limit!)

There are smaller ones in 52mm and 60mm (like our gauges&clock) though they weren't analog..

Thanks guys.. lots to think about..

MY current sprint speedo is only accurate up to about 30mph.. error progressively gets worse the faster you go which is not unexpected as the error will be exponential rather than linear.

It is definitely useable and once you are aware of the error then easy to work out correct speed. As you say Steve the rev counter is much more important on the track, and really no time to check speed too or you may be having a bit of grass or barrier action.

My only reason for going analogue was it is more in keeping with the car... I have found a few so will have a think about the best option for me. Hopefully we will be able to get out soon for a little drive or even a track day before the end of the year

I have a free gps speedo app downloaded on my phone. It can be watched in different formats and offers an additional trip . i have used it to check my speedo which isn't too bad just a couple of mph out.

Graham I find them useful too..

Storeyn: Spotted this 100mm version..they claim a gps module option..Hope you post pics of whichever you decide mate, sounds like a fun project..