GPS speedometers, all the same?

[gezzard]

The Speedo in my new motor is out by quite a bit, so I'm thinking of fitting a GPS Speedo unit.

Question is are they all sort of the same, will eBay or amazon ones be accurate? Anyone use one they can recommend?

I'm aware there are apps for Android, but I don't always have my phone on me.

Ta.

[Soopahfly]

I wouldn't put that much faith into a GPS speedometer. They all work on a calculated average over time, and the only time that will be accurate is if you're maintaining a constant speed along a flat, straight road.
If it goes up and down, or round a corner, the accuracy will change.
When they give the accuracy measurement of -/+ 1mph, that's for the above scenario. Flat road, constant speed, no curves.
They also require a constant number of satellites to work, the more the better. In built up areas or in wooded areas the signal will suffer and accuracy will drop. If you're new car is a Renault as your avatar suggests, they usually have the UV coating on the screen, which also can drop satellite reception. Look at the screen through polarised glasses and you'll see the screen looks like an oil slick, that's the UV coating. It may have a small square where it isn't covered over on the left hand side for the French toll card reader. That bit will have better accuracy but over on the wrong side of the car.

They also tend to underread, and I don't think they count if you get pulled over. I could be wrong there though.

In short, get your proper speedo fixed. Me and a mate did it with a pair of gps speedometers, took an average of the speeds and moved the needle while driving.

[gezzard]

The car in question is a 1977 Allegro, well a Vanden Plas 1500 to be specific,. I had the GPS app on my phone and the speedo is about 5mph under reading.

Thanks, I think the best solution is as you suggest and get it fixed, a digital modern instrument would look awful stuck to the walnut dash.

👍🏻

[rodharris83]

Perhaps speak to these guys?

link

[Clamity]

Depends on the quality I think. I tested one repeatedly when I was regularly driving between New Milton and Newbury and ISTR the trip counter always agreed to within a couple of hundred metres in both directions. I was impressed but it's all open country of course.

[slater]

I've not got any experiance with stand alone speedos but I use a digital dash/data logger with GPS and it is extremely accurate and precise.

I believe there are different 'levels' of gps as such. The level on your average navigation sat nav is much less precise than what these data loggers use. That said I use my phone for navigation and the speedo on that is far better then the one on the car for accuracy. It's just a little bit laggy.