GPS Driven Speedometer

[Mr.Gale]

Has anyone converted their speedo using a GPS to a mechanical speedo unit such as the SpeedBox? ( https://www.jegs.com/p/JEGS/JEGS-Mechan ... 4/10002/-1 )

Thanks,
Mr.G

[h20hamelan]

I will be updating mine, to a cheapie for now. This will give a base line, to make future calculations.

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

see

viewtopic.php?f=31&t=43530

&

viewtopic.php?f=31&t=40563

I know there are other posts about this, and what others have done.

[Mr.Gale]

Thanks. Too bad Classic Speed is no longer selling their unit.

Mr.G

[h20hamelan]

I think your mass produced, longer production run a better option.
If it is so important? vs, some originality!

[gjz30075]

Yes, I have, using the Speedbox. I didn't order from Jeg's, but rather right fom Speedhut. I ordered
without the cable and had a custom length cable made from a local speedo repair shop. I brought him
the unit, the speedo (I had a spare with the same threads) and the length needed.

I wanted to go stealth, so I mounted it in the glove box. I removed the original glovebox and made up
a 'custom' one using two really cheap Jeep glove boxes, found on Summit Racing site. The cable runs
out behind the dash and makes a somewhat gentle 90 degree bend to the speedo. All the wiring and
GPS pickup unit are out the other side and I have the GPS pickup secured under the fender / wing.

The unit reponds quickly if it were mounted on the dash but still works fine under fiberglass.

[My72Sprint]

I didn't care about absolute originality and updated to a new Smith Digital GPS Speedometer,
Other than the "Trip" mileage display looks like the original.

https://www.smiths-instruments.co.uk/lotus
Part Number pictured is SN6207-10CB.

Tim

[nigelrbfurness]

Fascinating. I'm all in favour of modern technology in an authentic looking wrapping. However whenever I use the Google satnav on my phone, the speed registered by the gps is generally much lower than the speed inducated on my dash instruments of any of my cars and that includes my very modern Alfa. So do these instruments use a different technique?

[ericbushby]

Hi, Your satnav will display the correct speed if you are on a straight and level road but as it works by timing between fixed points, then if your route is not direct point to point then it will display a lower reading than your road speed.
A car speedo measures the wheel rotation speed and while allowing for correct tyres and air pressure should be able to display road speed.
However there is a requirement that car speedos must never read low.
Manufacturers are allowed to have variations of + 10% and minus nothing. Therefore to comply with this, speedos are designed to read about 5% high. This figure may not be accurate, but you get the drift.
Everybody wins. the manufacturers comply with the law. The driver thinks his car is faster than it is.
The fuel consumption seems to be better than reality. It reduces the chance of exceeding the speed limit.
So, the chances of a satnav and a speedo ever reading the same are nil.
Eric in Burnley
1967 S3SE DHC

[My72Sprint]

Well Ok GPS may not be always precise.
Speedometer reading did exactly match radar readings 45mph was 45mph @ several plces.

My thought is GPS still better than a gearbox with speedometer drive gears mismatched to rear diff ratio & tire diameter where the speedometer is always 5% >10% off.

[Mr.Gale]

Almost all modern cars use a GPS speedometer, so I'm not sure why anyone would think they are not accurate. Or am I missing something?

Mr.G

[bitsobrits]

Almost all modern cars use a GPS speedometer, so I'm not sure why anyone would think they are not accurate. Or am I missing something?

Mr.G

I have three modern cars, none of which has a GPS speedometer. What makes you say this? I would be surprised if any car car has a GPS ONLY speedo as the odometer function would need a more reliable input to be legally valid. Perhaps a GPS speed reading ALSO via the satnav, but a GPS ONLY, seems unlikely.

[Mr.Gale]

I thought my 2018 Audi did, but now I'm not sure.
Anyway, anything would be better than my current speedo which reads ~20mph than my actual speed. My plan is to send my speedo in for repair/adjustment and then run it with a GPS drive.

Mr.G

[StressCraxx]

@Mr.Gale

Some things to check before you send your speedo in for repair.

Remove the speedometer cable and angle drive from the gearbox. Remove the clamp that holds the speedo gear in the transmission. What color is it?
There are three colors,
Black for 3:55 to 1
Green for 3:77 to 1
Blue for 3.9 to 1
If you have a black speedo gear in a car with a 3.9 diff, your speedo will read quite high.
If your tires are smaller diameter than stock, your speedo will read high. Check the stock tire size vs what you have on the car.

The last issue is the speedo itself. If the internals are dirty and dry, the speedo will read high.
Even when everything is perfect, the speedo will still tend to read a little high. TADTS (They all do that sir) but the speedo shop can adjust it.

[Andy8421]

Almost all modern cars use a GPS speedometer, so I'm not sure why anyone would think they are not accurate. Or am I missing something?

Mr.G

I have three modern cars, none of which has a GPS speedometer. What makes you say this? I would be surprised if any car car has a GPS ONLY speedo as the odometer function would need a more reliable input to be legally valid. Perhaps a GPS speed reading ALSO via the satnav, but a GPS ONLY, seems unlikely.

I don't know of any modern car that only uses a GPS speed sensor. Googling would indicate that modern cars either use an electronic VSS 'vehicle speed sensor' mounted on the transmission to monitor output shaft speed, or derive a signal from the ABS wheel speed sensors in the hubs.

[ericbushby]

Hi, Mr G
You are trying to drag me into the 21st century aren`t you.
It looks like I am way out of date here.
I usually buy daily driver cars about a year old and then run them for 10 or 12 years or 150,000 miles.
You have made me curious now.
I have recently bought two modern cars with satnav and GPS installed. I will compare the speedos with the `Head Up Display` App on the phone.
Next I will check if it works in the carpark under Tesco`s supermarket.
How do these cars cope with tunnels and forests where there is no GPS signal ?.
We need to know.
Eric in Burnley
S3SE DHC