My car’s speed – measuring – recording…

Wondering
what could be the simplest way to implement a basic sensor to measure
my cars' speed wrt ground, every 5 sec or so for a period of 1 or 2
hours time? Not precise but consistent.

Additional log implemented in a RAM with automatic time stamping plus eventual "bookmarks" at the push of a button.

No display but reading when at home just to evaluate my driving performance.

Power
supply, a 9V battery (by no means associated to the car's common).
Tapping into the car's wiring (whatever it is) is not considered.

Comments and caveats appreciated.

Agustín

Well there is always the bicycle wheel trailing behind the car...
My standalone Garmin GPS provides speed, but I would guess that there is no way to access that over the USB cable. I don't know whether the GPS apps on a cell phone provide you with speed as well, and accessing that from the app would also be problematic, I imagine. That would leave you with writing your own app using the on-board hardware of the phone. Fow what it's worth, having worked with Android, I would suggest you use Basic for Android (https://www.b4x.com/ ) rather than the standard Android development environment. They also have Basic for IOS if you have an Apple device.

The thought occurs to me that there must be an app for that- and there sure areHere's onehttps://play.google.com/store/apps/details?id=com.binarytoys.speedometer&hl=en

If you had you heart set on making hardware yourself then maybe someone else has a different idea

Augustin / Alan the only thing I can think of is to get one of those heads up displays - the standalone ones have a GPS built in.  You'd have to decode the segments of the display, then log them, but neither task is that hard, If you are In the UK you'd be in miles so you'd only need to decode the 10's display so you'd only get a figure of 10MPH, 20 MPH, etc.  But it would almost certainly be multiplexed which would give you a headache or two   Maybe  use the common line as an enable then read the segments.  In Km countries you'd have to decode 2 digits if you regularly go over 100 Kmh.  But you'd only really need to decode one segment of the hundreds digit, plus 7 from the tens digit.   Good luck with it.

A standalone GPS module usually gives you a serial data output that is pretty easy to decode. And then there's this Arduino shield from Adafruit which has the GPS module and the SD card.
https://www.adafruit.com/product/1272?gclid=E...6SZpfCb1wIVj19-Ch34YQVOEAAYAiAAEgLTvPD_BwEYou may have to write some software to calculate the speed from the GPS data but that shouldn't be difficult.Another option is to use an accelerometer but then you'd need a solid mounting to the car and you'd have to integrate the result to get the speed.In both cases, you may be best off just capturing the raw data and doing the calculations on a PC when you read it.

Some interesting ideas here, in the past couple of years I've used a Bluetooth module that plugged into my car's OBD2 diagnostics socket located just under the steering column.  An app on my smartphone displayed a configurable vehicle instrument panel and data can also be exported to plot the route on Google maps via GPS. It's really very good.The app I used was Torque, available via Google Play Store. I'm told not all OBD modules are compatible. The thing to look for is ELM327 compatibility. They are dirt cheap and worth a try.
eg http://www.ebay.co.uk/itm/Mini-Bluetooth-Car-...rque-Auto-Scan-Tool-OBD-II/162617059752  or similar.