My car's speed - measuring - reading

[dr pepper]

If you learn to read & understand live data from the Obd port you can use that to diagnose and asses the general condition of the motor too.
You can get apps for 'phones now that can read & show you all kinds of junk from the Obd via a bluetooth interface, I've cleared faults on my car with my android 'phone.

 

[tcmtech]

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.

The two simplest sensors I know of are teh ABS tire speed sensors and the transmission or rear end speed sensors.
Typically they are simple two wire magnetic reluctance sensors so tapping into them with a high impedance Op-Amp signal converter/booster circuit is pretty easy.