I am interested a 1/4mile setup with a decent precision - lets say ±0.1sec / ±2kmh. I already have an OBDlink LX. Will this be enough together with the cars GPS and other sensors? Or should I invest in an external GPS like the Dual XGPS 160?
OBD does not enhance timing accuracy, this is determined by GPS. The named accuracy will be achieved with a good internal GPS or an external GPS in most cases (manufacturers of GPS chips mostly state 95% of all samples).
GPS based lap timers use interpolation to derive accuracy far better than the update rate. In fact, increasing the update rate improves accuracy only slightly. The critical criteria for timing accuracy is GPS accuracy. Please check the slide "Basic concepts of GPS based lap timers" in the Train the Trainer document available from http://www.gps-laptimer.de/documentation
At 4 meters accuracy, and typical speed characteristics for start / finish lines, this results in accuracy better than 0.05 seconds for 1 Hz devices
