Before I had a real datalogger in the car, I used to use Android software (would probably be RaceChrono now, I used something else a couple years ago that was similar but not as good) plus a higher-res external bluetooth GPS module (this one:
Amazon.com: BT-Q818XT: Qstarz BT-Q818XT 10Hz Bluetooth GPS Receiver: Electronics ). For $80 + existing android phone, it's a nice cheap solution for pretty accurate results.