would 225 50 16 fit? Alternatively, what rev limit is programmed into gtec1? Maybe I can have mine raised to 7000rpm before I fit into car.

Just carried out a comparison test between Racelogic Vbox Mini 10Hz, and the RaceChrono software installed installed on a Nokia N80 and connected via Bluetooth to a QStarz Q-1000P 5Hz GPS.

0-100km/h - Most Vbox results around 0.6 to 0.7 sec slower than RaceChrono software. On one occasion it registered a time 1.7 seconds faster than the Vbox. Therefore on average, the RaceChrono results are almost a second too fast copmpared to actual timings. This is due to a combination of GPS receiver and software not being able to react quickly enough to a start. The stopwatch timer starts as soon as the software detects acceleration, or when the GPS registers a change in speed.

Some Bluetooth receivers have a static navigation mode which means that at very low speeds they register zero - which would not help the RaceChrono software. Speed is only registered as soon as it exceeds a given parameter such as 3mph or 5km/h. For some reason the Nokia N80 doesn't display speed at 5hz, rather just 1hz. A similar program on my SonyEricsson K800i does display speed at the full 5hz, which might help accuracy.

Hope this helps any potential users.

80-120 km/h test - On average this is a little more accurate with readings around 0.3sec faster on RaceChrono than the vBOX. So remember that RaceChrono 80-120km/h results are around 0.3 sec too fast compared with real timings.

If anyone wants to arrange a meet at a disused runway, I'll be happy to bring along Vbox to see what times can be achieved.

Using my own calculator...
215/35/18= 0.6% higher reading than standard wheels
225/35/18= 0.5% lower reading than standard wheels
215/40/18= 2.8% lower reading than standard wheels