Could be either. Start with the simple and check the wiring a connector for the speed sensor (which is located on top of the gearbox towards the rear of the crankcase).

You can check the sensor signal in the dash's diagnostic mode. Once in DIAG mode you will need to scroll through and select code 07. Raise the rear wheel off the ground and rotate it and check that the numbers on the display increase from zero at a steady and even rate. The display will go from 0 to 199 and back to 0 before the cycle starts over again. Any uneven intervals between pulses (counts), fluctuations or no readings at all, then the sensor will need replacing. It is not difficult to do and I don't think the part is that expensive either.

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