Consult a friendly dealer or an on line furum specific to the car. Modern speedos can be adjustable and it could be yours was set wrongly.

But a cambelt change will make no difference. You'd have to re-program the speedo or perhaps the ECU.

makes me wonder of the odometer is showing a too high mileage figure too?

My C5 speedometer shows 7 miles below real speed since I had the cambelt changed in October last year. 3 GPS systems agree with actual speed and radars that show your speed too. I recently made a long trip through France were before the sped cameras they had radars that advise your actual speed. I noticed that I was always over the speed limit, (even though it showed the right speed on the clock for the road) and that the difference between my speedometer and my 2 GPS was about 7 to 8 miles difference. This is not the usual 10% difference were the speedometer shows more than you are doing. It actually shows less than I am doing. Before I had the cambelt changed the revs were just below 2000rpm at 62 mph in 6th gear and now it shows just over at the same speed that's why I know I'm actually going faster.

Don't take too much notice of the actual speed displayed on the advisory speed dispalys in France, I have found that they can be wildly innacurate, both over and under reading.You should note that the under the French system allowances for error are only 5 kmh up to 100 kmh and 5% thereafter, so if you are doing 106 kmh in a 100 kmh limit you risk getting nicked.