Australian Design Rules (ADRs) allow a speedometer to have a tolerance of plus or minus 10 per cent, which means at an indicated 100 km/h you could be doing anywhere between 90 and 110 km/h although they are generally much more accurate than that. It could be that yours is on the low side of the tolerance, but it needs to be remembered that the speedo is a guide to your speed, absolute accuracy is not guaranteed. My advice would be to ignore what other drivers are doing, the law doesn't say you must drive at the speed limit, it's quite permissible, and acceptable, to drive below the limit.