By law, speedometers in vehicles must not under-read the actual speed at all, but they are allowed to over-read. In the UK I believe it is up to 10%, other countries may have slightly different rules.
Therefore, most vehicle manufacturers deliberately set their equipment to OVER-read by about 5%.
The TomTom will only update it's speed readings around once a second, so it can be 'caught out' for a short while if you are accelerating or decelerating strongly. It will also be slightly less accurate if you are on a very steep gradient or going round a tight bend.
But in most normal driving, it will be remarkably accurate.
(p.s. Topic moved from "introductions" to "General")
While you're at it, please check to see if the tires on your Toyota are factory spec size or not. 10% is an uncommonly large error, and if reflected in the odometer as well, is cheating you out of a fair bit of warranty mileage.