Who's going to find out?
Yes. Get it fixed.
e; You should fail your MOT and therefore the vehicle would not be insurable. As it would fail the MOT it's not roadworthy due to all cars legally having to have a speedo with something like 20% accuracy or something like that.
what you gonna do if a pc decides to drive it a few yards up the road?
mot man will test for a speedo either on a rolling road,that he tests for brakes with or when he drives it into the garage
How can they possibly test a speedo during an MOT?
You should fail your MOT and therefore the vehicle would not be insurable.
It is not a legal requirement for a car to have a speedo. It is your responsibility to ensure you do not break the speed limit. It doesn't matter how you do this.
Speedos are not checked during an MOT test.