Don't speed - Simples
For anyone who is employed, the HMRC now know exactly what you earn via a system known as RTI (Real Time Information) Lie to any government department at your peril! In the olden days, none of the government departments talked to each other. They are getting pretty shit hot at sharing information these days so not really worth telling porkies. I suppose if you are self employed (in the true sense, as in not a Director of a Ltd. company) you may get away with fibbing, but if you are claiming tax credits or claiming anything based on your earnings, you will be losing in the long run.
Murph - Slow the heck down you silly fudge