Speedometer not accurate?

[Bruce Wayne]

I imagine 110% and +4 can only really truly happen at one speed. You can have your speedo read 110% faster, or you can have it read 4 MPH faster. But only at one specific crossover point, will both be true. Am I thinking about this correctly?

at 30... 33 (110%) + 4 = 37

at 40... 44 (110%) + 4 = 48

at 50... 55 (110%) + 4 = 59

at 60... 66 (110%) + 4 = 70

at 70... 77 (110%) + 4 = 81

My Porsche speedometer over reads by close to 10%, at higher speeds the accuracy reduces, but the error seems linear. IE at 30 MPH indicated my GPS speed is 27 MPH and at 75 speedo indicated, the GPS speed is 70 MPH

Generally, the police will leave you alone on the UK M-way's at 80 MPH, so at 85MPH (speedo) i stay under the 10% error margin.

speeds high than that, no comment