Good Day! I have a problem regarding my.

Good Day! I have a problem regarding my speedometer, it seems to have an offset reading after I replace my car with a new set of tires (from 185/60/14 to 205/40/17). At first, I notice some changes in my speed (I think my car slowed down a bit) I just ignore it but I was alarmed when I was apprehended for overspeeding at 123 KPH. Since then I keep on looking at my speed and I base it on the RPM register after 5th gear (60 KPH at 2000 RPM is it right?). It run for a while but I notice another change in my speed this time when I replace my muffler (from stock to 2.5 inch). Notice that I'm running at 90 KPH (approx) but the speedometer reads only 60 KPH, When I ran slow at 30 KPH, my speedometer shows no speed reading. Could be the speedometer gauge is the malfunctioning? Or is there any other particular part or factors that can affect the offset reading on my speedometer (can the tire change and muffler replacement do affect?), Thanks a lot!