Dear Fellas,
Car manufacturers have an approximate 5MPH/8KMPH variation to speed. The speed-o-meter is always showing more. That is because of certain rules in Europe and Americas to comply with and avoid litigation.
Secondly, car speed (odometer) is designed with transmission revs and stock tyre and Rim Size. The change in RIM and Tyre profile further affects speed (+ or -) (with a bigger diameter/radius, at the same revolution, car is covering more distance)
Third, as would be aware of it, tyre/rim size would have an impact on overall torque that can be exerted by engine. This further rips of some speed. I guess, bigger tyre would put more pressure on engine to generate same torque. As you force to execute same torque, your horsepower would decline. They always cut each other at the peak.
forth is the case of radar guns.
Mercedes/BMW/ other good cars have normally engine cuts at finely tuned levels. They keep everything considering safety (deployment speed of SRS/belts etc)
Please don't make it formula 1. your life is important for many people.you would never want your son to drive that fast, would you? We ain't fly rockets. its just a car, how much it can go to? 300KM max. so what. you are not going to earn any reward for it.
Thanks for understanding.