It’s a fact that your car’s speedometer will not be accurate. Tyre size and wear play a big part. Fit bigger tyres to your car, your speedo will read slow. Even what pressure your tyres are inflated at, make a difference.
Now Australian Design Rules allow a +/- 10% tolerance in the speed that your speedometer reads. The folks who issue you tickets do not. So the key is to know how (in)accurate your speedo is.
I use GPS Speedo, as it’s free, and works on Windows Mobile.
With our Honda CR-V, it shows our speedo reads 6 km/h under. So if I drive at 86 km/h, my actual speed is 80 km/h. And I pass all the other traffic which thinks they are driving at 80 km/h.
Of course I don’t tell the my partner this, as she then will think “Oh, I can drive a 90 km/h and not get a ticket.”. And then wonders “Why did I get a ticket for speeding?”.