Apparently, the Japanese version of the Nissan GTR has software that limits the vehicle's maximum speed unless it is near a Nissan approved racetrack.

Source: http://en.wikipedia.org/wiki/Nissan_GT-R#Aftermarket_tuning

I don't know about your country, but in some countries there are private race tracks where you can speed legally for a few thousand bucks.

And in Germany you can drive as fast as you want on the Autobahn.

Seconed - in the US, motor clubs often rent out racetracks for High Performance Driver Education (HPDE) days; costs around $500 for a few hours of track time in your own vehicle, optionally with an instructor. It's a lot of fun and educational indeed.

I work in racing, not many people turn up with stock road cars / bikes.

> I have always wondered why one is able to sell a car that can go over 100mph when the maximum speed allowed on the roads in my country is 70mph and being caught going 100mph will land you in jail

What if you are rich and you have your own private road where you can drive at any speed you want.

Then you build yourself a not licenced for the road race car - just like so many other people who race. You don't even have to be close to rich to do that.

> Then you build yourself

Or you could just buy any car which you like and not waste time building one.

Because gearing that will let you go 100mph will also let you go 70mph at lower RPMs, leading to better gas milage and lower carbon emissions.

Modern cars are basically computers on wheels. It's not that hard to implement a hard speed limit.