Maruti Suzuki India — car is not going beyond 80 km speed

I have bought a new swift dzire tour model car a month back its a own board car not an yellow board. The problem is that our car is not going beyond 80km speed and after that its slows down gradually. The dealer where we bought the car has not provided proper information about the speed limit issue to us in detail. Later coming to know about this problem i was shocked we are unhappy because i liked maruti car very much then any other cars. One more thing we have found and confirmed that speed limit for yellow board cars only but we have bought it for our own purpose (Registered our car in white board) where we have put into trouble like yellow boards. Kindly provide alternative solution and procedure to sort it out. Reply me plz...