Hello, my name is ***** ***** I will try help you with this.
All speedometers have an inbuilt tolerance of 10% =- 2mph fast.
This means when you are showing thirty on a speedo, your actual speed can be 30 -10%(3) =- 2. So your speed can be anywhere between 25 and 29mph.
It can never read higher than your actual speed. If it did it would be faulty and illegal.
If you also fit larger wheels then you have to take into account the tyre profile. Suppose you currently have 225/60/15 and you fit 225/60/16 then top speed might be marginally increased but the torque would be reduced. This means acceleration would be worse.
It would also affect the calibration of the speedo and could make it illegal.
I assume you are reading the speed from a satnav at thirty and your speedo is showing about 35mph. This is normal.
It also actually protects you from ever getting a speeding ticket.
If you use the speedo to drive at thirty then you can never be going faster than thirty.
The same applies on motorways.
You can never be going faster than the speed indicated on the speedo.
If you rely on a satnav for speed indication then it is possible you could get a speeding ticket as satnavs don't have this factoring correction.
It could also land you with a ticket where they use average speed cameras over a set distance.
My advice would be to leave well alone.
If you have another vehicle you can check this with the satnav and you will see that it too will have some margin for error.
Providing the margin is within the prescribed band then the manufacturer will not accept it has a fault.