Speedometer calibration with larger wheels???

Hey everyone,
I have a quick question. I recently put new wheels on my Ibiza GT TDI 110PS, which originally had 6x15 inch wheels with 185/55/15 tires. Now it has 7x17 inch wheels with 205/40/17 tires. Before, the car would normally reach speeds of 215-218 km/h on the highway, but now it only reaches 205 km/h. Is this because the wheels are larger, and is the speedometer showing a lower speed? How can I calculate the actual speed of the car? What is the cost of a speedometer calibration?
Thank you for your help!!
Monchi.

Hi,

On Rainer's homepage, you can find the "Ufschen" tire size calculator (by Ulf + Ernst). On the third page, you'll find a calculator for the rolling circumference. Calculate it for both tire sizes, and you'll have the difference between the tire sizes. Everything else comes from the increase in air pressure, respectively. Rolling resistance.