Finding out current vehicle speed?

Hi! So how you see from the title i want to how to get the speed that a vehicle is driving. As tires I use simple cylinders with hinges and a basic VehicleSeat for the driver seat. I want to make a script which would increase the MaxSpeed on reached the old MaxSpeed. I just need to know how to find out the current vehicle speed now...

Thanks for reading. I hope you could help me out.