Microbit speedometer

I am using my microbic V2 to control a vehicle. I need to measure the speed of the vehicle. I know there is no GPS, but I can use the accelerometer to get approximate speed. But I don’t know how. I looked up on the internet and found the formula: v = a * t * 0.036
The problem is that I don’t know with the t (time), when it should run continuously and show the speed or did I get the completely wrong formula?