Suppose Bob is travelling in a spaceship at uniform velocity of 150000000 m/s in Michael's frame of reference.
Michael sees the spaceship length contracted and it appears to be only 86.6 meters long. Michael also notices that the spaceship clock is running slow. For every 1.2 seconds on Michael's clock, the spaceship clock only registers 1 second.
If the observers are moving relative to each other, their "meters" and "seconds" are different. So, if Bob is told by Michael that the spaceship is moving at 150000000 m/s, Bob would not agree on this speed because Bob sees Michael's 'meters' length contracted and 'seconds' dilated. I hope I have it correct. Bob would convert the speed as follows: (150000000*1.15473(/(1*1.2) = 144341250 m/s
Yes, each observer would agree on the speed of light and perhaps that's why in almost every such problem velocities are stated as fractions of 'c' like 0.5c to avoid confusion. But the problem is that I don't understand how Bob and Michael could find such a 'light-operated speedometer' and how it would work, or any other practical method which would let them agree on the speed.