Melssj5:
Its a 1979 car, the speedometer is driven by a rotating cable, So I must buy a device to transform that as pulses. 16 pulses per rotation..... But the question is, the faster the car moves, the faster the pulses will come. How can I be sure that I wont loss signals while reading ..... the pulse generator has its frecuency depending on the speed car, and the arduino loops in whoknows frecuency. What it pulses are generted faster than arduinos loop frecuency.
The Arduino runs at 16 MHz and should be able to count pulses up to 100 kHz without breaking a sweat (that's 375,000 rmp for your wire - 6,250 rotations a second). Likely your rotating wire explodes before the Arduino starts to think "oh, this is getting kinda fast".
Best way to detect such pulses is by using an interrupt. You won't miss any that way.
Then in your main loop() you just a few times a second how many pulses have been counted since the last time you checked, and calculate the speed from that.