TTL to Analogue display
I don't understand what you mean by "a pulse every 2.5mm (400 pulses per Metre)" However, I assume that you want to convert a pulse rate to an analogue value.
The way to do this is to use a monostable followed by a low pass filter.
The monostable generates a squarewave signal whose mark/space ratio is proportional to the pulse frequency. The low pass filter (a resistor and capacitor) creates an analogue voltage that is proportional to the average voltage of the square wave signal which is thus proportional to the input frequency. To scale the output in mph, all you need to do is to include an amplifier with the correct gain.
See the circuit I posted - Phase Detector, on 27 January 3.19 pm {Australian time (GMT + 10)} If you replace the XOR gate with a monostable, it is essentially what I described above.
There is also an IC that converts frequency to voltage LM2917, from memory, this does essentially what I described above.
Len