Dear Vinod,

In order to estimate the margins for the tachometer, take a sheet of paper and pencil and
make some calculations. Here is an example for 1/T measuring.
First, let say that we have a sensor, that receives one pulse per revolution:

wheel sensor
/-
( ( ]-----------|Int0
-/

So for max speed of 60000 rpm=60000/60=1000Hz we will have an interrupt every 1 ms.

Let's have a standard 8031 with 12Mhz clock and free running Timer0.
For 1 ms T0 will count 1000 ticks (T=1000)
In order to calculate rpms, we should do the following:
X=C/T, where C=[Clock] / [divider] * [60 seconds in minute] / [number of pulses per revolution],
C=12000000/12*60/1=60000000

Increasing rpm's there will be an increasing calculation error,
because we are counting integer ticks.
For 65536 prm we will count 915 ticks. C/915=65573.77 - 37.77 rpm error.

Now, what about the bottom margin. Lets assume Xmin=20 rpm, i.e. interrupt every 3 seconds.
T0 should count 3E6 pulses=2DC6C0 - obviously we should expand it in software to 24 bits.

From this you should be able to estimate the limits for your case.
About the sensor no one knows better than you your mechanics. What will you count -
blades, some hole(s) on the axis etc. For some designs
optical sensor will be the only solution, somewhere small inductive switch is
the easiest way.
Regards,
Fedia