wireless auto techometer

Please explain your reasons fully as this can affect the answer. Also, do you want digital or analog output display? What RPM range do you need to cover?

All details as you know them are needed.

i have gone through some magnetic pick up based RPM measurement , encoder based speed measurement , but not in automobiles , you have not given any closer explainations

Depends on what you want to measure.

There will be a switch to select the number of cylinders/rotors.

The pulse could be taken by a white line on the main pulley, or magnetic pickup on the crank/main pulley or flywheel or on the spark lead, there could be some batteries, a timing circuit like a LM555 for some timebase if needed.

So, what more are you looking for? there are various types, do you have 1 that your curious about or you have heard about them and are curious?