Speedometer not displaying values.

My opinion is exactly the opposite. Using interrupts introduces a lot of design issues and complexities and should be avoided as far as possible,

I'd agree with you, except that OP is using interrupts - timer based ones. David's suggestion, as I read it, was to junk the timer based polling, and trigger an interrupt when the sensor is activated.

On the other hand, if the pulses are slow enough, neither interrupt scenario is needed.