At low speeds, the time between pulses becomes more important than the Number of pulses per second.

That value is easiest to measure with the CCP module in "Capture" mode, using Timer1.

This way, the minimum refresh time for the speed value is only limited to the time between pulses, instead of a number of pulses across a fixed time base.

The low end can be extended by counting an Upper WORD for Timer1, giving a result with a resolution proportional to the circumference of the tires.

Not easy stuff, but PULSIN or COUNT won't do it.
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