> > 4MHz PIC has an internal cycle time of 1uS 
> > So thats 5000Hz or 200uS between edges

If you're using IntRC, be aware it varies with Vcc and temperature

(In fact an 18F452 with R = 3k3 will swing between 600 - 
550MHz, peaking at !! 750MHz !! with just 2.5V Vcc ! Check
out Fig 23-14 of DS39564B. Oh, if only it did......)

There have been a few threads about RPM meters for car engines,
might be worth looking up

> If you're getting that much resolution, then throwing most of it away

If this were a frequency meter you might be switching between
two methods to get the best accuracy

- for low frequencies, measure the period directly
- for high frequencies, sample and average

'low' and 'high' are relative to the PIC's speed

If the 5000Hz is the average and you're happy to have a 1Hz
update, perhaps

- count all the pulses and calculate at 1Hz

- count with T0CKI and compare TMR0 rollover time with
another timer, using this as data for a moving average, updated
at around 20Hz (TMR0 = 8-bit) that's calculated and displayed
at 1Hz

Use maths (slower, probably better resolution) or, as Dave
suggested, a lookup table (faster, but can be a pain to set up
and change)

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