hi,
I dont think this is a good idea at all.
We need only a vague idea of speed and not pin point accuracy what will you do with that extra decimals on your dash board.Your idea is very hazardous remember few things:-

1)Never make your vehicle dash borad fancy at least if you want to return home in good condition.

2)Accurate speed in decimal points will
do no good to your driving.

3)It takes more of your reflexes to read numbers then a needle.

wishing you a very happy and safe driving.

sanjeev