There is no such thing as perfect..... everything has a tolerance therefore your tacho will not be exact, however the tolerance is somewhat smaller to that used by car manufacturers.....
Your speedo is not connected tot he gears it is directly connected to the output sahft to the diff and is directly proprtional to the speed of the wheels going around...
Another point, as your tyres pick up speed and the wind goes under or over your car the rolling radius can change quite dramatically, look at the rear tyres on a dragster "rail" car as they spin they grow in diameter.
Most tyre manudfacturers quote wheel revs with a speed, ie wrpm at 30mph = 1080 etc....
Edited by mra-minis.co.uk, 11 May 2011 - 11:43 AM.
