seem to remember where you said new tyres would make a difference to the speed of a truck[ yes i understand that] but do you know how much a new set over a well worn set would make. 
the reason i ask is one of ours runs at 54mph on the speedo, mine at a smidge over 55, mine used to pull away from it, now since hes had new tyres he pulls away from me. im thinking 2/3 mph difference , does that sound about right, i must admit im suprised it makes that much differance .thanks pete
I am unfortunatly not technically minded ,but due to the old tyres being smaller than new ones the transfer of power /speed increases with
the new tyres,someone more wiser ,polite word for intelligent
wil come hear and explain it much better,IT increases the speed normally by about 3-5mph , as you pointed out in your post, Hope that helps you out,
Being sad, and having nothing better to do one day, I found the dimensions of new tyres, and max tread depths, legal minimum tread depths, and i worked it out to be between 2 and 3 mph differnece between, max tread, and legal minimum tread.
However, when the tacho is calibrated, it should be done based on mid depth tyres.
you need in each Time the Radius from Road to middle of Axle,multipliat with 2,and multiplicat with 3.15.
now you have the lenght the wheel needs to turn once around.
one kilometer has 1000 meter,and you can count how often the wheel turns around to do one km
do it that way with a good worn tyre and with a new one and see the difference