A bit of a poser?

[snakey]

I have a 750 sport as well as my Monster, the sport has 16 inch rims and i have eventually found tires to fit it, they are Avon Azzaros, these are a lot higher profile than the original Michelin's making the outside diameter of the tire the same as the monster, my problem is the speedo drive on the sport has packed up and being meant for the original tire size probably would not be very accurate with the higher profile tires fitted, I cannot get hold of an original one but can get one for a 17 inch wheel, which measuring the monster tires is the same outside diameter as the new tires that are on the sport. to my thinking, I should be able to fit the available speedo drive to the smaller rimmed wheel and get a relatively accurate speed reading , or is there something else that i should be taking into consideration that I've missed - the overall outside diameter on both tires is approximately 23 inches

any thoughts would be welcome
Pete

[Eeyore]

Mark the tyres with chalk, line up with a mark on the floor and then roll for three complete revolutions and mark the floor again, divide the result by three, this gives an average circumference, allowing for tyre and floor uneven-ness. repeat for the other tyre and compare.
This is how we check lorry tyre sizes/tacho calibration at the roadside.

[luckyskua]

I use an Avon Azaro 150/80ZR16 for the rear tyre and an Avon Vipersport 130/70ZR16 for the front. The speedo reads something like an indicated 42mph vs an actual 44mph - very close, as measured by those roadside 'check your speed' signs. I am not sure about trying a 17" wheel in the front because the tyre can contact the horizontal cylinder valve cover during braking, and whats the point with the super 16" CBR900 rubber thats available?