I have the sizes for the original zak slicks.

they are listed as

fronts 10.0/225x16
rears 12.5/27.5x19

I just need to work out the overall diameter of the tyre.

at first I thought the 27.5 number meant 27.5" total diameter but in that case wtf does the 225 number mean for the fronts ? if its section height in mm its 9" lol

 

There's a few online calculators but the tyre size will differ slightly to road tyres as the width is measured differently.

 

both width numbers seem to be in inches, and therefore 225 should mean 22.5 inch outer diameter

If you get a side view of a car with these tyres, you can easily compare the size of the fronts and rears.

 

ahhh I didn't think of that thanks mate

blimey 22.5 diameter front tyres, tiny !