31.10.50 R15
The 31 is the tyre overall diameter in inchs
The 10.50 is the tyre width in inchs
The R means the tyre is radial constution
The 15 is the diameter of the wheel.
These dimensions are nominal figures, for instace a BFG MT 31.10.50 R15
http://www.bfgoodrichtires.com/specs/mud-terrain-t-a-km/1152.htmlThe actual tyre measures 30.8 dimater but 31 is the standard size it is sold as. In AT pattern the tyre diameter is reduced to 30.7 inches.
Remold tyres generall come a bit bigger than a new tyre due to the extra rubber added.
To put it into prespective though the differeance is about 1% at most so windup is not going to be noticeable on soft ground and on the road your going to be running in 2wd or centre diff unlocked (not sure on TLC 4wd system) anyway so its not going to be a problem on high grip surfaces.
One of the most important things is tyre carcus construction, a mix of radial and cross ply tyres on a vehicle is not safe. They should all be of the same type.