MTRs do come in a 31 inch.
Steve explained the imperial sizing. Yours are 265x70x15 right.
The 265 = 265mm across the tread
The 70 = 70% of the tread width, this is the sidewall hight from rim to outer edge.
15 = 15inch rim (go figure

)
Confused?????????
As for a suggestion............. 31x10.5x15 Firestone ATXs. Did 35000km on a set back in the boring days with the Surf, still had 10mm tread on them. Loved them.
