I use 13/43 gearing with the very tall Pirelli MT43 trials pattern tyre. Initially I used a Dunlop D606 120/90x18 which is lower in profile, and suits the WR very well, I think. I got 14,000 km out of mine before the the sidewall knobs started breaking out. All offroad use.
Gearing is higher for sure with the Pirelli, but still fine. I'll be going back to a Dunlop simply because the Pirelli makes the bike very tall at the back - and it alters the steering a bit, as well as making the sidestand too short.