4-5mm on the 3.0. (something ridiculous like 12mm on a Surly Bud)

Hmm, thought it might have been more. Interesting!

Thread hijack – does anyone know of a 27.5+ tyre and rim combo that will fit the rear of an on-one inbred 29er?

A 2.35″ Rock Razor on an i35 rim does not fit in a Sherpa Mk 1

Going the other way, could one expect to just stick a 29″ wheel/tyre into a 650b+ and head happily into the sunset? Just wondering about deep mid winter when the mud gets really deep/thick

Depends on the tyre. A 29er tyre/rim will be taller than a B+ (could be 15mm or so) thereby raising the BB. You could also have seat tube/front mech clearance problems.

Thought it might fit a Banshee Prime but still no luck, looks like I’ll strip the rims and build them onto some fat hubs for my Bucksaw.

I’ll stick with my Instigator Rocket as my plus (hoonigan) bike.