My MT4, after a quick spin around the city this evening. Its currently set up with Michelin Country Rock 1.75 tyres front and rear, so it's quite quick on tarmac. I'm planning to return it to fully rigid, as I managed to salvage a catalogue correct Raleigh cromoly stem from another bike, and I've got some X Lite bar ends to add too.
I was a bit bored during a Zoom meeting a few days ago, so I took some measurements of the M Trax Ti 3000 frame I've got in the spare room at the moment. It turns out the frame has amazing amounts of clearance, you could fit a 26x2.5" tyre in there comfortably. There's also loads of vertical clearance, and there's space for a 650bx 2.0 tyre, and even a 700c x 38-40 mm. Obviously, the brake posts are in the wrong place for 650b or 700c rims so it's all academic. Even with a 650b rim the brake posts would need to be 12.5 mm further up the seat stay (or you'd need to find cantilevers with lots of room to adjust the pad height).
The things you do when bored in a meeting. ..