For those following this, the BM2 ends up at a fraction under 25 inches with those fat tyres, and with 7mm of running clearance between tyres and centre section.
That's 130 tyre, 7mm gap, 90mm motor, 175 wide centre section, and then another 90, 7, and 130mm.
629mm or 24.76mm overall tyre edge to tyre edge.
Even with these on it...
It could be 12mm narrower (half inch) , but I allowed 7mm clearance room to add the 145 wide and even lower profile tyres at a later date if needed.
If you do the maths, it means that I could build a narrow powerchair for a small person, child, that used stock powerchair 3 inch wide x 14 diameter tyres, and with full group 24 batteries, and still end up just 20 inches wide!!! And 37 long inc footplates/anti tips.