A full set consists of 4 x T42 bogie, 2 x T201, 2 x T202 driving wheels and 6 x T31 tender wheels.

Suitable for Gauge One Fine or Standard.

DRIVING WHEELS: The scale diameter for 6' 8" at 10 mm/ft is 66.7 mm.
The G1MRA 'Dee' book recommends that the wheels are turned undersize at the tread (at 65 mm) to compensate for oversize model flanges.

65 mm = 6' 6" diameter represents 1" of tyre wear, about two thirds of what would normally be allowed.

I suggest that you make up your own mind what to do taking into account the particular depth of flange you are using and also whether you want the tread to retain its 'ex works' appearance.

T31 4' 0" wheel tyre looks thick because it also suits the 4' 1" GER tenders with more rim overlap (and hence the same internal diameter).