2.03.02 Rear wheel

From Sidecarwiki

15" rear wheel conversions are popular. These allow the fitment of a car tyre which will last much longer than the original bike tyre, the contact patch is also bigger. Wheels can be adapted by fitting a car rim to the original centre, or sometimes by using adapters to bolt a complete car wheel in place.

Depending on the size of the original tyre, a 15" car tyre conversion might also help to lower the overall gearing of the bike, this is a good thing as the bike fitted with a sidecar is now pulling much more weight than it originally was, lowering the gearing will reduce the load on the bike a bit.