Good morning DrPete, We have a 2020 Plus Six which runs the 18" speedster "pepperpot" wheels. Interestingly, I prefer the design of the speedster 18" wheel to the multi spoked 19" variant. We did not like the 19" wheel as we wanted the larger sidewall tyre you get with the smaller 18" wheel, this helps with poor road surfaces. The roads in our area are just plain terrible and as we also run a Volvo XC90 with 22" alloy rims/ low profile tyres I did not want a repeat of the hassle to constantly consider pothole damage.
I am surprised your dealer said the 18" wheel will not fit?, this used to be standard fitment on the Plus Six and I was not aware of changes to the Plus Six wings since it launched? The Morgan configurator still shows them as available for new builds?.
