Hi, it could be difficult in the future. I understand from the British Historic Racing Facebook page that Goodyear have taken over Avon. It's said that they plan to close the Melksham factory and cease production of some tyres. One range that may not be available from 2024 is the AM23 range which is used extensively in historic motorcycle racing and by all the top racing Morgan threewheelers. So, there has got to be a doubt as to whether Goodyear will want to continue producing a low volume 130/90 x 20. Vintage cars do use 20" diameter wheels. It might be worth doing some research on Blockley's and Vintage Tyres' web sites. The issue might be that they are wider, starting at 5" or 5.5" wide.
