Fred Rednor linked us to the Conti website:
http://www.conti-online.com/generator/www/de/en/continental/bicycle/them es/tips/dimension/tubulars_en.html (or http://tinyurl.com/24o6xw )
Where I noted the following equation in the "Remarks" column: 27" = 28"
Isn't is a crazy, mixed-up world we live in?
Mark BulgierSeattle WA USA