What type of speedo do you have? When I bought my Westfield (the first time) the speedo read very high. It's a Smith's mech unit, like MGB.
Dave Bean put me onto a shop on the west coast --maybe Palo Alto Speedo--
they asked how many turns the cable did in a given distance, and re-geared the unit to suit
So, if this service is still available, get the tires you want, and match the speedo to them
also, back in "the day", a shop could build you a correction gearbox that went in the cable