Rob -- Have replaced tires twice on my TR6 (after purchasing redlines
from outside sources) and have had no objections from local tire dealers
when I bring them to be mounted on my stock rims. Last time was two
weeks ago and the company doing the mounting was Olssen Tires here in
south Fla (Their main line in Firestone -- does this suggest anything?)
I mounted 215/65R15 tires on stock wheels with no tubes -- no problems.
If your new tire size has a total diameter width within 1/4 inch
of the old size, it will not affect your speedometer accuracy
significally, provided the speedo was calibrated to the old size.
Compare the two diameters.