I presume that they come calibrated for an 18" rear wheel.
calibration screw accessible from the outside of the gauge for those of us with 19" rear wheels?
With respect, surely you know by now the diameter of the wheel rim
makes only a limited difference to the diameter of the complete wheel with tyre?
You're talking about an (A65?) 18" rear wheel with a 4.00 tyre and a triple's 19" rear wheel with a 4.10 tyre. There's absolutely bugger-all difference in overall
diameters; what use would a "calibration screw accessible from the outside of the gauge" be?