Ball speed rarely exceeds about 1.5× clubhead speed, even on a perfect driver strike.
Smash factor (ball speed ÷ clubhead speed) is capped by the physics of the club-ball collision — 1.5 sits close to the legal-driver ceiling. Shorter, more lofted clubs run lower, around 1.1–1.4, since more energy goes into spin instead of ball speed.
That ceiling is for a driver, where the COR limit is 0.83. A wedge is nearer 1.2 because loft sends much of the energy into spin rather than ball speed, so a 'poor' smash factor with a wedge is simply what a wedge does.
At 0.83 the rule says 1.5× and the exact answer is 1.488× — 0.78% high. It holds to within 2% from 0.81 to 0.88, and drifts outside that.
1.5 is not a coincidence, it is (1+e) shared between a 200 g head and a 46 g ball at the legal COR limit of 0.83 — which works out at 1.49. The rule is really the rulebook: the governing bodies capped the face, and 1.5 is where that cap lands.
The rule against the exact answer, computed across the range. Inside the shaded band the shortcut is close enough to use; outside it, reach for the calculator.
Estimate with the rule, then check it against the calculator that models it properly.
Open Projectile Motion & Golf Ball Flight →Ball speed rarely exceeds about 1.5× clubhead speed, even on a perfect driver strike. Smash factor (ball speed ÷ clubhead speed) is capped by the physics of the club-ball collision — 1.5 sits close to the legal-driver ceiling.