We have measured the branching ratio for the superallowed 0 + → 0 + β transition from 34 Ar to be 0.9448(8), and determined its f t value to be 3058.1(28) s, a result with ±0.09% precision, which is a factor of three improvement over the previous result based on current world data. The f t-value ratio for the mirror pair of superallowed transitions 34 Ar → 34 Cl and 34 Cl → 34 S, becomes the most precise yet measured and, in a sensitive test of the method used to calculate the isospin-symmetry-breaking correction, δC , it agrees well with the ratio as calculated with Woods-Saxon radial wave functions. This confirms the method used in the most recent survey of superallowed decays to extract V ud , the up-down quark-mixing element of the Cabibbo-Kobayashi-Maskawa matrix. In addition, our branching-ratio results for the four observed Gamow-Teller branches to 1 + states in 34 Cl are shown to agree well with shell-model calculations based on the same effective interactions that were used in the determination of δC .