A measurement of the partial width ratio using a method which tags the decays through the lif etime of the produced heavy hadrons is presented. This method relies on the tracking precision afforded by a double-sided silicon vertex detector. The tag algorithm makes a probabilistic interpretation of three-dimensional track impact parameters, using the data to measure the resolution. By tagging the two b hadrons separately, both and the tag efficiency can be determined from the data. For a 26% efficiency of tagging a single b hadron within the vertex detector solid angle coverage, a purity of 96% is achieved. A value of is found. Combining this result with other recent ALEPH measurements gives a 95% confidence upper limit on the Standard Model top mass of Mt < 228 GeV.