NCT Controller described a loss of wake separation event on the SFO final when after issuing a go around; radar data indicated the separation was less than required prior to the go around instruction; contrary to PRM Data.

Narrative

Aircraft Y; a B757; was following Aircraft X a Heavy B767 on final. Winds were a factor and aircraft were slow to slow. Aircraft X was inside the final approach fix and the ground speed dropped to 110. This was 20-30 KTS slower than all other aircraft had shown inside the marker. My attention was on the two aircraft the whole time. I slowed Aircraft Y down to avoid a go around situation. The Controller I relieved was on his two minute overlap and was monitoring the PRM scope and counting down the separation and advising me of the 1 second update on speeds and separation. I canceled the approach clearance on Aircraft Y; climbed him to 4;000 via runway heading; in which I thought was a timely manner according to the PRM readout. The go around was reviewed by the supervisor performing OMIC duties and found that I only had 4.84 miles between the aircraft when Aircraft X read back the clearance. I was never less than 1;000 FT from the preceding aircraft; but when on an instrument approach procedure; wake turbulence cannot be compromised. I have advised my supervisor that there may be a discrepancy between the PRM readout on mileage and/or time. He agrees and is looking into it. Be that as it may; I accept responsibility for running them too tight and will give a little more of a buffer when dealing with overtakes in the future; while running things more conservatively. Also; I could have canceled Aircraft Y's clearance earlier and stepped them down on the ILS behind the heavy jet until I was sure the spacing was/was not there.