Hi there, It may surprise, but I'm thinking a little over 160mph. In 1978 in the final of the Palm Springs tournament he was clocked at 153mph. Now it is clear if you have been around for a while in tennis, that the speed guns have been clocking faster and faster over the last 15 years. McEnroe at 50 serves in the 125's, when he struggled to get past 120 at his peak in the early/mid 1980's. Back in the early 90's Boris Beckers top speed was on 131. Sampras serve until the mid-90's never broke 130. Now he regularly does it at the age of 38. I also remember that in the early 90's when Roscoe was around 40 he could still serve a 135 to 140 on occasion. So I can easily see Tanner clocking over 160 if he was at his peak now with the current speed guns and the best modern rackets. Phenomenal yes? Still faster than anyone alive. He wasn't a giant either - 6ft to 6ft 1"