Best guess is ~145mph actual (corrected for speedo error) for that indicated 158.
Get a grip folks. I think Triumph got things pretty much right with the S3 gearing. It'll be right at redline in top gear, and it doesn't get better than that. Pump up the power a LOT (and 5hp won't buy you more than 1-2mph, so that $1000 pipe ain't sh&t), change the gearing a perfect amount to match the power boost, and you might get an honest 155 - but it'll cost you several thousand dollars.
LOL - my Aprilia Falco said I went 176mph. Sheeyeah, right.
AMA# 289558, COP#0001, BIR#47
'97 Trophy 3, '98 Tiger, '03 Speedie