When Truro did her run,it was daytime - departure from Millbay was 09.23am. So Rous-Martin was NOT working by a paraffin lamp,trying to read mileposts in the dark!
By my own calculations,if Rous-Martin's timing of 80mph at the exit from Whiteball Tunnel was correct (and of course it is just as likely to be low as high), then provided Truro was providing enough power to overcome friction and wind resistance, gravity should have been enough to take her to over 100mph down the bank. Bear in mind that Rous-Marten's timings are average speeds and Truro was still accelerating until the brakes came on after the end of the last quarter mile.
So whilst 102.3mph may not be correct,there is no doubt in my mind that 100mph was achieved.