seen as youve done all hese maths youve forgot to notice that it will give more strain on the engine having it higher geared so that will prob knock a bit off aswell, bearings and other transmission parts could be looser or tighter than the stock car you based your whole estimate on, no way of getting it exactly right but if you think of every possible little thing then you could get close but still not perfect
so get a radar gun or venom speed meter to see how fast you really do go, only real option tbh