I've used this calculator for years just for shits&giggles..if you know the exact weight of your vehicle w you in it,plus fuel(in other words,race-ready) it's been pretty accurate for me.
I used it all the time when I was drag racing my bikes and it was always close to the dyno rwhp
Gives you a ballpark figure,and you can calculate drivetrain loss. It's not scientific by any means and YMMV
http://www.wallaceracing.com/et-hp-mph.php
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-'24 X5MC,'22 430i
-'09 Busa,'05 GSXR-1K(GT28R turbo)'00 Busa(GT3582)
Gone but not forgotten:
-'22 X3MC,'20 M550i,'18 X2,F30 328xi,'07 E63 AMG (1st W211 63 in the 11s),997 C4S Cab,CLS55 AMG,E55 AMG,C55AMG