Interviewer: "Write a Θ(log(n)) approximation algorithm for converting n miles to kilometers that's accurate within plus or minus .01*n kilometers."
There, did it in O(1)
In other words, your algorithm is asymptotically too fast and you failed the interview! :D
That said a fixed-factor mutiplication can probably be done faster AND [more] precisely as a sum of some shifts. Or many other ways with a lookup table.