While I can do this for 4km, I can't for different values like 9km. I've done the multiply-by-1.6 thing often enough by now to be fast enough at it, so I'll likely be sticking to it. This is a cool trick nevertheless.
9 is 8 + 1, so that's 8->13 + 1*1.6 = 14.6
Or any other combination, but using a higher Fibonacci number is going to be more accurate than combining smaller ones.
Or in this case, as it's only 1 off a Fibonacci number you can convert that one and add 1.6 without multiplying it by anything (multiplying by the 1 off).
Because knowing a km is 3/5 of a mile is more useful.
It follows that a mile is 5/3 of a km. From there it's basic math. 4 * 5 is 20. 20 / 3 is 6.666...
3 miles ~=5km
1 mile = 1.6km
So 5km + 1.6km ~= 6.6km