How about just 1.5 + 0.1? I.e. the same amount plus half, plus a tenth?
So, 55 mi -> 55 + (25 + 2.5) + 5.5 -> 88km
And inversely, 0.5 + 0.1, so half plus a tenth:
80km -> 40 + 8 -> 48 miles
And for most purposes, just (one and a half = 1.5) and (half = 0.5) is close enough. You can mentally add a little more.
55mi -> 55 + (25 + 2.5) -> 82 (let's say around 85)
80km -> 40miles (let's say around 45 miles)
I've found that calculations are faster when you know when and when not to split calculations like the way you describe (even though at least 80% of the time it's faster to split them up).