I don't convert anything myself. I enter the miles driven and liters filled into my fuel economy app which does the conversion when I specify US gallons on it.
I'm obsessive about tracking my mileage and economy because a deviation from this could indicate that something is wrong, as had been prior to restoration. Skepticism or not, currently all is good in my engine bay (attached)
I am in the process of renewing my W115 240D, detailed on this
thread. To date, best I can do is 30 mpg US highway, despite being a 4 spd manual and an OM616. Feel free to follow along as I try to get it to match or beat its younger sibling.