I just did a unit conversion of the OP comment. 500 miles = 800km. The 3.5 is to convert weight in kg to lb, then convert miles to km to match the unitless numb

I just did a unit conversion of the OP comment. 500 miles = 800km. The 3.5 is to convert weight in kg to lb, then convert miles to km to match the unitless number in the original (2.2*1.6 is ~3.5)... the units don't make any sense, but it's just a rule of thumb I don't necessarily agree with the rule of thumb, I was just converting it into units I could understand and thought I would share it with the community. They do line up approximately with injury-avoidance running shoe wisdom (replace every 400 to 600 km), so it seems plausible. I think it would depend a lot on the individual (i.e. I hike in trail running shoes, so I should replace more frequently than someone who uses heavier duty boots - might just want to replace insoles occasionally?)