Getting percentile of a combination of values

Question

I am working with daily speed data on R. So, I have separate columns for months, days, and their corresponding average vehicle speeds. So, for each month, there are multiple speeds (rows). I am trying to get the 85 percentile of speed for all possible combinations of months (taking 2/3/4 months at a time). So essentially, I am trying to retain all data rows that includes the months coming from those combinations. Say, all speeds corresponding to January and February combination, and so on. Functionality-wise, for example, getting the percentile of all rows of speed column for a subset of data based on the Month column that correspond to the different combinations of months (taken 2 or 3 at a time). I am absolutely clueless. Can you please help? Thanks much