I am trying to calculate the time expressed in hours and minutes based
on the average speed and distance travelled, but I'm having trouble
getting it to display correctly as e.g. 00:16 for 16 minutes.

For example I might have 16.8 as the miles travelled in cell A2 and 64
as the average Mph in cell B2. In cell C2 I have =A2/B2 which gives
0.2625. So I am trying to convert that decimal to h:mm. It should come
out something like 00:16. If anyone has any ideas on that I'd
appreciate it.

Since 0.2625 is the fraction of an hour, multiply it by 60 to get the number
of minutes... 15.75 where the .75 is fraction of a minute (so you apparently
will want to round this number to get your 16 answer).

Rick