Newer diesel with least problems

I never can tell when regeneration happen on my vehicles (sometimes the only indication is smell) but 1 time driving long distance I was resetting mpg display to observe how winds affect it.
So once the mpg dropped from 12 down to 9 mpg on 20 miles stretch. There was no noticeable difference in wind to explain such difference, so only conclusion was that I catch it on regeneration.