Drafting: stream of cars vs semi

[FLAteam]

I've been driving about 132 miles per day on a long stretch of highway for about a week now and I've noticed that drafting behind a stretch of normal consumer vehicles (cars, trucks, suvs) may be helpful for increasing fuel economy. Consumer cars are also generally going faster than semis so drafting has a larger effect. I've found that even being 3 seconds behind any vehicle is better than hitting the air alone.

Any thoughts on what is better for fuel economy? I admit that while driving behind other consumer vehicles I tend to pulse and glide more behind a stream of cars due to the large variation of speeds, which may attribute to the increase in fuel economy. I realize this will depend a lot on the type of car driven (larger cars probably have more to gain behind a semi) but if all cars were equal in size and drafting in both scenarios were at the same speed, would it be more beneficial to go behind a semi or a stream of cars?