Yesterday I was riding my bicycle on the road. I was in a headwind. Riding 12-13 mph. A spray vehicle passed me. These things are like big insects. They sit high to get around in a field with mature corn in it. The sprayer was like the one in the photo but didn't have as big of a spray head as in the photo
Sprayers - John Deere
The bottom of the sprayer is about 6 feet off the ground the tires are fairly narrow to fit between corn rows.
Anyway as soon as it passed at 25-30 mph I got a terrific draft and got up to 21 mph for a couple of hundred yards.
The point is that there is a huge amount of interference drag below the vehicle. It was almost as if the sprayer was filling up the entire space to the ground. As if it was semi truck.