Draughting.
 

While travelling from Edinburgh today I had an artic tractor trailer (semi), with an enormous bulldozer on the back, join the city bypass just ahead of me. He was accelerating fairly easily up to 50 mph so I felt comfortable slipping in behind him. That way I wasn't the stopper in the bottle.
What I found was a sweet spot, a comfortable distance behind him, where my mpg suddenly improved from around 50 mpg up to about 70 mpg. He was making a huge hole in the air, and because of his size I could benefit from it without getting so close that I was tailgating. I was easily two/three seconds behind him and he was still allowing me to throttle back and gain the mpg.

If you think bulldozers are great, wait till you see a van trailer!

I have heard of that sweet spot behind big rigs. I follow generally a couple car lengths behind. Got tired of the vigilance, assholeness, and the risk of running closer behind trucks. Cool if it is also just as good, or nearly so.

I drafted every big rig I could when I was driving Ron Burgundy out on I-4 every day between Tampa and Orlando.

I can tell you from experience that I was normally way over a hundred feet behind the semis and still in the bubble. I never had to get into what I would consider an unsafe distance to get the extra mpg boost, so I hung back at the furthest distance that would give me the higher mpg on my instant mpg meter.

Worst part of drafting the big boys was the amount of pebbles, etc . . that kept
taking paint off the front of the car. And occasionally I had to dodge something in the road that the trucks could drive right over

I found I could drop back and drop back, then it was as if a switch had been thrown. The car lost steam and the mpg fell. I would then have to accelerate back into the bubble.

I'd have more patience for drafting if I had adaptive cruise control. Set that baby to a close follow and then just listen to podcasts.

draughting, mmmmm.....BEER!!!

100 feet is short. I take 50 meter distance (150 ft) as standard for the 50 - 55 mph semis over here.
Anything the semis tires kick up has already landed back on the tarmac before I run over it, so no pebble showers. Plus, you generally see stuff on the road before the truck runs over it if you are far back enough.
Once a truck I was drafting lost a pellet. It slid to the side of the road without ado, but I'm certain I would have avoided it or stopped in time if it came straight at me.

If you go faster the distance needs to grow too - squared with speed.
With no wind, or straight ahead or aft, the draft will extend that far anyway.

Golden rule: always be able to see at least one of the semis mirrors, so you know they know where you are. Hmm, beer.

Following the two second rule means about 150 ft at 50 mph so I would be at least that far back. With the width of the bulldozer, I'd have had to be about quarter of a mile back before he had a chance of seeing me in his mirrors!

I find following trucks (or busses) useful, to help keep me at a lower steady speed as most trucks are using cruise control.

As for the mirrors most drivers are pretty aware of what is behind them or just generally dont care. As both followers and the truck will 'wander. you will get seen.