The last time I measured lane stripes, decades ago in a land far away when I was more bored, they exceeded 50 feet.
It is very unsafe to tailgate, especially a truck. DON'T DO IT! It is not an approved hypermiling technique.
I used the super scientific counting methodology. One one-thousand, two one-thousand, three one-thousand..... The gaps should be 30' and the strips are 10'. I counted a number of times using different markers on the side of the road and I always counted the same. I didn't bother trying to use a stopwatch though. The point is, this is a pretty big gap.
I was drafting a big rig once, about 6 lengths behind at least, at 70 mph, while incidentally another big rig came up and started drafting me. Then the rig in front of me threw a mud flap. I saw it coming, and saw the other rig on my tail, and did NOT apply my brakes to somehow avoid the flap. It was flying horizontally, like a frisbee. It careened off my hood where it put a nice crease, and removed my right front parking light assembly. Nice clean job. I just had to keep going. FWIW.
I learned my lesson in 1983. Returning from 2 yrs in England, I picked up my Dodge 2.2 Charge (medium gray metallic) and drove it home to Missouri. Passing through St Louis, too close behind a truck, I caught a face full of shredding tire. I left black marks all over the hood and windshield. I got out all but one streak, which stayed with the car the entire time I had it. Never could get it out. I've seen mattresses, shovels, chairs, ladders, cows, deer, dogs, and even pidgeons appear as if from nowhere. The best safety classes I had were the regular and advanced Motorcycle Safety Foundation riders classes. Stay clear, know what's around you 360 degrees, and don't fixate on any one thing. Staying clear being the relevant item for this thread.
Once a long long time ago (1970s) I was driving on the highway in my 1962 Ford Falcon. There was a Corvette in front of me which looked like it dropped two bolts from under the car. Well, it was like a surreal experience as I watched these two bolts come towards me in perfect unison bouncing along the road. They took about two or three good bounces and bounced right over my car. I can still replay it in my head. It's a good thing there was nobody behind me!
Drafting leaving aside safety issues, can still reduce your MPG. If you need to make an sudden stop, all that accumulate MPG increase is gone in a second. Being far enough back to anticipate problems and avoid braking to get out of them might well improve trip MPG, which is, of course, more important than instantaneous MPG.
Something similar: Driving past a golf course on a beautiful day, I noticed these golfers..... looking at me! Then I picked up the golf ball hanging in the air. Only it wasn't hanging. It was b-lining for me. Slowly it dropped, then pinggggginnnggg, it hit the road 30 feet in front of me, & popped over my car. I looked at the golfers, who were still looking at me, except for one fellow who decided to turn his back to me. Give you a guess who hit the ball....... ////////////////////// Felipe Massa, formula one driver, was following another racing car, which lost a small spring. The spring clubbed Massa in the helmet, destroying the helmet & would have killed Massa without it. To return to formula one, Massa had to have a titanium plate put in his head to strengthen his skull.