Originally Posted by racertim
To draft or slow down?
To draft. I did three stripes back from the tractor trailer and he never got buggered and I was amazed by the difference--*and* it was raining, so hydroplaning wasn't helping, although he was plowing two tracks for me to follow in the rain. Understand I drive this stretch of the Mass Pike (I-90) on a regular basis. Coasting down any of the slight hills westbound in neutral has me gradually dropping speed and leveling out at about 50-55mph, 60mph for a short portion near the exit, but the following didn't happen there. So I'm drafting behind this truck at 3 stripes back--throw it in neutral--70mph... waiting.. waiting.. waiting... 70mnph.. waiting.. waiting.. 70mph.. waiting.. finally it starts to drop to 69 and then slowly to 65 at which point I put it back in gear to close the sight gap that's grown. I didn't time it, but this in neutral draft lasted about 30 seconds. I found it pretty remarkable and it confirmed my other experience I've mentioned a couple times on this forum drafting with the Prius. So it is my belief I'm using roughly the same amount of fuel at 70mph drafting three stripes back (roughly 125 feet) as compared to not drafting at 60mph. I wouldn't be surprised if I'm using less fuel drafting at 70mph. If I was in neutral at 60mph and not drafting that truck I would be steadily losing speed--at 70mph, it took about 20 seconds before I started to lose speed, and I didn't pulse right before putting it in neutral to create momentum--I was driving along at a steady rate and felt that I was being pulled so I put it in neutral expecting, at 70mph, the draft wouldn't be strong enough to hold me in place and I would slowly drop back. It sometimes felt like there was a sweet spot, as in this just mentioned example. I wonder if there is and how many feet variance it has. (say 110-140 feet)
All this is confirmed by the post Holycow referred to:
"notes from this mornings commute...
on a 2-3 mile stretch on a constant downward grade at ~68mph (via gps)
I noticed the MDF stated I was getting ~57mpg's in an open lane.
Behind a box truck with a 1.5 count I dropped to ~50 mpg's.
Behind the same truck with a 3 count I jumped up to 75-80mpg's and sustained that until I pulled out from behind it.
The grade was very constant and I was moving within the draft and keeping a specific distance and the exerted effort of the car was noticable.. but I tested this a few times, followed that truck for ~18 miles and noticed the 3 count was critical to making the drafting work; too close and it actually hurt performance slightly, too far back and it hurt alot.. that 3 count rule..
So, too close meant less effective draft for him.
So there you have it. 3 stripes the charm.