since installing the scangauge, i noticed sometimes drafting seemed to have vurtually no affect at any safe distance, and sometimes i could pick up a draft at a great distance.
i'm thinking its the wind, drafting seems to work extremely well when the wind is calm or near calm. naturally with a side wind i would expect drafting to be less effective. but it just seems like if theres much wind at all regardless of direction drafting is more often a waste of time.
anyone else had similar or contrary observations or ideas?
Don't know about in practice but I'll have a poke at the theory:
A tail wind would basically low the relative speed between you and the air you're moving through and therefore make drafting less effective (since drag is proportional to speed). A cross wind would move the lower pressure zone from directly behind the vehicle to somewhere diagonally on either side (meaning you may have to be in the adjacent lane to affectively draft). A head wind would make drafting more important/useful as relative speed is increased.
This is going to be totally dependant on what you consider to be a safe distance. What I feel safe travelling at, and what my car is capable of safely travelling at, is going to be different than your case.
That being said, even at distances that I would consider safe, I have seen unbelievable gains. This was driving my parents Lexus 400h, and I saw a jump from 25-30mpg to 40-45mpg. If only the truckers would use their cruise control, or pick a more stable speed.....
When I draft I tend to keep back a ways. I see a large variation also and tend generally to agree with your observations. The effectiveness of drafting seems to depend on the shape of the lead vehicle too. Rounded corners reduce the draft.
One of my best drafts was behind a pickup that was set up with a huge open back shell to protect something that was not there. It wanted to suck me in if I got too close. Only lasted a few miles though.
I sometimes draft 30+ miles behind a truck commuting on highway 101. I'm not sure if a scangauge will register lower gas consumption if you are drafting, but I can honestly say I get about 8-10mpg more in my VX.
Before drafting the best I can get was around 48mpg. After drafting, 52-56mpg easily. Before these results by yours truly, I would think otherwise about drafting (eg. you ain't gonna get more than 1 mpg drafting trucks son!).