Some months ago, someone posted a simulation showing how much difference wind could make in Prius fuel economy. I can't find it now, but here's an anecdotal report. Tuesday night, the south fringe of the giant storm--no precip, but temperature of 23 degrees F, headwind reported about 25 mph, buffeting the car around pretty good--I drove about 22 miles on I-40 (after about a 10 minute warm up), maybe a slight elevation loss, at my usual 72 mph or so. Indicated MPG 39.8, meaning about 37.4 real. Today (21 degrees F), another 22 mile drive on I-40 (after about a 9 minute warm up), maybe a slight elevation gain, at 72 mph or so, the big difference being very little wind: Indicated MPH 46.9, about 44.5 real. That's about 7 mpg difference that can reasonably be attributed mostly to wind.
I can believe it. Every now and then, I will have a head wind communing and can see the instant FE gauge go up and down with the gusts!
Sorry to sound stupid but does it really make all that difference to follow a semi or another large vehicle?
Yes. Their draft zone can be quite large and you can still get some benefit while keeping a safe distance. If you're not on cruise control, you may feel a "pull" once you get into the draft zone because now that there's less air resistance, the amount of power you're requesting to move forward is now enough accelerate further.
Winter time drivers beware though. You may get a sheet/chunk of ice through your windshield from the top of the trailer. I've seen rigs that got an over night accumulation in New Mexico that didn't come off until they got to us two states away. Personally, I never drive behind a big rig unless I have to temporarily. I want to know what's down the road and they're hard to see around plus those tires pick up and throw rocks etc. I never thought the mpg increase was worth the risk.
You could almost think of the headwind as equivalent to driving faster from a fuel mileage point of view. If you were facing a direct 15 mph head wind while driving 70 mph, I assume it would be the rough equivalent of driving 85 mph. Obviously crosswinds are another thing entirely (taking in vectors and such that don't make me miss elementary functions/triganometry). About the only thing you can do is slow down a bit to lessen the hit. Drafting behind a large moving brick (big rig) does make a big difference. I've experienced drafting from cycling as well as swimming, and it makes a huge difference.
