Tailgating a lorry/semi (modelled in CFD)
 

I conducted a little experiment this evening to see the aerodynamic effect of driving close to a large vehicle and where a good position to slipstream is.

The red/orange zone is where the air is at lower velocity and the idea would be to get the car in that zone so that the car is entering air at approx 20mph instead of 60mph (less resistance).

http://i4.photobucket.com/albums/y14...comparison.jpg

Looks like around 1 car length is the place to be at 60mph, however painful death is a risk and the grim reaper has to occupy your passenger seat.
http://i4.photobucket.com/albums/y14...experiment.jpg

It's a great way to wind up wearing the semi's rear bumper as a new facial implant. It'll look real stylish in a coffin...

Seriously, those trucks can stop a lot faster than you would think they can, and your reaction time is generally a lot more than 1 car length at 60 MPH.

Saving fuel is one thing, but doing it in an unsafe manner is entirely another.

-soD

That's an excellent representation of how close you need to draft to make any significant improvements in fuel mileage. Indeed, if anyone was contemplating this as an actual method to practice, there is next to zero reaction time for braking or obstacle avoidance.

decapitation
 

Jane Mansfield lost her head this way.If you've grown tired of your cranium I recommend it.

Yeah I probably should have added a disclaimer on there somewhere. I was just experimenting since I have the modeling software and the skills to use it. You can see the yellow section goes further back and would still allow some fuel saving while mitigating the risk of painful death.

Deja vu over again, deja vu over again...http://www.youtube.com/watch?v=lttgT1XZVvE

...or go to 8:45 mark.

That's an urban legend...it was either her wig came off, or a portion of her scalp, not her whole head Jayne Mansfield - Wikipedia, the free encyclopedia

Head, wig, scalp, doesn't matter if your dead.

Notice in the second simulation the drag appears worse than in the first one.

Can you give us a % of Cd improvement at 1, 2 and 3 seconds behind the truck at 45, 60 and 75 mph? :)

Viio, could you do another round of simulations, with the truck's wheels removed? There is always a portion of air flowing under the truck which would slightly alter the wake's shape.

(Not that I'm going to be reading any truck's tags with my nose, just curious about the advantages of staying at 2-3 car lengths.)