Hi Marc,

The benefit of "drafting" is the reduction in relative wind speed  
lessens the load on the engine.

I have a cylinder head temp gauage on my aircooled baywindow and have  
been driving a LOT of highway miles in the past few weeks. When I can  
get behind a semi at a range of 3-4 carlengths at 60MPH I see a drop  
of about 50F on my head temps within 20-30 seconds, every time.

The advantage of the wind break (reduced HP requirement from the  
motor) definitely compensates for the reduction of cooling capacity in  
the reduced wind speed. If there's enough wind to keep my aircooled  
engine happy then I can't see it being a problem for the waterboxers.

Another plus:
Over two recent 4hr runs I documented a considerable fuel economy  
improvement, achieving 23MPG on a vehicle that would normally run in  
the 18-20 range.

It helps a lot that Ontario recently mandated the use of speed  
limiters on all trucks keeping them to a maximum of 110kph. It's a lot  
less stressful driving on the bigger highways now that there are no  
more big rigs bearing down on you at 140KPH (85MPH)... In the past I  
found it hard to stay with a truck going faster than 115 KPH.

The downside of drafting is the reduced reaction time available in  
case of something unexpected. It's not something to be done lightly.

Greg

Quoting Marc Perdue <mcperdue@GMAIL.COM>:

> Regarding running behind semis, it was my understanding that doing so
> would affect the airflow across the cooling system and make it more
> likely to overheat.  Given the Vanagon's "natural" tendency to do so
> on a good day (joking), this doesn't seem like a good idea.  Having
> said that, I suspect that this effect would be fairly minimal over a
> short period of time, less desirable to do over a great distance.
>