How do I fix a sail to my van?

We at Liverpool Man and Van are always looking for ways to be more green, use less fossil fuels and help prevent global warming. Whilst driving through the rain, being nearly blown over by the wind, my helper, "lift it up johnny", came up with yet an other brain wave. 

Why don't we have sails on the van? It seems a silly idea, but they have sent men to the Moon so surely with a little computer help, an automatic sail system must be possible, Speeding down the  motorway when suddenly the automatic system set the mainsail, keeping the same speed but using less fuel?

Is this possible?