I started doing a little research on VGs after I started noticing them on a number of large trucks. Seems they are most useful in getting airflow to smooth out when crossing some kind of gap, like that between the back of a truck's cab and the front of it's trailer. But I did find an interesting video on the subject and some pictures that may help understand their effects and applications better. I think that I'll probably be adding some to my towcar. Just a matter of deciding the best type to try and the best place to put them. Since I go through an entire tank on a daily basis I should be able to provide a fair bit of practical use data on them.