Are you looking to fly a flag from your trucks, or to have flag decals on your trucks? Most F.D's here in Chicagoland use actual U.S. flags that they secure to their rig, however some Depts use decals on both sides of the rig.
Your flag choice is limited to nylon based materials, not cotton. The grommets that come on the flags work just fine for your application. You just have to fabricate your own flag pole. My fire station routinely has "highway speed" winds, for hours... and hours...
We stopped flying them per our chief's request. Guys were bringing in heavy weight flags and zip tying a pole to the rear grab handle. If you use your apparatus alot, they don't last long.
Why did we stop. We had a bunch of comments from veterans saying that the flags being displayed on our trucks in a dirty, grimy, tattered or weathered state were disrespectful to the flag and what they had fought for. One went on to say that they should only be displayed in that state, if it went through an actual battle.
If I were you, I'd go with what Mike has shown and FETC has said. There are so many pictures showing vehicles adorned with your National Flag, and so many of them show dirty ragged flags. A dirty, ragged National flag is a disgrace, an insult to the flag and the nation. And take note, it's not my National Flag we're talking about, but I still see the insult.
Please go for the decal option, much easier to keep clean.
take a look at this web site we fly these poles on all the rigs in orange county fire dept about 95 pumpers they fly up top of the rig and don't get black like if you fly them off the back http://flagspring.com