I've had several friends ask me about the ability of police to search their automobile when they get pulled over, no doubt worried about the cases of moonshine they keep stashed in the trunk or just curious about the extent of their constitutional rights.  Confusion in this area of law is rampant, and for good reason.  Automobile search law is a mess.  The Utah Supreme Court basically admits it in this case from 1990, saying "the fourth amendment, especially in the context of automobile searches, has been the source of much confusion among judges, lawyers, and police."

Can Police Have a Dog Sniff My Car Without My Permission?

Unfortunately, yes they can. The Supreme Court has held that a drug dog "sniff" is not a search, basically because they're not invading your privacy (unless the dog "alerts" and they do a search). However, there are some important limitations. You can't be detained against your will on the side of the road for a drug dog to sniff your car. It has to be done while you're being held for another legal reason. So if you're pulled over speeding, and one cop is getting your license and registration, and typing it into the computer, and getting a background check, and another cop is walking around your car with Ajax the drug dog at the same time, that's probably legal.