It's annoying to drive down the road and suddenly find yourself riding over a road patch that includes tar. Once the tar is on your tire, it quickly picks up other debris such as gravel. Removing tar and gravel from your tire is usually an easy process, but it is very important to do it correctly, and follow a few precautions. Other than perhaps your brakes, the tires are the most important safety consideration on your car.
Scrape off tar with a plastic picnic knife. Use a plastic knife to avoid damaging (or puncturing) the rubber. You can use a screwdriver or other pointed object as well, but be extremely careful to keep it from coming into contact with the tire itself.
Scrub off tar with a stiff scrub brush, a little detergent and a lot of elbow grease.
Apply some linseed oil to the tire. Let the linseed oil seep into the tar for at least 15 minutes. Scrape off the tar with a plastic knife.
Apply a tar-removing product to your tire (see Resources and References for examples of these products). Check the instructions (or warnings) on the label to make sure the product is safe for removing tar from rubber.
Show some patience. Chances are the tar and gravel will fall off by themselves before long, especially if they are on (or near) the tyre's tread.
Even though they will remove the tar, do not use strong chemicals that could damage the tires. Flammables such as gasoline should also not be used because they could cause the tire to catch on fire.