Solution 1: Make sure that your baby is comfortable. Are the straps too tight? Obviously, you don't want them to be too loose, but make sure that they aren't so tight that they become painful for your child. Is her clothing bunched up? Loose, bunching, or pinching clothing can also cause your baby to hate riding in the car. Consider putting your child in a onesie or loose-fitting clothing so she doesn't feel constricted and has some breathing room for her belly. Does he have enough support? If this might be the problem, try placing some additional cushioning around him in the seat, such as two small rolled-up blankets on either side. Is she too hot or too cold? After spending nine or so months in a perfect climate, a temperature that is even the tiniest bit too hot or too cold can be extremely upsetting for your child. Try climbing in the backseat yourself to check -- it may be hotter or colder in the back seat than in the driver's seat.
Solution 2: Time your trip carefully (if possible). Your baby's discomfort over a wet diaper or a little bit of gas can be a problem all on its own, and traveling in the car can make it worse. Timing your tips just right can make all the difference in the world. Try different "routines" and pay attention to what does and doesn't work. For example, you can try feeding him, burping him, and then rocking him until he drifts off a little bit before strapping him into the car seat, and see if that works.
Solution 3: Keep your baby entertained. There are so many toys and gadgets on the market now that are designed for this exact purpose -- keeping your baby entertained in the car. Take advantage of this! You usually can't go wrong with a toy that has lots of lights, colors, and sounds. You could also try attaching a mirror to the back of the seat, since many babies love to look at their reflection. However, keep in mind that some babies need something to calm them down in the car, not keep them entertained. If this is the case, you could try buying a toy that makes soothing sounds (such as ocean waves, heartbeats, or music) or putting a picture of your happy and smiling face in a place where your baby can see it easily. You might feel a little bit silly with a large picture of yourself in the backseat, but seeing your face may be just what your baby needs to calm down.
Solution 4: Calm your baby with music or white noise.This one goes along with some of the things mentioned in the last tip. Trying out different sounds while you drive may help your baby calm down. Just as loud noises at home (such as the vacuum cleaner) can be oddly soothing for some fussy babies, loud noises in the car can have the same calming effect. Try turning up the music, tuning in to a static radio channel for the steady noise, or playing a CD of soft music or lullabies. Talking and singing can also help to reassure your baby that you are still there. Your baby wants to make sure you are still there, especially when she can't see you.
Solution 5: Talk to your pediatrician. If you've tried all of these tips and tricks and are still struggling with a miserable baby during every care ride, it might be time to call in reinforcements and have a serious talk with your pediatrician. This can be especially important if your baby is having trouble breathing or experiences other physical symptoms such as vomiting that might be caused or worsened by riding in the car. Medical conditions like reflux or other tummy upsets might be the source of the problem. If your baby is particularly fussy on curvy roads, motion sickness might be another possibility. Don't let the car get too hot or stuffy; a cool breeze can help, so try cracking a window open. No crazy race-car driving, especially around the curves. Motion sickness can come and grow and your baby will usually outgrow it naturally. Your doctor might be able to help by providing medicine for persistent motion sickness, especially on long trips. Try traveling at night, and make sure the baby's stomach isn't too full or too empty.
Information taken from: http://www.circleofmoms.com/article/5-solutions-babies-who-cry-car-03773