Temperatures inside a vehicle can easily exceed 100 degrees even with the windows cracked and the outside temperature well below. These temperature would be unbearable for a person, imagine how a dog must feel.
Dogs cool themselves primarily through panting. Trying to cool off by breathing in hot air in a car is not going to be easy, and ultimately can lead to heat stroke and even death for you dog. If you are going somewhere dogs are not allowed it is best to leave your dog at home where they will be more comfortable.
A lot of dogs love the outdoors. We have a dog that almost never wants to be inside. Be sure to leave your dog plenty of water should your dog be outside, especially when it is hot or warm. If you know that the temperature is going to be exceedingly hot and you are leaving for an extended period of time try to get your dog to come indoors while you are gone.
If you like to take your dog for a walk try to wait for later in the evening when the temperatures are cooler than in the middle of the day.
These are just a few tips that we follow with our dogs. You should always use your best judgement and if you have any questions always contact your local SPCA or humane society.