I have traveled from Indiana to South West Florida with 3 cats, a total of 1000 miles. Then from South West Florida to West Illinois after losing everything in the hurricanes with 9 cats in the car, a total of 1100 miles, WITHOUT problems.
First off, do NOT feed kitty 4-6 hours before leaving. Make sure the litter box is used before leaving. Make sure to place a soft towel or favorite blanket in the carrier / cage and a familiar toy. be sure to use the seatbelt shoulder strap to hold it in place incase of sudden stops.
The first 1/2 hour or so is the worst, until kitty figures out that its safe and going with mommy. Try not to pamper kitty too much if the meowing is bad, try your best to ignore the "protest" something as simple as your finger thru the gate so kitty can smell you is enough until he / she settles down. Make sure an air vent is providing a gentle breeze to kitty as well.
I've found that once things settle down kitty can be allowed out of the carrier if your trip is really long, but be prepared to make an emergecy stop to replace kitty to the carrier should it become too scared. (I've had as many as 6 out at once and all they did was lay in the back window, or passenger seat and watch the world go by.)
If your trip is over a couple hours long, make certain to stop and offer a small drink, and a chance to use the litter box. (a small one tucked behind the seat on the back floor is best) and NEVER NEVER open your doors or windows without first placing kitty back in its carrier. The last thing you want is to loose your baby at some rest stop in a moment of panic.
If your kitty becomes a little car sick at first, don't panic, just pull over and tidy things up without making kitty feel guilty or ashamed. Car sickness usually will happen in the first hour or so of travel.
Above all.. Cats will pick up YOUR emotional state while traveling. If you are calm and confident, kitty will reflect your mood and settle in to the new experience just fine...
Any questions ??? Contact me on Yahoo or AIM instant messengers screen name as posted in my profile ... thawks120