PILLOWS PILLOWS PILLOWS! They are your FRIENDS!
I agree that you should gather the softest smooshiest ones you have, take them from your parents, siblings, everyone....everywhere....and have someone help tuck you in so you cant move when the car turns or goes over bumps. That's usually when I hurt the most when in a flare. Amazingly, I feel better when i'm the driver, cuz I feel more "in control" of myself for some reason. But as the passenger, I feel more "wobbly" like I have less control over my body, so pillows are awesome.
And....ifit's a really long drive, hopefully be nice and tired, and wear some earphones and listen to music annd hopefully you can sleep for most of the ride. Take pain meds about an hour before you get in the car, or do whatever it is that you do to make yourself feel a bit better, and get a tad more relief.