Do as Sewell suggested - practice. Another threader posted some time back to put plastic on the seat as it allowed you to swivel about more easily. As you practice, have a pillow or whatever is handy, between your legs. This will give you an idea as to what mobility, or lack thereof, you have, as you have to keep your hips even, moving them together as you swivel.
No doubt you will be brought to your vehicle in a wheelchair, so you might try using a kitchen chair in its place, and getting from that into your vehicle. Hand holds on the vehicle will be a big help. Mine doesn't have any, so I used the roof as support as I lowered myself into the seat.
If you find your vehicle too unhandy, you may have to beg the use of a friend's or family member to help out.
Another thing, make sure to take your pain meds about an hour or so before the trip; helps a whole lot.
I found the bumps to not be much of a problem, but going around curves did cause discomfort. After the first curve, I asked my driver to slow down around the curves, just ease into them and no more problems.
Keep asking if you have questions; we are here to help each other.
Best of luck with your upcoming surgery. I know you will be the better for it. I love my new hips and have absolutely no regrets at having the surgeries done. Each day is better and brighter.
Glenys
LHR 5/28/09
RHR 7/23/09