Buy a Car That Encourages Good Posture
Good posture is important for preventing back pain. This is particularly true when sitting in the same position for long periods of time, i.e., when driving. We just discussed how to position your body and adjust your seat to maximize support; now, we're going to look at some of the features in cars for which you should be looking to ensure good posture.
Make sure that the seat has independent height and tilt controls, preferably electronic ones that are easy to reach. The base should adequately support the thighs and not be too long or too short and be wider than the hips and thighs.
The back rest should reach the shoulders without obstructing your view and be wide enough to give your shoulder adequate support. The back rest should support the entire length of your back, preferably with an adjustable lumbar support with up/down and in out/adjustments.
The steering wheel should be adjustable and positioned centrally to avoid spine rotation.
The pedals should also be centered, and your feet should be able to reach the pedals without stretching.

Comments
I would think that a car which encourages good posture is no car at all.. perhaps an alternative to consider would be ... a bike! The activity encourages blood circulation, and the exertion strengthens back muscles, along with many others. And, being able to serenely roll past grid-locked commuters works wonders to lower stress levels.
I do not understand how people can hope for good posture in the most all of the cars, some which have deep curvature to their seats, and almost all force heads forward with the headrests necessary topass the collision tests. I have to take my Toyota head rest off and put it on backwards in order that I can sit up straight at an135 degree angle.