I had to buy an automatic car, manual transmission just killed me! Also I now have a 2007 car which is lighter to steer than my old 2001 car, it makes such a huge difference. The seat is higher up too so can get out of the car easier.
Sitting in chairs really hurts, the pain builds and builds till I just have to get up. Hard seats are the worst, also any seat that tilts backwards. I fidget alot and get up and down to help minimise pain. What helps me to sit for longer is having the feet up too, so using a foot rest or a second chair to have the legs up on really helps.
I am also a huge fan of memory foam, I have a little memory foam cushion which I use on the kitchen chairs as they are sooooo hard!
You are not alone, I think it is very common that people with AS cannot sit for long.
