I’m no expert but I’ve had 11 back operations, so I sort of know what I’m talking about. What works for me might not work for the next guy. It could be a sitting position and I can adjust my seat ever so slightly, forward, backward, tilt etc., you get the idea. What else helps is resting my left foot on dead peddle and using cruise control as much as possible thus allowing me to move my feet about and not continuously on the go peddle. What else really helped me was a wheelchair cushion. They are very thick and soft, exactly what a person in a wheelchair would need. I got mine from the VAMC and used it in my truck, sitting at the kitchen table (I had two) and even at the movies. For those who need more lumbar support there is a rolled-up cushion called the “Lumbar roll”. I used that everywhere, especially driving and at the movies. I drive a truck now because it’s so much easier getting “up” into the truck, rather than getting “down” into a car. I used to own a Corvette but had to get rid of it because it was hard to get into it and impossible to get out of it. I also found that "bucket seats" were more comfortable than "bench seats".
Amazon.com: The Original McKenzie Lumbar Roll - Firm Density - Low Back Support for Office Chairs and Car Seats : Home & Kitchen
And here : Amazon.com: LOVEHOME Lumbar Support Pillow for Chair and Car, Back Support for Office Chair Memory Foam Cushion with Mesh Cover for Back Pain Relief - Black : Home & Kitchen
Amazon.com : wheelchair cushions
Good luck and I hope these suggestions help.
In fact I just bought this new cushion today.
Amazon.com: Kolbs Gel Extreme | Wheelchair Cushion Seat Cushion | Coccyx, Sciatica & Tailbone Pain Relief Cushion | Seat Pad Ideal for Office, Kitchen Wheelchairs and Auto | 16 x 16 Inch Seat, 3 Inch Thick : Health & Household