See how to sit comfortably and healthily from the back experts BackCare.org. ┬áBut to maintain this sitting position you’ll need a SHOFT fitted because if you don’t you’ll probably end up slipping down an inch or 2… putting strain onto your protective back muscles…

http://www.backcare.org.uk/wp-content/uploads/2015/02/Driving-and-Back-Pain-Factsheet.pdf

good vs bad posture with lap belt highlighted