I'm a smidge under 6'5 and I drive 40 minutes to work every day. I've done close to 18000 miles in the last 12 months. No back problems. And I get 45mpg from my Focus C-Max (blegh). Previously, I've owned a Pug 205. Had no back problems with that either. I regularly drive my better half's Ka, and the only problem I have there is down to the seat being too hard.