Specs for a 2000 Integra GSR:

Front:
Head room: 38.9"
Leg room: 42.2"
Shoulder room: 52"
Hip room 50.7"

Rear:
Head room: 36"
Leg room: 32.7"
Shoulder room: 50.3"
Hip room: 49.9"

I'm about 5' 10" and have plenty of room. My buddy is about 6' and is very close to the ceiling. Don't even think about sitting upright in the rear. I can't even do that. I do have a sunroof.

FWIW