No idea how much it saves but I always hit the button to turn it off when I get in the car (UK cars have it set to default position on, need to get it coded off) along with the lock door button... It may not hurt the starter motor but I'm too old fashioned perhaps thinking it will, added to that I figure its a V8 so why try and save fuel.
__________________
///M3 | E90 LCI | Space Grey | Fox Red with CF Structure | 6MT | 260M | My build
|