| I have a 10lb bottle and that is not considered big, still it would be near impossible to put it under a seat unless the seat is fairly high and you plan to have no cushion or any form of decent padding on it. 
I saw this in Japan while I was there and I am planning on doing the same thing. The bottle is placed in between the tunnel at the back. That is a 10lb bottle as well so as you can see its pretty big. If you drive a 2+2 I think its a great place to put it...saves you from buying a remote opener as well!
___________________________________________________________________________ There was a time where I wasn't sure whether the Z would be the right kinda car for me, that is till I saw this one morning...
 ...Yup. I drive a Z now. |