well there is no doubt that this Arrangement saves space but dont be fooled by the pictures. its doesnt include the spare tyre that takes about 20 % of the space plus creates awkward space left I mean that you dont get whole chunk of rectangular space.
during my one year experience with exec GV I noted
94 kms with A/C city driving
123 kms max without A/C city
plus you need to get the cylinders filled earlier than 55kg big one
but at any given time I would prefer santro's arrangement over one big cylinder
p.s there is not market solution available becuase all cars that I know off have circular space for spare tyres while santro has square space to accomodate the cylinders though if someone has comeup with a solution without damaging the original body do let me know