ALTO has RUBBISH A/C even on PETROL. My cousin has got one and he drives it on PETROL but its A/C is simply rubbish.
Coure has also not a good A/C on CNG.
Chevy JOY (Never expierienced).
SANTRO simply ROCKS as concern of A/C with CNG. Also it has trunk space available with CNG. I hav one and i really enjoyed it.
VITZ is also knocking the door in this case. My friend has Automatic VITZ 2003 and believe me its A/C is awesome. But don't know about A/C with CNG.