Are we kidding me???????? Don't know if you've made up your mind, November is still a couple of weeks ahead, so RETHINK a bit bro on your calculations.
Kindly note - and read my lips - Altis will bring your fuel expenses down, not up!!
If not by 5-7K's monthly as you have calculated in the reverse direction, at least by 2-3k's LESS per month in the right direction of your calculus!
As for CNG, well, I'm of the strong opinion, humbly, that Gop better pay back all investors in this biz - inclusive of all black kick-backs - and send these shareef people back home on pension or wish them best of luck with some other ventures, I can't really see how the heck they're gonna survive this biz, not all of them have the well located gold mines!!!
If any doubts on Altis fuel efficiency vs Gli, check my personally conducted review below and beat it with the relatively under-powered 88 Hp'ed GLI.
(My "Chevy" already failed, so best of luck here):
Anyhow, the Altis fuel efficiency, for the pleasure of your drive: