well sir i have one corola 1996 gli 1.6 litre manual.. and one se saloon 2005 automatic....... both were on petrol til recently when i converted gli in to cng. before getting on to cng definatley manual had an advantage over automatic gears slightly........ both cars were 1600 cc but u can push each gear more in manual......... as far as milage is concerned my gli 1.6 litre gave me about 11km/litre (in city lahore) with range varied from 10.5km to 11.5
on highway it use to give me max of 13.5km to litre
where as se salon again 1.6litre engine but with automatic gears gives me consumption of 9 to 10km/litre in city with ac .........on highway its touches around 14km/litre......
so in city there is advantage of roughly 1 to 1.5 km litre on petrol..... but am ready to pay it considering the trafiic situation in city lahore.........now comes the cng part i have recently converted my gli to cng 3 days ago so i have not much experience in it definatley there is reduction in pick of about 15 to 20% specaily noticeable with ac on specaily in second gear after that its ok........
automatic car has already little less pick than manual.. so getting it on cng wil decrese the pick by same percenatge........donot compare automatic cars pick with manual car pick .......if u can manage changing gears in rushy traffic and u seriously want pick then change gear ........ but if the trafiic is same as in lahore just get some tunning and adjustment of ur cng kit. and enjoy automatic gear there is nothin like that........
the diference of milage on petrol is not more than 1 to 1.5 km/litre....... and one more thing i donto know about cng but as far as petrol is concerned efi cars like ur borthers 1997gli 1.6 litre has much more better fuel consumtion than carborator driven cars i gues ur car is not efi....1.6litre efi engine can give better fuel economy than 1.3 litre non efi engine........