Apologies, don't have the exact figures, but probably Alto has a better average because of small body / engine size. You can further confirm in their relevant threads.
But I wanted to ask, if you are going for a new Cultus AGS, why not a Swift? They are almost overlapping in the price department, but there is a day and night difference between them, be it the drive, comfort, acceleration (specially MT), handling / stability etc. Yes, the fuel economy is not like an Alto but I think it is comparable to Cultus, the CVT gives 15-16 with AC in city driving (not with the light foot), while on long - it can give you around 20-23 if you drive it between 100-120. Depends on the driving style tbh.
But yes, if strictly talking about Alto and Cultus, Alto would definitely give better fuel economy.