Brother,
Cultus is simply the best from all aspects if compare with Alto.
Well just for the informations, the speed really does matter.
If you drive with the speed of 80~90 Km, every car will give you best mileage.
For example,
GLI will give 15~16 Km/Liter
XLI will give 16~18 Km/Liter
Cultus will give 17~18 Km/Liter
Mehran give 15~16 Km/Liter
VITZ will give 19~20 Km/Liter
Passo will give 20~21 Km/Liter
Zest will give 18~19 Km/Liter
Hyundai give 14~16 Km/Liter
Cuore will give 16~17 Km/Liter
CITY will give 17~18 Km/Liter
Belta will give 19~20 Km/Liter
Now, if you increase the speed upto 100/120 KM, then above given averages will come down upto 2~3 KM
Mileage depends upon the Engine’s Condition, Quality of the Engine & Gear Oils, etc.