vitz would give you 17 if you drive it VERY calmly and if the engine is running at 100%.
I'll suggest you get all the fluids (engine/gearbox/radiator/powersteering fluid) changed, get the intake cleaned, change plugs change all the filters (oil/fuel/air). Aslo items like belts should be checked and please ensure the tyres are no larger than 578mm (overall diameter) and that all unneeded weight is removed from the car (subwoofers, bodykits, extra items from the boot/trunk etc.). These small things will make a HUGE difference to the way your car drives overall and will also give ou peace of mindin terms of reliability.
Hopefully after all of this you should get approx 15-17 average unless you car is an auto in which case you should be getting 14-16 average.
Good luck and let me know if there is anythgn else we can help you with.
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