Firstly is the drive; due to changes in suspension and minor tweaks in chasis, newer vitz handles brilliantly. The car has much more grip and suspension is perfectly balanced between softness and hardness. Moreover, the car has a sharper steering response, 2nd gen vitz has a numb steering with no feedback at all. Lastly new Vitz is perfect for bumpy roads i.e it absorbs bumps easily and has a better cushiony effect.
Speed and mileage wise 3rd gen vitz is better. It is more responsive and on highways it's really agile and good for overtaking cars. The mileage is also better with AC in city 3rd gen vitz does 13.5 km/L and older vitz does 12.5 km/L
However interior of 3rd gen vitz is a mixed bag. It is really spacious then second gen and has more features such as auto lights and multi information gauge cluster ( temperature, mileage, average speed etc ). However, 3rd gen vitz dashboard is less impressive then 2nd gen, it has barely any cabins plus the build quality is slightly inferior, there are also less cabin lights ( jewella has more lights ). But on positive side, 3rd gen is less noisy then 2nd gen.
Lastly, brakes of 3rd gen vitz are awesome. The car responds instantaneously. I am a fan of its brakes.
I hope this makes it clear for you
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