Weight of the car has nothing to do with high speed stability. Swift is even lighter than City. It's all about aerodynamics. Some super cars weigh less than 1500 kg, and they are still stable at 300+ kph speeds.
I suggest you do two things to improve stability.
First, change tires to anyone of the following.
Yokohama es32, Continental CC6, Dunlop SP Touring R1, Pirelli CINTURA P1, Bridgestone Nextry
I did a lot of research before shortlisting the above tires because I booked City for motorway driving and high speed stability is very important for me. I will change my City's tire once I receive the vehicle.
You don't need to change the OEM tire size. I experienced the improved stability with Alto after changing the tires to Bridgestone Nextry. I didn't change the OEM tire size, but still massive improve in stability on motorway. It's super stable at 120kph.
The second improvement you can do is use a lip spoiler. It will reduce both lift and drag at high speeds. This is how a lip spoiler look like
Don't use a wing. Wing on passenger vehicles is a big NO because of limited engine power. Wing is useful for performance cars or in motorsports application.
Just to clarify, this is a wing.
