Question

what decides the top speed of a car-torque or horsepower

Answer 1

Top speed and acceleration does not COMPLETELY depend on HP and torque. There's a lot more things you might want to consider :
1. The type of fuel - Diesel has a slower burning rate than petrol. This explains the lack of throttle responsiveness of diesel cars.
2. Type of transmission used. : An automatic transmission is built in a way such that it changes gears beyond a certain RPM. This RPM range in which gears shift changes depending on which mode you are driving in. The sports mode of your automatic will rev a bit higher than your normal'drive' mode. In a manual transmission though, you have the liberty of changing the gears at whatever RPM you feel like. This way, you can try to stay within the power band of your engine, thus aiming at a higher top speed a lot quicker.
3. Tires : If your car has a high amount of torque, and your tires are not up to the mark, you will end up in a burnout.

Theoretically, at 5252 rpm, the torque and HP become equal. So basically, up to 5252 rpm, Torque determines who's faster. Both in matters of top speed and acceleration. If your car revs much higher though, the HP will determine which is the faster car, provided other factors like transmission, tires etc doesn't change much.

The Swift and i10, both have their power band engine below 5000 rpm. So the higher torque will mean a higher top speed and better acceleration. That explains the swift having a higher top speed and a better acceleration.

Hope that answers your question. :)

Answer 2

The topmost speed of a car depends upon it's Horse Power.