Motorists misjudge bicycle speed
That's a hard question to answer because there is no real official way to solve this issue, and it's often up to the driver to estimate your speed, you can't really do anything about this.

One way to show motorists how fast you're going would be to highlight part of your bike like the wheels or the pedals, to actually highlight how fast they are spinning. Fast spinning wheels and pedals make you thing "This guy goes fast".

To do so you could try to wear yellow overshoes (I don't know if you can find these in India, we have these in France), and your feet will instantly pop out. You can also just put reflectors on your pedals or install colored pedals, but it's harder to notice.

For the wheels, you can also try to put reflectors on the spokes so that it gives it a reference point that people will see spinning. Some people just put stickers on the spokes though.

Finally, as you mentionned your gear can induce the fact you're going. If you look "legit" with a helmet and proper cycling gear, people will unconsciously admit the fact you are going fast. Wearing bright clothes will also help you being noticed by drivers.