Motorists misjudge bicycle speed This question is probably a little too location-specific but I hope it attracts some good answers
Okay, here we go...
Most cities in India do not have dedicated paths/lanes for bicycles 
We are expected to ride our bikes on the roads sharing space with a variety of vehicles
(Except for expressways, where two-wheeled/three-wheeled vehicles are prohibited anyway)

In my experience, most motorists are pretty good at estimating my speed when I cruise under 25 kmph
But I often cruise at 35 kmph, occasionally touching 40 kmph for short distances

Some motorists, especially people riding motorbikes, cut me off quite close when they overtake me from behind
Sometimes, motorists attempt to overtake me and realize that I am moving at only a slightly lesser speed than them, after driving in parallel with me for several metres

This is even more common on roads which are 2-lane undivided carriageways 

The other scenario equally common is oncoming traffic, where one vehicle is trying to overtake another assuming they can finish the maneuver before they cross me

I solve this problem by occupying the entire lane and slowing down as necessary
The difference is that both the oncoming traffic and i can see each other and we can make each other aware of our intentions

The question is how can I warn the drivers intending to overtake me, that I am moving at faster speeds than an average commuter?

It is interesting to note that I get more attention and consequently more space when riding with a helmet and bright coloured t-shirts on weekends but not when I am riding with normal work clothes

PS: I finally decided to post this question after increased vehicular traffic in my area made these kind of encounters almost a daily affair
Please feel free to edit/comment for clarification
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.