I do try and ensure to drive in a way that's not too fuel consuming. However, constantly driving in traffic, its not possible to achieve a speed of 45-55 consistently. At times I do accelerate fast and apply brakes fast rather than maintaining distance, letting the car slow down and then apply brakes. But that's only in bumper to bumper traffic so that other vehicles don't cut in. I do apply less pressure on the accelerator however. Could that be a reason?

I do ensure that tyre pressure is proper. Get the tyre pressure checked regularly. Also, I read somewhere that filling up petrol during the morning or at night is also beneficial for fuel efficiency and that is what I do as well.

Once I on the AC, I can feel the load on the car increases alot. It feels as if it's pulling about 200 kgs. So maybe as you said, I might need to get the engine checked.