Increasing MAX speed of car

Better stability and better cusion.

Asheesh, its just not about the above two factors. Underinflation results in higher rolling friction, thereby making the car spend more effort in just keeping it moving forward. At the speeds you are interested in, even small amounts of residual power can work wonders. So the objective should be saving those small amounts from the general setup of your car wherever it is possible.

So if your posted results are with the 24 PSI, you certainly need to pump them up for getting closer to the speeds you want to achieve.

Krishna i agree, have changed tyres to 155R60 12" , also have increased tyre pressure to 26 PSI. Had a word with my works manager , he still insists 24 PSI is good enough.

Since past 4 yrs car has been serviced only in Mumbai dealership i suspect changing things without perfect knowledge could make things bad.

Also works manager suggested pull the car to atleast 80KMPH in 2nd gear and after that i can feel an increase in Pick up (now i drive as usual). Is it a mindset or really it helps pulling car in low gear at higher speeds?

Asheesh, what the works manager has suggested is essentially redlining the engine - taking it close to 5k+ RPM, which is not done in normal course of use. This will result in a very high rate of flow of fuel through the engine (cleaning?), as well as will exercise it so that it kind of 'loosens up' its moving parts, thereby performing better after this exercise.

While doing it only once in a while it is not very scientific and i would not recommend it. As I am not all that an expert (in fact i'm not an expert at all ) lets also hear from the Gyani Jan about this too, before concluding on the issue.

In normal course of operation, though, you should change gears whenever you reach peak torque. I checked up at marutizen.com, but it only has estilo specs at the site. There the peak torque is @3500 RPM. Check your owners manual, it may be different for your buddhi zen. So change gears to the higher ones at or just before 3500 RPM for best acceleration and good FE while accelerating. Of course, while cruising, use 1.5 to  2x of your idling RPM in whatever gear is required for your target speed.

Krishna my Zen might be buddhi but it can put a shame on some of the young cars who charge something 3-4 times the Zen.

Yes, she is quite some atheletic, in-shape  zen. You've maintained her very well (from what I've read in the thread). The buddhi remark was only for age - the specs will change between models, you know.

Was wonderering 12 inch low profile tyres might be the reason for car vibrating at higher speed as every road disorder comes to steering.

Increasing the rim size will help ?