Delhi to Ahmedabad in 13 hours, I'd fun!

Oh really? I didn't know that. I thought I can drive at 100 and plus, and car will perform nicely. And will keep on performing. But since you are cautioning me, I shall take care.

I used to fill ‘Speed’ petrol, but then I read some threads on ‘Carwale’ on which petrol to use, so I switched back to 40+ a litre fuel from 42+; but this change is after this DEL – AMD trip. Your comment on the better performance with the lesser RPM in the fifth gear, is because of the ‘Speed’? Or some other reason?  

The car would perform nicely even above 100 or more.  I guess Krishna meant to say that it is better to drive at 80 for various reasons he has explained & also discussed by others. The added advantage of putting less burden on the engine (speed/rpm) is that it will help longevity (vis-a-vis a car that is always driven at higher speeds).

Longevity... ummm... only very very slightly, I admit. But I'm not outright wrong there... (heck, allow me one small point in a long post man... )

Kabir, lesser rpm is not always most efficient, particularly when accelerating...

In fact best engine efficiency is when you are accelerating with wide open throttle (WOT is the geek term for it) in the range of ~ 3k RPM.  However, when cruising, unfortunately you have to use the same engine that you use for accelerating. These two things are very different in their requirements and you would need two very different engines if you want peak efficiency in both  conditions - for accelerating your regular engine, and for cruising a very small teeny engine (~20 HP? maybe less!) as the only power required is to counteract tyre friction and aero loss.

So if you keep your throttle wide open when cruising, you are burning more fuel than required. But if you close it, the engine slows down, dragging the vehicle with it. To avoid this it can be put in neutral when cruising, thereby disconnecting it from the wheels.

Maximum efficiency therefore is obtained by speeding to 90 kmph, shifting through gears between 2500 to 3000 rpm and then shifting to neutral, coasting down to say 70 kmph. When the car hits 70, jam back your gear stick to 5th, and accelerate to 90 back again. This technique is called pulse-and-glide, and can give ~20% more fuel efficiency than normal, without changing anything else.

There is a discussion already on speed/normal petrol. Search and go through. I won't divulge the result, so that you go through the thread

(1) So that means when I was cruising at 145 kmph and 3500 RPM - the rated PS of D’Zire petrol of 87@6000 - if I do a pro-rata calculation I was getting ~51PS. Am I right?

(2) Since Delhi has lot of flyovers, I generally put the gear in neutral when about to reach the top of descent on that overpass so as to use gravity to go down, but never tried this pulse-and-glide thing. I am planning to go to Chintpurni and Shimla soon, I shall try this.

(3) The result? O.....o..... I thought ‘Speed’ won’t give you speed, mileage, and all those that they claim. So stopped using it in spite of the Bharat Petroleum’s recent advertisements on the reduced price, I now fills the normal petrol. But now I certainly search that thread to know the outcome.

Have a look at this thread - go through the discussions from page #7 onwards.

I am planning to go to Ahmedabad by car from Noida in the 3rd week of December with my family. I also plan to include a night stop as my son is 2 years old and a long car trip will be trying for him.

I have a Hyundai Accent 2008 model which has tube tyres. Would you recommend shifting to tubeless tyres.

Any other recommendation/accessories for the car to avoid back pain?