[antz.bin]

Originally Posted by carZest

thanks all for the replies. To clarify, I was driving alone with minimal luggage (one large suitcase and a baby pram).

@vinit: "So, even though it is a bigger engine, it still returns better FE between 80-110 then in the 120-160 range."
Hmm! So bigger engine size doesnt help too much.

The question at hand was which car will give the best FE at 140-150. The above only says that FE reduces when you drive faster, which is true for almost all cars. The proper answer to this is The engine which is inherently fuel efficient AND remains relatively unstressed at 150kmph will return the maximum FE. So theoretically, an Accord 2.4MT can give you the best FE at 150kmph. For a diesel, maybe an Audi A4 / BMW 320D

But then again, as dhawcash points out "BTW, i remember an episode of TopGear, where Jeremy compared the FE of a Toyota Prius and a BMW M3 at their track. the Prius was driven as fast as it would go, and the BMW just followed it. result? the BMW returned much better FE than a Prius in those conditions as it had a much bigger engine, which just cruised at those speeds- which the Prius gave everything it had to achieve."
Clearly a larger engine should do those speeds more easily. That should translate into lesser fuel burnt.

In the above case, the Prius was going flat - out and the Beemer was cruising relatively unstressed. Hence the final result.

@nitrous: "To maintain that 140-150 in the i10 (big task for it), you had to use a lot of rpm and probably downshifts - Formula for high fuel consumption.".
RPM yes, downshifts no. I don't think I downshifted from 5th in that entire hour or so. RPM, might b 4K or so. I was alone in the car, next time I will ask a co-passenger to look at the value.

@misquitas: "Just to add another query to this discussion on drag factor. In theory, which of the two highway scenarios would generate better FE on the I10:".
My experience has that with windows down the car will easily climb to 150 in 5th gear. Strangely at this sweet spot, switching the AC on-off has NO effect on its being able to do that speed. I do the same speed even with the AC on. But with windows down, the car will do that speed only after spending significant time in 4th. Am not sure how this translates to fuel-economy, but this is my observation.

There was another Top Gear episode when Jeremy took a car on the Motorway, rolled down the window with the car doing motorway speeds, and started throwing currency notes out of the window.

Such an act implied that contrary to proper wisdom, keeping the Air-Con ON on the Highway will in fact improve FE, because when you keep the windows rolled down, you effectively make your car less aerodynamic/streamlined and you waste fuel in overcoming this extra drag instead of making the car go further / faster i.e. you literally throw money out of the window.

@suhaas:
Mea Culpa! And I don't take the road for granted. Just that it happens sometimes!! All I can assure you was it wasn't with any adrenaline pumping. My usual quiet driving, rising sun, trucks pulling over for the morning cuppa/nap emptying the road even more, a lonely stretch from Dhone-Anantpur-Penukonda devoid of too many villages, car in fine kettle, hmm!

^^ Such a thing tends to happen many times because the kind of concentration required for an overnight drive, one tends to speed up when we see twilight approaching. Such an approach can get dangerous while on the road purely because twilight timings are the worst in terms of overall visibility. And a concentration lapse at such a moment coupled with "Adrenaline Pumping" + "Empty road" can lead to very serious consequences. The best thing to do at such a time is to take a 20-30 minute break and wait for it to become fully bright / dark(if driving in the evening).

Compare this scenario with a batsman about to complete his century. This is the time he will concentrate the most, try to get through the darkest part(the 90s). As soon has he hits the 100th run and the celebrations begin, thats the time he sees the light at the end of the horizon (twilight approaching). Thats the time even the strongest of concentrations can lapse and could lead to something very bad.

Other Answers In Line. Marked by Bold. (Is replying like this allowed in forum rules? Pardon the noob if I went wrong somewhere.)

P.S.: All I meant to say in the above is, there is no good time to speed up to 150-160 on a Normal Indian road (not an Expressway). If you still feel the need for speed, you could just head to MMSC / Buddh International Circuit and get it over with in a safe manner

[dhanushs]

Originally Posted by misquitas

which of the two highway scenarios would generate better FE on the I10:

No doubt, its the 70-90 without AC.

Originally Posted by carZest

Clearly a larger engine should do those speeds more easily.

Yes, very true. But, not at the speeds you mentioned.

Originally Posted by misquitas

(big task for it), you had to use a lot of rpm

Yes, I guess what nitrous meant was that, you have to be pedal-to-metal most times, which is bad for FE.

Originally Posted by misquitas

My experience has that with windows down the car will easily climb to 150 in 5th gear.

Windows down and i10 will easily climb to 150??!!.. .. I doubt it.

[nukeblitz]

Originally Posted by carZest

I own an i10 1.1 done 65K kms. 3 days ago I drove from Hyderabad to Bangalore, leaving Hyderabad at 2.30 am. I drove at speeds of 80 to 100 kmph for the first 3 hours. Mostly found long-distance truckers (HR, GJ, TN registrations) en route, making for consistent right-lane driving.

I was 100+ kms short of Anantpur (not sure of the exact odo reading) at 5.30 am when suddenly sunlight increased my visibility greatly, with no parallel increase in villagers, bikes, cattle etc. Essentially I had a 1 hour window where I increased my speed to 140-150 kmph (speedo reading) consistently. I had my A/C on all the time. Tank was near half-empty when I had refilled at BP COCO Gooty.

After maintaining these high speeds for an hour I saw that I might have burnt as much petrol in 1 hour as I had in 3 hours. This has happened many times before when I do these speeds. The engine is never strained and very easily climbs to 150 kmph. Maybe half its life has been on the highway so it has 'opened up' better. Again, to make it clear: I let the road (and all on it) choose my speed. For example, Mysore highway: never > 80 kmph etc. So I don't always do such speeds.

My question to the gurus was if a higher capacity engine/car give a better mileage at these speeds? Would, for example, the i10 1.2 (kappa/kappa2) give better mileage than my 1.1 at speeds >= 140 kmph? Or say the Ritz 1.2?

First off, I don't blame you for doing those speeds on that particular highway, even if I do think those speeds are unsafe. I do lots of Mysore-Hyderabad runs and that road is very tempting. But I just wanted you to know that I have had a couple of close calls with the locals there which made me reconsider any speed above 100kph. The closest one was when this little village girl runs across the road. There wasn't any village around either. Worst was that her parents were on the other side and were encouraging her to do this. Fortunately, I was cruising along at my usual 80kph and was able to swerve in time missing her by about 2 inches. If I was any faster I don't think I would have been able to avoid her. There also a lot of bullock carts and bikers going the wrong way. But I'll admit tho, there are a few sections with the most incredible range of visibility and they are hard to resist...


FE, as far as I know, depends on mainly on throttle position. This is because the ECU injects fuel into the engine depending on the amount of air coming into the engine to ensure complete combustion.

So, to give an example, going uphill at 50% throttle in 3rd at 3000 rpm will consume more fuel than going on a flat road at 25% throttle in 3rd at 3000 rpm.

So, to get the maximum FE, try and use the gas pedal as little as possible. That also means no lugging as trying to get a car accelerate from 50 in 5th will require more throttle than trying to get a car to accelerate from 50 in 4th.

When you are at high speeds, the wind and tyre resistance are high - which means you'll have to use the throttle even more to overcome these resistances - ergo more fuel consumption.


I'm sure my deductions are only one side of the coin. Can anyone else elaborate on the details between fuel consumptions? Specifically, how does rpm affect fuel consumption?
I'll give an example.
25% throttle, 80kph, 2500rpm, 4th gear
vs
25% throttle, 80kph, 2000rpm, 5th gear

Notice that only rpm and gear changes here. How is FE affected? What's the reasoning behind it?

[SCORPION]

Originally Posted by carZest

My question to the gurus was if a higher capacity engine/car give a better mileage at these speeds? Would, for example, the i10 1.2 (kappa/kappa2) give better mileage than my 1.1 at speeds >= 140 kmph? Or say the Ritz 1.2?

IMO, every car has an RPM range within which the car is tuned to perform its best, and this includes FE as well. This is the range where the torque and power are most usable. Most cars have a sticker saying what speeds you should be doing to obtain best FE, in each gear, which in effect, is the same rpm range.
Larger engines, inherently consume more fuel, depending on their state of tune. So you cannot compare 2 engines with a large capacity difference directly. A larger engine will give you relatively better fuel efficiency at higher speeds than a smaller one as it generates enough torque/power to do those speeds at a much lower rpm (state of tune, gearing, vehicle weight, aerodynamics, etc are not being considered here.)
However, adding an additional gear (6th) will give you better fuel efficiency with the same engine.

[mayankjha1806]

Originally Posted by carZest

My experience has that with windows down the car will easily climb to 150 in 5th gear. Strangely at this sweet spot, switching the AC on-off has NO effect on its being able to do that speed. I do the same speed even with the AC on. But with windows down, the car will do that speed only after spending significant time in 4th. Am not sure how this translates to fuel-economy, but this is my observation.

Windows down does translate into fuel economy. On highways my car gives 15+ if i drive around 100-120, but with windows down this figure also goes down as the air flow across the car makes it difficult to engine (A car with closed windows cuts through the air).

To look at this a little differently, when i drive with windows rolled up there is usually a need to switch on the AC as it becomes hot inside (especially when the Sun is out). So whatever advantage i get with windows rolled up gets negated by switching on the AC.

On our recent trip i had to drive back from Chennai-Bangalore at night and it was pleasantly cold, hence no need to switch on the AC and i was able to drive all the way with windows closed (well, open them once in 5 mins just to let the stale air out and fresh air in). The fuel efficiency i got was 15+ again.

FIAT EcoDrive recommends optimum speed of 90 to 95 kmph for best FE. Anything above will increase the fuel consumption exponentially.

[Sheel]

3,000 revs for a petrol Car and 2,000 revs for diesel Car in top-gear with windows rolled-up will give you good FE with decent cruising speed.

Driving a Car at the rpm where it starts to make its torque shall result in good cruising speed & decent fuel economy.

I recently did a trip in my diesel Car, one way I was pedal to metal, with 4 folks on-board, A/C fan running at position no.2 with thermostat positioned at maximum cool position.
The fuel needle fell by two bars/marker.

On the way back, we were two folks on board, doing 80-90kmph with fan at position 1 and thermostat at 9'o clock position (middle of the cold and red zone) and the bar/marker went down by 1 notch.

[coolclouds]

Originally Posted by Sheel

3,000 revs for a petrol Car and 2,000 revs for diesel Car in top-gear with windows rolled-up will give you good FE with decent cruising speed.

I feel like 2000 rpm in 5th gear (80-90km/hr) returns me the best fuel efficiency for my Lancer Petrol. FE figures went down drastically, when pushed to 130-140-or even more km/hr.

For more clarity, Rs. 3000 (approx) fuel required for Bangalore-Kochi with speeds 130-140 whereas only Rs. 2000 required for light foot economical drive (80-90km/hr)

[nukeblitz]

I would say that top gear with minimum possible accelerator pedal usage to maintain constant speed would give the best fe. But, in a hypothetical situation in which opening up a throttle a little more gives higher returns in speed when compared to increased fuel consumption, it would be benificial to do so...

I suppose this speed will differ from car to car.

[neel385]

Originally Posted by SCORPION

IMO, every car has an RPM range within which the car is tuned to perform its best, and this includes FE as well. This is the range where the torque and power are most usable. Most cars have a sticker saying what speeds you should be doing to obtain best FE, in each gear, which in effect, is the same rpm range.

Totally agree. In the Indigo, if one checks out the RPMs at which the manufacturer's advised best FE speeds are obtained at various gears it comes to around 2000-2100. So that is the best FE RPM for the engine.

[Mr_Bean]

High speed driving and good fuel efficiency are possible but it all depends on:

1.) How one reach those speeds
2.) How much time the car stays at those speeds.

Let me give you an example, if one has to maintain speeds of 120km/hr, there are two ways to reach those speeds on my car (baleno):
1.) shifting 1st gear - 35, 2nd gear-90, 3rd gear - 130, 4th, 5th gear - 120
2.) shifting 1st gear - 10, 2nd gear -20, 3rd gear-30, 4th gear -40, 5th gear-50 all the way to 120

Option 1 and 2 should have variable fuel consumption for a while after reaching 120 in 5th gear. I've experienced this diff during my drives.

I drove from Chennai-blore at speeds of 80-90km/hr with AC OFF & occasional speeds in the range 110-120 all in 5th gear, my baleno gave me a mileage of 18.3kms/litre.

When I am in the top most gear with the intended speed, I try to release the accelerator slowly to a point where the speeds don't actually decrease, this is also evident by the reduced coarseness on the accelerator.

[misquitas]

Originally Posted by dhanushs

Yes, I guess what nitrous meant was that, you have to be pedal-to-metal most times, which is bad for FE.

Windows down and i10 will easily climb to 150??!!.. .. I doubt it.

Are you sure have quoted me correctly in your post? I don't think I made those statements, which you have attributed to me.

[revintup]

Dont know about whether consistent speeds of 150kmph+ will lower FE or not but it will certainly lower the LE (Life Expectancy )

Driving an audi with 8 airbags on the autobahn may be safe but certainly not an i10 on our roads which simply are not built for that kind of roads.Drive safely and responsibly.Remember its a public road and you are putting others at risk too.Be responsible for yourself and your family.

It may give you a high but being safe seems to be a better alternative.Please find other means to get high without being a safety hazard to others.

I should know,for i saw 4 of my friends die in a RTA 50 mtrs ahead of my car.Everything can change in a split second.

TO MODS : Shouldnt such an irresponsible thread be deleted?Its not just a query he says he travelled at those speeds.Hope someone else doesnt try it after reading these threads.

[evilshantanu]

Originally Posted by revintup

Dont know about whether consistent speeds of 150kmph+ will lower FE or not but it will certainly lower the LE (Life Expectancy )

Driving an audi with 8 airbags on the autobahn may be safe but certainly not an i10 on our roads which simply are not built for that kind of roads.Drive safely and responsibly.Remember its a public road and you are putting others at risk too.Be responsible for yourself and your family.

It may give you a high but being safe seems to be a better alternative.Please find other means to get high without being a safety hazard to others.

I should know,for i saw 4 of my friends die in a RTA 50 mtrs ahead of my car.Everything can change in a split second.

TO MODS : Shouldnt such an irresponsible thread be deleted?Its not just a query he says he travelled at those speeds.Hope someone else doesnt try it after reading these threads.

I agree with you totally. I was unfortunate enough to witness a i10 accident on the Bangalore Goa road. He was doing speeds like 140-150 km/hr. It was scary and when I reached the site and what I saw, still haunt me. Though this has brought in a lot of maturity in my driving. But I agree that this thread should be locked /deleted

[GTO]

Simply put, if you care about FE, stick to this side of 100 kph. Anything over, and your engine is working too hard to fight drag.

I guess this topic has been discussed enough, and there's some fantastic replies. Closing as we don't permit threads on high speed driving.