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Old 26th August 2011, 18:20   #1
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Default High-speed driving & fuel consumption

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?
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Old 26th August 2011, 18:56   #2
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Default Re: high-speed driving - fuel consumption

AFIK, irrespective of the engine size, the fuel consumption would increase as the speed increases. If you happen to do the comparo across different manufacturers or with the same manufacturer and same car but a differently tuned engine, the results vary. Hence it would be only fair to compare with engines of same state of tune. Also, other factors like load, tyre pressure,condition of car will also influence the results.
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Old 26th August 2011, 19:20   #3
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Default Re: high-speed driving - fuel consumption

Hi,
At higher speeds , more power is needed to overcome the aerodynamic drag. now the question is where ( at what rev range ) the engine develops that power most efficiently. small engines are generally designed to run with peak efficiency at low end to midrange of the rev range. so a bigger engine may provide better efficiency: with factors like gearing, aerodynamics, etc thrown in. I don't think going up by a 100 CCs will make much of a difference. basically is a complex equation, depending on various factors.

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.
Try U-tubing the Video.
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Old 26th August 2011, 19:41   #4
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Default Re: high-speed driving - fuel consumption

Maybe question can be changed as, which Indian car (petrol and diesel separately) gives a better mileage when driven in 140-150 consistently.
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Old 26th August 2011, 20:03   #5
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Default Re: high-speed driving - fuel consumption

Number of revvs per kilometre matter here.
Also, aerodynamic drag increases exponentially at 3 digit speeds.
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.
So, driving at 120 in the 5th gear with minimum throttle would have helped you save a lot of fuel, but also maintain a higher average speed.
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Old 26th August 2011, 20:29   #6
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Default Re: high-speed driving - fuel consumption

I have a Palio 1.6. My experience is the same - If it is between 80-110kmph, i get a better FE as compared to 120-160 kmph.
So, even though it is a bigger engine, it still returns better FE between 80-110 then in the 120-160 range.

FE of two different cars (say i10 & Palio 1.6) at speeds in the range of 120-160 is a different aspect altogether, and it is difficult to calculate the exact FE, unless one intentionally takes the car out there to measure it at those speeds.

Last edited by vinit.merchant : 26th August 2011 at 20:30.
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Old 26th August 2011, 21:24   #7
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Default Re: High-speed driving & fuel consumption

High speed driving definitely leads to much lower fuel efficiency.

Case-in-point: (in a 2008 Hyundai Verna Diesel)
Driving on the Mumbai to Lonavala Expressway, at an average speed of 140-170 kmph on the straightways, yielded me a fuel average of 10.5 kmpl.
Driving the same expressway at consistent speeds of 90-110 kmph gets around 15kmpl.

This includes the distance taken to get out of Mumbai and onto the expressway, and the ghat sections. I'm sure on a pure highway run, at 90-100 kmph, the car could return close to 17-18kmpl.
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Old 26th August 2011, 21:40   #8
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Default Re: High-speed driving & fuel consumption

To get the best FE figures on your highway runs, try and stick to 5th gear and hover between 70-90 km/h. You're bound to get the best mileage that your car can deliver! Not only will you cover distances, you will get good economy.

If you go past 110 km/h, in 5th gear, theoretically, you'll be throttling a lot more. The deeper you bury the throttle, the lesser the FE figures will be. Plus, tyre-pressure, aerodynamics, luggage/passengers, and weather conditions play a role too.

So by doing 140 km/h, you might reach 10 minutes earlier (or maybe more, I'm not a mathematician), but your FE figures would have dropped by a considerable amount.

P.S. I know it's none of my business, but doing 140 km/h is not safe on Indian highways. Many on this forum might disagree with me and others would want to challenge me on this, but it isn't the best idea, is what I'm saying. Our roads aren't built for cars to be able to do such speeds, with constant repair-work and undulations, diversions and so on creeping up all the time.

So, with all due respect, Sir, I'd advise you to try and stick to the highway speed-limit, which is 80 km/h. On some roads, that might be hard to do so. But don't go beyond 130 km/h, especially in a city car like the i10.

Last edited by suhaas307 : 26th August 2011 at 21:43.
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Old 26th August 2011, 21:41   #9
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Default Re: High-speed driving & fuel consumption

We own a Santro(1.1L,6 yr old) which has done 45k kms and a Ritz(Vxi,1 yr old) done 7.5k kms.Within the city,same driver,same driving cycle,same roads,the FE of Ritz is way better than Santro.And on highway,i just cant compare both of them,there is remarkable difference between the FE figures,even at similar speeds.The max i got in Santro till date is 17kmpl driving at around 60kmph average which went down to 12kmpl at speeds over 80kmph.Now when i compare that with Ritz,at speeds around 60-70kmph(engine rpm 1500-2000),i got a whopping 20.8kmpl and last month i did a 1100km one way trip to Gurgaon in my Ritz,FE was 18kmpl with speeds ranging from 60-130kmph(most of the times,we maintained a speed of about 100-110kmph,engine rpm 2700-3100).
What i feel is that there are few factors that needs to be taken into account,firstly the maintenance of the vehicle.Secondly,yes the engine size does matter or maybe the newer engines are much more efficient than older ones.Lastly,with the age of the vehicle,things dont look that rosy.My Santro gives a fantastic FE of 8kmpl in city now!And i have not taken it on highway considering a few things.


Quote:
Originally Posted by suhaas307 View Post

P.S. I know it's none of my business, but doing 140 km/h is not safe on Indian highways. Many on this forum might disagree with me and others would want to challenge me on this, but it isn't the best idea, is what I'm saying. Our roads aren't built for cars to be able to do such speeds, with constant repair-work and undulations, diversions and so on creeping up all the time.
Quite right.And you'll find morons in all parts of our country.

Last edited by BoneCollector : 26th August 2011 at 21:49.
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Old 26th August 2011, 21:41   #10
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Default Re: High-speed driving & fuel consumption

We have a i10 1.1 in the family and what I have noticed is, engine gets buzzy after 90 kmph (must be doing over 3500 rpm but there is no RPM meter to tell). At 140-150 kmph the engine must be operating close to redline and there is no way it can be at it's most fuel efficient zone.
Engines are normally most efficient where they develop peak torque and this must be around 2500 rpm for i10 1.1 which should translate to 60 kmph in 5th gear.

Last edited by Guna : 26th August 2011 at 21:43.
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Old 26th August 2011, 21:49   #11
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Default Re: High-speed driving & fuel consumption

The major fuel sucker at high speeds is the Drag. I was thinking it was exponential increase at high speeds, but recently came to know that the load due to drag increases 3 times at speeds over 120km/h. With normal passenger car design, these days, IMO, below 120 is where drag/rpm/safety combo is right. Anything above it, and the engine is stressed more.

Other than this, engine capacity too matter, but when compared to drag, its minimal.
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Old 26th August 2011, 22:21   #12
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Default Re: High-speed driving & fuel consumption

Quote:
Originally Posted by suhaas307 View Post
So, with all due respect, Sir, I'd advise you to try and stick to the highway speed-limit, which is 80 km/h. On some roads, that might be hard to do so. But don't go beyond 130 km/h, especially in a city car like the i10.
Totally agree. I don't think the I10 (a tall-bodied hatch) is safe at speeds exceeding 130 kmph.

Quote:
Originally Posted by suhaas307 View Post
To get the best FE figures on your highway runs, try and stick to 5th gear and hover between 70-90 km/h. You're bound to get the best mileage that your car can deliver! Not only will you cover distances, you will get good economy.
Quote:
Originally Posted by dhanushs View Post
The major fuel sucker at high speeds is the Drag. I was thinking it was exponential increase at high speeds, but recently came to know that the load due to drag increases 3 times at speeds over 120km/h. With normal passenger car design, these days, IMO, below 120 is where drag/rpm/safety combo is right. Anything above it, and the engine is stressed more.
Other than this, engine capacity too matter, but when compared to drag, its minimal.
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:

1. Speed of 70-90 kmph without AC.
2. Speed of 90-110 kmph with AC

Last edited by misquitas : 26th August 2011 at 22:23.
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Old 26th August 2011, 23:03   #13
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Default Re: High-speed driving & fuel consumption

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.

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.

@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.

@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!
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Old 26th August 2011, 23:07   #14
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Default Re: High-speed driving & fuel consumption

Quote:
Originally Posted by BoneCollector View Post
We own a Santro(1.1L,6 yr old)...


What i feel is that there are few factors that needs to be taken into account,firstly the maintenance of the vehicle.Secondly,yes the engine size does matter or maybe the newer engines are much more efficient than older ones.Lastly,with the age of the vehicle,things dont look that rosy.My Santro gives a fantastic FE of 8kmpl in city now!And i have not taken it on highway considering a few things.


Quite right.And you'll find morons in all parts of our country.
This is uncanny.

I own a 7-year old Santro that has done 40500 on the odo. The tyres are due for replacement, otherwise it runs fine, apart from one niggling issue that has cropped up after the previous service. Anyway, that's OT.

So I get not more than 10 in the city. If I try and drive as sedately as possible, the figure jumps to 12 (with the AC on 20% of the time). After the previous service, my car managed to do 400 kilometers on a tank-full (33 odd liters) which I thought wasn't bad. But my new driving driving style seems to be taking its toll on the engine now. I rush through the gears as quickly as possible and hit 5th gear even before the car is doing 40 km/h. I barely allow the engine to be on the boil.

I've never taken it on the highway. But I'm sure I'll get somewhere around 14-15 if I do.

In comparison, my 1 and a half year old Honda Jazz delivers exceptional fuel economy. Averaging around 80-100 in 5th gear, we managed to achieve 20 km/liter, which we thought is brilliant, for a large hatch that makes 90 horses when wrung a bit.

Coming to the factors you've mentioned.

As you've rightly pointed out, the maintenance of the vehicle is oh-so important. A car, with worn out tyres and a wheezy engine that hasn't been serviced, on a highway drive, is a recipe for disaster.

Engine displacement has a lot to do with FE, I'm guessing. The larger the engine, the more fuel it will consume.

The newer engines are more refined and are technically more sound than older ones, so expect them to deliver better, and not just in terms of FE. More over, your Ritz is a Maruti Suzuki. That itself speaks volumes of the car's efficiency and capabilities.

Age, too, is a huge factor. The older the car, the harder to push it on highways.

Quote:
Originally Posted by misquitas View Post
Totally agree. I don't think the I10 (a tall-bodied hatch) is safe at speeds exceeding 130 kmph.

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:

1. Speed of 70-90 kmph without AC.
2. Speed of 90-110 kmph with AC
You'll definitely achieve better FE figures if you do 70-90 with the AC off. However, it depends on the car. A small city-car like the i10 will not give you great efficiency at speeds over 110. It would deliver goof FE at 70 in 5th. However, a larger car with a larger engine will probably give better efficiency, if driven at 110 in the highest available gear.

The AC, when switched on, will eat into the mileage figures slightly, whatever the car and however fast you're going.


Quote:
Originally Posted by carZest View Post

@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!
That's good to hear!

I know, we do tend to get a little excited and see how far you could push it. Better sense goes flying through the passenger window as the revs begin to climb. It happens to the best of us. Just remember, please be careful, I'm sure you are!

Also remember, you can never be too careful!

Last edited by suhaas307 : 26th August 2011 at 23:10.
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Old 26th August 2011, 23:27   #15
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Default Re: High-speed driving & fuel consumption

Quote:
Originally Posted by suhaas307 View Post
This is uncanny.

I own a 7-year old Santro that has done 40500 on the odo. The tyres are due for replacement, otherwise it runs fine, apart from one niggling issue that has cropped up after the previous service.
I got the tyres changed at 38k as they were pretty worn out.There are lots of body rattles inspite of car being well maintained.Here i would like to blame the HASS of my city.Moreover,the roads in my part were not as good as they are now so the Santro has had enough of beating.

Quote:
Originally Posted by suhaas307 View Post
The newer engines are more refined and are technically more sound than older ones, so expect them to deliver better, and not just in terms of FE. More over, your Ritz is a Maruti Suzuki. That itself speaks volumes of the car's efficiency and capabilities.
This is what i feel as far as my experience is concerned.
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