Team-BHP - 6th gear for a car - Need, criteria, pros and cons

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Yes, I agree Shan2nu. As I mentioned, fuel burns more on the one hand as speed increases, but on the other hand it is covering more distance. There has to be some trade off somewhere. But theoretically, you have to agree that higher the speed, higher the amount of energy to travel the same distance (because resistance is more) for a given weight of vehicle. But the combustion engine does not allow that. It is a completely different game for hybrids. Their best fuel efficient speed could be as low as 20 Kmph. For instance, a Toyota hybrid will give more mileage in city (less resistance) than a highway (more resistance due to higher speed).

There is def a trade off, but the engine speed and vehicle speed required to minimize this trade off will differ with each car & driving condition. What driving technique does wonders on one car might not work on another.

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6 ps required to reach 60 Kmph was an indicative figure on flat road (again not mine). If we have to factor in all conditions, there will never be a thing like top speed. Isn't it?

Actually, even "top speed" only indicates what that car was capable of doing on that particular day, under those conditions (unless it is electronically/mechanically limited).

Which is why record breaking top speed runs need to be done twice within an hrs time and the average speed is considered. (still not a perfect method but better than doing it once).

So if a manufacturer says your car has a top speed of 200kmph, your car might max out below it (if the conditions are more worse than when they tested) or even above it (if the conditions are more favourable than when they tested).

Shan2nu

@opendro
I had made a fairly long post, inline commenting in your post. Guess it was deemed too long and removed. Don't feel like going through the whole exercise again. So will keep it short and sweet (and in small doses) this time!

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Originally Posted by opendro (Post 2076669)

Minimum number of gears:
This is an irrelevant question as of today as most cars have 5 forward gears as standard. But theoretically, it would be number required where each upshift would be done at the redline in lower gear to 1000 rpm in next gear till we get such a gear where we can no longer reach the peak power rpm. This number could be mostly 2 or 3 gears in most cars. For instance, most cars reach 40 Kmph in 1st gear. 2nd gear will be 40 Kmph at 1000 rpm. This 2nd gear will unlikely reach the redline before it hits the top speed.

Say two gears. What would be the effect on drivability, and FE? And why?
If the figure (number of gears) can be found theoretically so simply, why should we consider anything else?

Top Speed:
Top speed is roughly a function of max power, vehicle weight and other variable resistances (such as wind resistance, rolling resistance, etc. which

Can you give the relationship between max speed and weight?

Regards
Sutripta

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Originally Posted by .anshuman (Post 2075252)

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Originally Posted by kadanaJ (Post 2075243)

The primary purposes for a 6th gear in these cars (and especially a hatch like a diesel i20) is to provide maximum fuel efficiency, (not top speed as someone mentioned), and that is in my opinion to real purpose and advantage of a 6th gear.

KadanaJ, You are wrong in the case of diesel cars. The top speed of diesel hatchbacks will get affected after addition of a sixth gear. Here is How: Most modern common rail hatchbacks(except i20) hit redline in fifth gear(4k rpm, the max power point, not the peak revving point in case of Multijets). Even my Elantra with tall gearing hits redline in fifth cog.

With the addition of a sixth gear the Diesel cars will obviously experience a better top speed. I feel the ability to cruise in a more relaxed manner is worth the effort. Personally i miss the sixth gear in my Punto on open highways.

What I meant to say was, while the sixth gear will definitely improve top speed, the main benefit for the average buyer will be lower NVH and improved fuel efficiency.
Top Speed is most certainly also a benefit, but not one that most people will take advantage of.

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Originally Posted by kadanaJ (Post 2088689)

Exactly.I own a i20 CRDi 6 speed.Though i hardly cross 3rd gear in city driving,on highways the 6th gear is a boon.I engage 6th above 100 kmph and the NVH levels are very less which enables relaxed driving and you can speak with co passengers comfortably .Also the rpm will be around 2k,so its right in the start of the turbo sweet spot and you can surge ahead with a light tap on the acc pedal.And the 6th gear returned unbelievable FE on my NH4 trip cruising aroung 120-130kmph mark.I got 23.5 kmpl!!!

The maximum efficiency (thus acceleration) will always come when you are driving at peak torque. As pointed out Power = Torque x RPM x Constant. So power would increase beyond the peak torque, but finally fall off as the torque drops.

Thus if a sixth gear can keep me in the peak torque while cruising at higher speed, it is beneficial, otherwise not.

From purely performance point of view for engines with a fairly flat torque (detuned), less gears are required, while for engines with peaked torque (highly tuned) more gears are required. This is the reason for fewer gears in the original petrol Geep (3), and a large number of gears (7+) in a highly tuned racing car.

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This is only true from a performance point of view. When you need instant acceleration.

On regular cars, the last gear is usually designed with fuel economy in mind and would use a relatively tall gear ratio, which lets the driver cruise at a low rpm at higher speeds.

So both designs are beneficial under different circumstances.

Shan2nu

Excellent discussion guys....

I too feel the need for a sixth gear on my indica tcic. In city obviously not. But on highway drives to Silvassa / Baroda / Lonavla / Pune, i really feel the need for a sixth gear. My reason is not further power or speed, but merely lower engine rpm, therefore improved and relaxed engine state, therefore better NVH, and of course better mileage.

I would ideally like to drive between 90-100 in 6'th. This would be very relaxed indeed.

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Originally Posted by Shan2nu (Post 2154787)

Should the question of a Petrol or Diesel vehicle not come into play ? I mean a petrol needs to reach or come to its peak high rpm to get the maximum Bhp's out which is not possible in the 5th gear because it has less torque and cannot overcome the drag of wind/tyres etc and hence the top speed in 4th gears.

Where as most of the Diesels reach their max rated bhps in 5th gear, Swift Diesel rated at 4000 rpm @ 76 bhp easily reaches above 4000 rpm in 5th gear and attains its top speed. If it was given a 6th gear with a bit lower ratio than the 5th gear it could easily cruise at 120 + with engine ticking below the rpm's it would tick in 5th gear at that particular speed as it has the torque to pull through and at the same time get a better mileage and relaxed cruising speed.

A 6th gear would hold way more advantages in a Diesel than in a petrol IMO

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Why go in for an extra gear if fuel economy is what you're looking for. Just get a taller final drive ratio. This will automatically bring down the cruising rpm in 5th gear at 120kmph and let the car hit top speed in 4th.

By altering the FD, any car can be made to achieve it's top speed in 4th or 5th gear, irrespective of what fuel it runs on.

Shan2nu

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Originally Posted by Shan2nu (Post 2259250)

A taller final drive will increase in gear acceleration in every gear, with a 6th gear all the motives of acceleration, higher top speed and fuel economy can be achieved with a 6th gear designed to just bring the rpm to 2200 ( just above the sweet spot) @ 120 rpm in a 'Diesel' car.

Everything can be achieved by adding a gear, Altering a final drive in a petrol car to a taller one will kill the in gear acceleration to no extent as petrols already lack the high torque at lower rpms the the diesels have, adding a 6th gear with ratio's spaced correctly will make it sweet.

Can you please explain or point to some link on how to alter the final drive ratio? Or is it a complex and/or laborious task?

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Originally Posted by Shan2nu (Post 2259250)

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The same theory holds true for petrols as well.

The OHC VTEC comes with a very tall 4.058:1 FD. If it had a 6th gear, honda could have used a 5.06:1 FD, so that the car can benefit from a shorter overall gearing for quick acceleration, while still maintaining a relatively low cruising rpm for better FE.

So a 6th gear is as important for a petrol, as it is for a diesel.

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Can you please explain or point to some link on how to alter the final drive ratio? Or is it a complex and/or laborious task?

You will need to order for a suitable shorter final drive gear from someone like M-Factory. Replacing it is not a major issue, but make sure you get it done from an experienced garage/tuner.

http://www.teammfactory.com/final-drive-gears-road-race

But then again, it also depends on what you need the FD for. While a shorter FD can improve acceleration in a straightline, it can sometimes work against you on a race track, that suites the factory ratios.

Shan2nu

All technical discussion aside, I have done enough traveling in my i20 to confirm that riding in sixth gear does increase the fuel efficiency by a great margin. Riding continuously in 6th gear and cruising at an average of 70-80 kmph, I was able to extract a mileage of 22+ in my i20 which was far better than an average of ~19 on normal highway cruising.

The Sad part: You need to wait for a hartal to have the opportunity to drive all continuously in 6th gear on Indian highways. :Frustrati. Just like I did.

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Originally Posted by Ricky_63 (Post 2075374)

I don't know if this is revelant - I would have loved a 6th gear on my Pajero. It seems strained over a certain speed in 5th.

If its the Pajero SFX, then its strained due to two reasons:

1. 120 bhp engine which is underpowered for a car above 2 tonnes.

2. Due to the above reason, it has been geared short to improve drivablity in city. So even at 110 kph it does 3000 RPM. Which is high. Anything above 2.8K is overkill for diesel cruising.

3. So a 6th gear would have helped to be more relaxed. But the disadvantage would be that the gap between the 5th and the 6th would have been even more. As it is, the performance is nothing to speak of, it would have accentuated the lack of power even more. You would have to downshift to 4th to overtake. My $0.02.

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Originally Posted by searchingheaven (Post 2995326)

You have said the right things, but I still feel the 6th gear would help to ride smooth & easy once over the 100 kmph mark. :thumbs up