4 speed vs 6 speed automatic transmission chevy

GM has begun production of a new, modular family of Hydra-Matic six-speed automatic transmissions for rear-drive applications. The 6L80 six-speed transmission debuts in the 2006 Chevy Corvette, Cadillac STS-V and XLR-V, and several models of GM’s all-new 2007 full-size SUVs.

Production of the transmissions is the culmination of a $450-million, 3-year investment at the Ypsilanti Transmissions Operations (YTO) facility. By 2010, GM will introduce 10 variants of six-speed transmissions, including front-drive models, which can enhance vehicle fuel economy by up to 4% when compared to four- and five-speed automatics.

The six-speed transmissions feature two overdrive gears and a wide, 6.04:1 gear ratio spread to improve performance and fuel economy when compared with conventional four- and five-speed automatic transmissions. With two overdrive gears, engine rpm is reduced by approximately 9% at 60 mph—a reduction to about 1,500 rpm.

Lower engine rpm can bolster fuel economy because less fuel is used. A lower-rpm cruising speed also enhances smoothness and reduces noise heard in the vehicle’s cabin.

GM estimates the wide ratio spread can help cut 0-60 mph times by as much as 7% and enhance fuel economy by up to 4%.

Engineering the all-new Hydra-Matic six-speed transmission with a modular architecture enabled engineers and designers to design a transmission that is easily adapted to a wide range of vehicles. Equally important from an operational perspective, the new six-speed automatic’s modular design means any of the four primary variants can be manufactured in the same assembly plant.

As many as 47% of all components are common for all four transmission variants. In theory, different variants could run sequentially down the same assembly line. The new six-speed automatic’s manufacturing plan dovetails completely with GM’s Global Manufacturing System strategy to implement a common manufacturing process and procedure at every worldwide GM assembly plant.

YTO’s six-speed facility is currently configured to produce up to 1,500 transmissions per day. The six-speed RWD transmissions will be featured in 25 different models globally by 2007.

In addition to the Hydra-Matic six-speed RWD family, GM recently introduced the new Allison 1000 six-speed automatic for Duramax diesel-equipped heavy-duty trucks. GM also will introduce a Hydra-Matic six-speed automatic for front-wheel-drive/all-wheel-drive vehicles; it debuts in the 2007 Saturn Aura sedan. GM also will produce six-speed transmissions at additional facilities in Michigan and Europe. GM Powertrain and GM Daewoo are working to develop six-speed transmissions for front-drive global applications.

GM and Ford cooperated on the basic design for the front-drive six-speeds, but they will produce their own versions of the transmissions at separate facilities.

An optimal gearbox will have a great compromise between acceleration and fuel economy, which means it should ideally have about 6 or more gears.

BHPian Rocketscience recently shared this with other enthusiasts.

This is mainly a FYI or public interest thread for confusion in minds of millions of car enthusiasts and journalists alike, many a times I have heard/read from even senior BHPians and industry veterans in car magazines about lack of 6th gear, gear ratios or absolute lack in knowledge about the same so I'am trying to simplify the facts so that they reach more and more people to clear their confusions.

The fact of the matter is 5th, 6th gear or even higher gears, having them or lack of, have nothing to do with how relaxed your car feels at highway speeds or its acceleration or top speed.

The bottom line is it is strictly on case by case basis in each car/bike and it is strictly at the manufacturer's discretion to implement it as they wish to.

Let me take an example to explain it better:

My Chevrolet Cruze had 5 gears and my Creta on the other hand has 6 gears, ignoring the difference in their power output and engine characteristics for a moment (you can hardly get an Apples for Apples example in these cases anyways).

The Cruze even with 5 gears never felt the need for a 6th gear on the highway infact its 5th gear was so long that you got useable acceleration post 120 Km/h at which point it was doing just 2000 RPM.
Creta even with its 6 gears , in 6th gear the engine is spinning at 2100 RPM at 100 Km/h with the engine alive and a little too eager, I have always felt a need for a higher gear in Creta when doing speeds above 100 despite it offering a 6 gear transmission simply because its gears are too short.

Lets take another example, the previous generation Toyota Innova which would scream its lungs out post 100 KM/h in its 5th gear with many people rightfully wishing for a higher gear.

Now what do these 3 examples mean? Is it getting even more confusing and pointless? Let me explain.

The thing is no matter what the engine, there are optimal gear ratios for extracting the most out of it. An optimal gearbox will have a great compromise between acceleration and fuel economy, which means it should ideally have about 6 or more gears.

Why more gears? Think of it like a Pizza with more slices for better division, the Pizza is your engine's power and the more the slices or gears the better it can be spread.

As most of you would know, Internal combustion engines don't have a flat torque curve and gears were used for this exact purpose, the engine can't directly run your car from standstill so the lower gears multiply its torque at the expense of speed, so your engine for example spins 3 times but the tyre spins just 1 time in its first gear thus increasing the effective torque. Now the more the number of gears, the shorter the gears can be for the initial gears, the closer their ratios will be and thus you can get over the initial dull response of most engines today and quickly move to the meat of the power band.

So, coming back to our ideal engine - gearbox combo, it will have a better spread of power thanks to more gears which can effectively use most of its power band and it will also have a tall overdrive gear for relaxed cruising on highways for good fuel economy, relaxed engine with less noise and wear and tear and less wastage of power (which is the reason for more efficiency and increased fuel economy).

What is the reason then the 3 examples I have listed have such wildly different gearbox characteristics despite having 5 and 6 gears and why the 6th gear doesn't mean anything for your highway driving as is proved in Cruze's case.

You see, the choice of using 5 gears over 6 is largely down to cost. Many manufacturers already have 5 gear gearboxes which they don't want to invest more money to upgrade and some of them don't want to spend money on adding that 6th gear or the additional engineering it will require, so instead of using the ideal gearbox, you get a compromise.

What this does not mean is your car having 6 gears will automatically make it relaxed at highway, a manufacturer can choose to make 5 gear car more relaxed than your 6 gears one by simply making the last gear extremely tall.

So when do you need a higher gear?

In case of the Innova, Toyota India apparently did not want to invest on a new gearbox, they also did not even want to invest so much as to provide a taller 5th gear which would have solved the problem, a 6 gear gearbox like I explained is mostly a luxury and not a necessity, it only provides a better spread of power and not higher top speed or relaxed highway cruising, a taller 5th gear can do that too, what Innova had was to accommodate the high weight of the car in a relatively underpowered engine, they took some old gearbox and did not optimise it enough and just adjusted the final drive to make all the gears shorter.
That resulted in a higher than optimal RPM in top gear (5th) at lets say 100 Km/h which resulted in a busy engine which was wasting precious fuel, it is like driving your supposedly well optimised car at 100 Km/h in 3rd gear, it will run fine but at a higher RPM, this is useful if you want to overtake or want to drive hard but when you are cruising, you simply don't need all that torque and acceleration so you trade it for relaxed driving by shifting to 5th/6th gear and all is well in the world. (You get less noise and more fuel economy).

Not in the case of the Innova where they failed to provide a tall 5th gear, it felt like a normal car in 3rd or 4th gear and hence the justified complaints.

You can't however complaint the same in a car like the Cruze and this is precisely the point of this thread. In Cruze, had they provided more gears, it would have taken care of dull low rpm response and lag but it would not have changed its highway driving characteristics as they would have simply used the same 5th gear as 6th and would have added shorter gears before that, it ran super relaxed in 5th gear in highways with zero need for a higher gear. Had it been a 7 speed transmission they would have simply used the same 5th gear as 7th gear, I hope you get the jest of it.

In the Creta too despite 6 gears Hyundai did a sort of jugaad and hence I feel I need another gear despite being in 6th, somewhat like the Innova but less intense. Here Hyundai used the exact same gearbox as the Verna with identical gear ratios but to accommodate the higher weight of the Creta and it being less aerodynamic, they simply changed the final drive ratio which made all the gears substantially shorter, this solved 2 issues.

The Verna despite having 6 gears still had a somewhat sluggish response in lower RPMs (turbo lag), this was solved by shortening all the gears, then the higher weight too was well taken care of, but this resulted in a short highest gear too (6th) which is providing acceleration and torque which is not needed for cruising (one can always downshift for that) and as a result the engine is running at higher than optimal RPM at highway speeds and feeling more busy (humming starts at 120 despite it being a very refined engine) and providing less fuel economy than it is capable of simply because the 6th gear is shorter than optimal, while most cars don't have this problem, in Creta anyone can catch this as even my dad who is not an enthusiast is looking to upshift gear at around 110 despite the car being in 6th because it feels like the car is in a much lower gear. (Feels because after having driven many cars which have the correctly optimised gears one knows when to upshift even without looking at the RPM needle).

This could have been solved by re-engineering of gearbox specifically for Creta and optimising the gear ratios accordingly so as to have shorter lower gears but longer higher gears but that would have required more cost than Hyundai was willing to invest so they chose not to. They could have also solved this by using the same setup as Verna but instead of different final drive they could have also in addition to that used a higher 6th gear but they did not even want to spend that much.

So you see, 6th gear is not always needed and even it can't guarantee you an optimal highway cruising experience. It is much more nuanced than that as I tried to explain.

There is much more to this topic, but it is getting very complicated as it is, so more including gearboxes for bikes vs cars later.

Here's what BHPian Turbohead had to say on the matter:

Bring a mechanical engineer, I totally understand your point- sadly, for everyone else 6>5. Hyundai knows it best.

Similarly, tire diameter also affects the gear ratios. Greater the tire diameter, longer each gear feels. An Innova primarily running on the highway will have a more relaxed cruising with a larger tire diameter at the cost of low speed driveability- should be fine since the 1st gear is really short and often avoided by a lot of drivers. This is something that's rarely discussed even in our forum. I learnt it the hard way and sort of regret upsizing the tire since it ruined the perfectly tuned ratios.

Here's what BHPian deetjohn had to say on the matter:

The biggest plus of a 6 speed tranny is not the overdrive, but the shorter and closer ratios low down. For example, a shorter 2nd or 3rd is very useful in our congested cities OR when it's speed breakers & potholes galore. Makes the drive much more smoother, faster and efficient. And that's the case for both AT or MT.

So, I for one really like it when there are more ratios to play with. But the tranny shouldn't be dimwitted when it's an AT. Some good AT boxes are the 6AT of Ecosport, 10AT of Endy, 7/6 AT of VAG.

Here's what BHPian Kosfactor had to say on the matter:

The point of 6 Speed is not to keep the engine speeds low - engines do not work efficiently at low speeds, they have a certain range of speeds at which they are most efficient as well as would last longer.

A 6 speed gearbox would give you nice spread of 1-4 speeds for traffic and 5-6 for highway cruising. Less clutch burn, less fatigue and higher FE at the end of the day.

Just drive a Hyundai and its equivalent vehicle back to back through heavy city traffic as well as crowded highway at rush hour , you`ll know the difference and that its not just the gear ratios, there is a whole lot more. Everything that enthusiasts dislike about a Hyundai in general are actually very useful qualities of their vehicles.

Here's what BHPian fiat_tarun had to say on the matter:

Nice explanation, but like you said it totally depends on the implementation.

I for instance really miss a 6th gear in my Ecosport diesel. Ideally I would have liked a shorter 2nd gear (1st is very short and 2nd is long) and the addition of a 6th for highway cruising.

Ideally 100 kmph should be under 2000 rpm for the best efficiency in a diesel. My highway mileage hardly ever crosses 17-18kmpl as at 110 -120 kmph the rpm is in the 2500 - 3000 range and mileage takes a big hit.

Check out BHPian comments for more insights and information.

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