4 speed vs 5 speed

[XTO]

The only difference is the spacing. The most important is the final gear ratio. During traffic or medium speed no matter what gears you select, the rpm will be the only basis whether you’re a guzzler or not.

[mbeige]

The only difference is the spacing. The most important is the final gear ratio. During traffic or medium speed no matter what gears you select, the rpm will be the only basis wheather you’re a guzzler or not.

I think you mean that engine speed is the effect that several factors have to provide driver input. These factors include overdrive gear ratio, final drive ratio, and powerband relative to speed and gearing.

What I mean is some cars can run at 100kph, let's say, for around 3,000rpm but have a tall final drive ratio (say, 3.xx). On the other hand, the same car can be fitted with a lower final drive ratio (say, 2.xx) to drop the engine speed down for the exact same forward speed.

Some cars there, like the Tamaraw FX mentioned in a recent thread, are said to have really tall gearing but low engine power. So the engine makes up for its lack of power by having a taller final drive ratio, so that it can have the mechanical advantage to transmit whatever little power is produced into mechanical work efficiently. This comes at a price, however. Their tall final drive ratio will result in high engine speeds during highway cruising, and are best suited for city driving instead (fuel consumption wise).

So all else being equal, having an extra fifth gear over the fourth gear, and given that the fifth gear has an even lower ratio than the fourth gear, allows the vehicle to consume less fuel for the same speed by way of lower engine speed/RPM as a result of using that fifth gear instead of the fourth gear.

However, when driving in the city, lugging the engine will result in more fuel consumed. Lugging the engine is described as using a gear that's too high for the specific conditions such as speed and engine RPM, so there is a need to downshift to a lower gear.

In essence, that fifth gear is only helpful on highway drives when the momentum of the car is enough to propel it forward with the tiny amount of help that the fifth gear can provide to add to the momentum by way of minimal fuel consumption.

[pup2]

"In essence, that fifth gear is only helpful on highway drives when the momentum of the car is enough to propel it forward with the tiny amount of help that the fifth gear can provide to add to the momentum by way of minimal fuel consumption."

I agree. Yung MT 4-gear Lancer SL 1.4 ko nung araw mas mabilis sa arangkada (perhaps up to 3rd) kesa dun sa GSR/GT nung pinsan ko na 5-speed 1.6 -- pero siyempre pag rekta na, iwan na ako. heehee.

[niky]

Unfortunately, not all gearboxes are created equal, so there's hardly a time when a manufacturer creates both a 4-speed and a 5-speed box wherein the first four gears are the same.

It's simply because the five gear car doesn't have any more power than the four gear car, so a higher overdrive gear isn't required, as 4th gear is already designed as an "optimum" overdrive for that car.

Thus, in common practice, both the four gear and the five gear box have similar overdrive ratios (4th = 5th), with the 5 gear box having closer gears.

On some performance cars, 5th gear isn't even close to the overdrive ratio on the automatic 4-speed version of the car. On most cars, 4th gear or 5th gear is so tall that you'll never hit redline... on my car, top speed is limited by redline in 5th...

The 4-gear Lancer SL is a smaller car, so it can be quicker off the line than the GSR. You need to compare like-to-like cars here.

There's little advantage, nowadays, to building a four-speed box over a five-speed one... just a difference of one gear inside the box. The advantages for building a five-speed box lie in flexibility and fuel economy.

For six speed boxes? That's when you start getting higher and higher overdrives... as giving extra-short ratios at low speeds on a 6-speed MT means the driver has to row like crazy at traffic speeds. That's why 7-speed and 8-speed boxes are mostly automatic. Those extra gears make for more smoothness, but they'd make it a pain to shift it yourself...

[rion]

*rion: Not really.

What you're looking at is not the time it takes you to row through the gears, but the amount of power you're using as you accelerate in gear.

The shorter the gear, the easier it is for the engine to accelerate in that gear.

Here's a good example... take your basic mountain bike.

Start off in lowest gear (we'll ignore your front gearset and assume your rear-set is eight cogs)... As you start off, you don't have to pedal very hard... then shift, then shift, then shift... as you go faster, you're still using the same amount of power.

Now, if you have less gears... start off in 2nd... then 4th... then 6th... then 8th... you need to pump harder to get up to speed. So even if you lose less time to shifting, you're using more energy to move yourself.

Most modern 4-speed automatics are geared so that you lack for power on the road, but they have to sacrifice somewhere, and it's often in that first gear, which is much longer than on a 5-speed car. That long first gear makes the engine work harder from a stop... it's like starting your bike in 2nd gear... it takes more muscle to start the bike rolling, which is equivalent to more gas wasted in a car.

-----

Hell, thought of an even simpler example. Staircases. More stairs, with a shorter step height = easy climb. Less stairs with a taller step height = puff... puff... puff...

Oh I see po. but a average driver with a normal car can put atleast a 2 secs delay in a shift diba?

[niky]

Actually, it's probably between 0.5 seconds for your common driver, if they aren't really in a hurry. I've got timing data that show shift times of 0.1 seconds (or possibly better) are possible with a good driver and a good box.

Older ATs take longer than that to shift, and, worse, take some time to "make up their mind" about when to shift. Newfangled DSGs, SMGs, CVTs and the like have shift times that most drivers can't beat, and some economy cars are getting ultra-fast boxes, too.

I'm pretty sure I could shift a five-speed MT Ranger as fast or faster than the five-speed AT box can, but not on every single shift at every single stoplight. And there's no way I can shift as fast as VW's new DSG (8 milliseconds? I can't even blink that fast!)

But the point isn't the time you take to shift, but how much power you use and how long you use it to get to a certain speed. And therein lies the advantage of boxes with more gears, and the advantage of manual boxes over their heavier, inertia-challenged, torque-converter equipped automatic brethren. CVT's another story, though...

[rion]

I see. but how about the delay of the driver in the clutches pedal in a common box ride?

[OTEP]

Mabilis lang mag shift.

And consider that in some uber powerful cars (e.g. a Corvette), you can already be at 100kph without having to be in top gear (6th).

[niky]

Again. the delay that occurs while pressing the pedal isn't as significant as the fact that the automatic box is often heavier and usually has longer gears.

That 0.1 second shift time includes clutching. With flat-shifting, there's almost no perceptible interruption of power.

Most common autoboxes still have a delay, simply because an automatic with no delay would be jerky and uncomfortable to use on the road (BMW's SMG is an excellent example of this). The exceptions to this rule are VW's DSG box, the CVT and the more expensive 6, 7 and 8 speed automatics coming out now... but the DSG is not a torque converter automatic. It's considered a robotized manual (because it uses clutches instead of a torque converter).

But arguing which is better is an academic point. Simply put, a common manual is more efficient and faster than a common automatic, but a robotized manual is often faster than either, as it combines the manual transmission's superior clutch-system with an automatic's computerized gear-change response.

[suysuy]

CVTs for the world!