4 speed vs 5 speed

[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.