"Why does acceleration change with different gears, even with the same engine?"
"Does a lower gear improve acceleration?"
"Does a higher gear increase top speed?"
When considering car performance, "gear ratio" is a crucial factor.
The force generated by the engine isn't directly transmitted to the wheels.
Between the engine and the wheels, there's a transmission that uses gears to convert rotational speed and torque.
Therefore, even with the exact same engine, a change in gear ratio alters the car's characteristics, such as:
Strength of acceleration.
Engine RPM.
Speed achievable in each gear.
RPM during high-speed cruising.
Fuel efficiency and quietness.
This article will clearly explain what gear ratio is, the difference between low and high gears, and how they affect acceleration and top speed.
What is Gear Ratio?
Gear ratio is a number that expresses the relationship between the rotational speeds of the input and output gears.
In automobiles, it's easiest to understand it as:
How many rotations the transmission's output side makes for a certain number of engine rotations.
For example, if the gear ratio is,
3.500
Simply put, it means the output side rotates once for every 3.5 rotations of the input side.
On the other hand, if it's,
1.000
The input and output sides will have nearly the same rotational speed.
A larger gear ratio generally indicates a "lower gear."
A smaller ratio indicates a "higher gear."
This can be a bit confusing if you only consider the terms themselves.
"Low gear" doesn't mean the gear ratio number is small.
Generally, lower gears, such as 1st gear, have larger numerical gear ratios.
Why is 1st Gear Strong for Acceleration?
When starting a car, 1st gear is typically used.
The reason is that 1st gear significantly amplifies the engine's torque and transmits it to the wheels.
For example, if the engine produces,
200N・m
of torque. If the 1st gear ratio is,
3.5
the torque after passing through the transmission would be, in a simple calculation:
200 × 3.5 = 700N・m
In a real car, there is also a final drive ratio.
If the final drive ratio is 4.0, then:
200 × 3.5 × 4.0 = 2,800N・m
In reality, there are losses in the drivetrain, so this exact number doesn't reach the wheels.
However,
the basic principle that gear ratios can greatly amplify engine torque
remains unchanged.
This is why a car can start powerfully in 1st gear.
So, why can't you drive at high speeds in 1st gear?
If torque can be greatly amplified,
"Wouldn't it be fastest to stay in 1st gear all the time?"
you might think.
However, while gears amplify torque, they also result in a lower wheel rotation speed.
In 1st gear,
The engine rotates quickly.
The wheels rotate relatively slowly.
This relationship means that as vehicle speed increases, the engine RPM quickly approaches its limit.
That's why you shift up to 2nd, 3rd, 4th gear, and so on.
As you shift to higher gears, the torque amplification decreases, but the wheels can rotate faster at the same engine RPM.
In other words, the transmission's role is to:
Increase torque at low speeds at the expense of rotational speed.
Reduce torque amplification at high speeds to increase vehicle speed.
What is a Low Gear?
A low gear generally refers to a gear with a numerically larger ratio.
For example, if a transmission has the following ratios:
1st gear: 3.500
2nd gear: 2.100
3rd gear: 1.500
4th gear: 1.100
5th gear: 0.850
In this case, 1st gear is the lowest.
Low gears have characteristics such as:
Easily transmitting high torque to the wheels.
Providing strong acceleration.
Making it easy to climb hills.
On the other hand, they also have characteristics such as:
Engine RPM tends to be higher at the same vehicle speed.
The speed range achievable in a single gear is narrow.
What is a High Gear?
Conversely, a high gear refers to a gear with a numerically smaller ratio.
In the previous example,
5th gear at 0.850
would be on the higher side.
High gears have characteristics such as:
Lower engine RPM at the same vehicle speed.
Ability to achieve higher speeds in a single gear.
Potentially beneficial for quietness and fuel efficiency during high-speed cruising.
On the other hand, the torque transmitted to the wheels is amplified less in a high gear, making it disadvantageous for strong acceleration from low speeds.
That's why in automatic cars, when you press the accelerator deeply, it automatically shifts to a lower gear.
This is known as kickdown.
How Gear Ratio Changes Acceleration
When considering acceleration, it's not just about:
How many N・m the engine produces.
In reality,
engine torque × gear ratio × final drive ratio
greatly changes the torque transmitted to the wheels.
For example, let's compare two scenarios with the same 200N・m engine.
If 1st gear is 3.5,
200 × 3.5 = 700
On the other hand, if 4th gear is 1.0,
200 × 1.0 = 200
Assuming the same final drive ratio, 1st gear provides greater torque amplification through the transmission.
Therefore, even with the same accelerator pedal position,
Acceleration is strong in 1st gear.
Acceleration is milder in higher gears.
This difference arises.
Honda also published an example in its CIVIC TYPE R development documentation, where 1st to 3rd gears were set to the lower side to achieve sharp acceleration.
What is the Final Drive Ratio?
When understanding gear ratios, one must not forget the "final drive ratio."
It is often called the Final Drive Ratio in English.
The engine's power flows through:
Engine.
Transmission.
Final gear.
Drive shaft.
Tires.
Therefore, to consider the actual overall reduction ratio, you need to look at:
Transmission gear ratio × final drive ratio
For example,
1st gear: 3.5
Final drive ratio: 4.0
Then,
3.5 × 4.0 = 14.0
On the other hand,
1st gear: 3.5
Final drive ratio: 3.0
Then it would be,
10.5
Even if the transmission's 1st gear is exactly the same, a different final drive ratio will change the torque transmitted to the wheels and the vehicle speed at the same engine RPM.
What happens if the final drive is set to the lower side?
Increasing the numerical value of the final drive ratio generally makes it a lower gear.
For example, if changed from,
3.5 to 4.1
If the engine torque and transmission are the same, more torque can be transmitted to the wheels.
Therefore, there's a possibility that you'll feel:
Stronger acceleration from a standstill.
Improved acceleration in each gear.
However, there will also be changes such as:
Higher engine RPM at the same speed.
Lower maximum speed achievable in each gear.
Higher RPM during high-speed cruising.
In other words, it's not a case where only one thing improves; there's always a trade-off.
Does a high gear always increase top speed?
This is a point that is very easily misunderstood.
Theoretically, a higher gear allows for higher vehicle speeds at the same engine RPM.
However,
a higher gear does not necessarily mean an increase in actual top speed.
Near top speed, there is significant air resistance.
As vehicle speed increases, more power is needed to overcome it.
If the gear ratio is too high, the driving force transmitted to the wheels becomes smaller, which can result in:
The engine not being able to reach its maximum RPM.
The speed not increasing further due to air resistance.
In other words,
the theoretical top speed based on gearing
and
the actual top speed the car can achieve
are not always the same.
Top speed is determined by a balance of factors such as:
Maximum power.
Air resistance.
Tires.
Gear ratio.
Engine RPM.
It's not "low gear = fast" or "high gear = slow" either.
While a low gear is advantageous for acceleration, the engine quickly reaches its RPM limit.
This necessitates shifting.
On the other hand, an appropriate high gear allows for a longer use of a single gear.
This means that for actual acceleration, not only are the following important:
Acceleration force in each gear
but also:
Where to shift up.
Where the engine RPM drops after a shift.
How long the shift takes.
Simply lowering 1st gear doesn't necessarily shorten the overall acceleration time of the car.
What is a Close Ratio?
A term often heard in sports cars is,
"close ratio"
This refers to a setting where the ratios of adjacent gears are relatively close.
For example, when shifting from 1st to 2nd gear, it prevents the engine RPM from dropping too much.
Engines have:
An RPM range where they can produce high torque.
An RPM range where they can produce high power.
One of the purposes of a close ratio is to make acceleration smoother by keeping the engine within its effective RPM range even after shifting up.
Mazda also described its 6-speed automatic transmission for the RX-8, stating that by expanding the gear ratio range while simultaneously shortening the intervals between each gear, they aimed to achieve both strong, smooth acceleration and quietness/fuel efficiency during high-speed driving.
What is a Wide Ratio?
In contrast to the concept of close ratio, there is also the concept of,
"wide ratio"
This involves having a wide range of gear ratios to ensure sufficient driving force in lower gears while keeping engine RPM low in higher gears.
For example, a configuration where:
1st gear is quite low.
The highest gear is quite high.
This makes it easier to achieve both:
Strong performance from a standstill.
Lower RPM during high-speed cruising.
This approach is becoming more feasible with multi-speed transmissions such as modern 8-speed, 9-speed, and 10-speed automatics.
Why are there more gears?
In the past, 3-speed and 4-speed automatic transmissions were common.
Today, multi-speed transmissions with 6, 8, or even 10 gears are also seen.
Increasing the number of gears allows for more precise settings of:
Low gears for starting.
Intermediate gears for acceleration.
High gears for highway cruising.
It also allows for a wider overall gear ratio range while minimizing the RPM drop between gears.
In other words, it makes it easier to achieve different requirements:
Strong performance at low speeds.
Utilizing the engine's effective RPM range during acceleration.
Cruising at low RPMs at high speeds.
Relationship Between Gear Ratio and Engine RPM
Even when driving at the same speed, engine RPM changes depending on the gear used.
For example, at 60 km/h, there's a difference like:
3,000 rpm in 3rd gear.
1,800 rpm in 5th gear.
While the actual numbers vary by car model, in general, higher gears allow for lower engine RPM at the same vehicle speed.
Therefore, on highways, higher gears are used to cruise at lower engine RPMs.
This serves purposes such as:
Reducing noise.
Reducing fuel consumption.
Keeping engine load within an appropriate range.
Mazda also explains in its technical documents that setting higher gears to lower engine RPM in the common operating range improves fuel efficiency and quietness.
However, "lower RPM means better fuel economy" is not always true.
This also requires caution.
Lowering engine RPM doesn't always improve fuel efficiency.
Attempting to accelerate strongly at low RPM in too high a gear can put a significant load on the engine.
Fuel efficiency varies depending on a combination of factors such as:
RPM.
Accelerator pedal opening.
Load.
Combustion efficiency.
Vehicle speed.
Mazda also explains that gear setting is a crucial factor that influences not only driving force but also which operating range of the engine is utilized.
Therefore,
"Always using the highest gear possible"
is not necessarily the correct approach.
Relationship between Low-End Torque Engine and Gear Ratio
The "low-end torque" discussed in the previous article is also deeply related to gear ratio.
An engine that produces sufficient torque from low RPMs will accelerate more easily even with relatively higher gears.
Conversely, an engine that only produces significant torque or power at high RPMs will require selecting a lower gear and increasing RPM during acceleration.
In other words, a car's acceleration feel must be considered by combining:
The engine's torque curve
and
the gear ratio.
Even with the same maximum torque, if the gear ratio is different, the driving force delivered to the wheels will change.
Even with the same engine, driving performance changes with gear ratio.
Let's assume there are two cars with the same engine.
Car A has a lower gear ratio, prioritizing acceleration.
Car B has a higher gear ratio, prioritizing highway cruising.
Even if their maximum engine power and peak torque are the same, the driving experience will differ.
Car A will have characteristics such as:
Feeling powerful when the accelerator is pressed.
Engine RPM rising quickly.
Potentially more frequent gear changes.
Car B will have characteristics such as:
Milder acceleration.
Easier to increase speed within a single gear.
Easier to maintain low RPMs during highway cruising.
This is one of the reasons why:
cars with the same horsepower rating in the catalog can feel different to drive.
Tire Diameter Also Affects Gear Ratio
The tires are the final component that transmits power to the road surface.
If the outer diameter of the tires changes, then:
Same engine RPM.
Same transmission.
Same final gear.
But the distance traveled per revolution changes.
If the tire outer diameter increases, the distance traveled per revolution increases.
Therefore, theoretically, it shifts towards a higher gear ratio.
Conversely, if the outer diameter decreases, it moves towards a lower gear ratio.
However, changing tire size is not done simply for acceleration performance, as it also relates to:
Speedometer.
Vehicle inspection and safety standards.
Tire load capacity.
Interference with the vehicle body.
Electronic control systems.
Does a CVT have no gear ratio?
A CVT is called a "continuously variable transmission."
Unlike conventional multi-speed ATs or MTs that switch between fixed gear ratios like:
1st gear.
2nd gear.
3rd gear.
A CVT continuously varies the gear ratio.
Honda also explains that for CVTs, by changing the transmission diameter of two pulleys and a metal belt, the gear ratio is continuously varied according to vehicle speed and accelerator operation.
In other words, CVTs also have:
A lower gear ratio range.
A higher gear ratio range.
However, their characteristic is the ability to move continuously, not in steps, between these ranges.
Why CVTs increase engine RPM during acceleration
When you press the accelerator deeply in a CVT car,
you might feel the engine RPM rise before the vehicle speed increases.
This is not necessarily a malfunction.
It's because the CVT continuously varies its ratio to match the engine RPM that provides the most efficient power for acceleration.
With a fixed-gear transmission, the sequence would be:
Accelerate in 1st gear.
Shift to 2nd gear, RPM drops.
RPM rises again.
A CVT eliminates these steps between gears, making it easier to utilize the engine's optimal RPM range.
Why do ATs automatically change gears?
Modern automatic transmissions shift gears by monitoring factors such as:
Vehicle speed.
Accelerator pedal position.
Engine RPM.
Load.
Driving mode.
For example, if you press the accelerator deeply while cruising at high speed, the AT might shift from a high gear to a lower gear.
This is a control mechanism where:
The driving force to the wheels is insufficient in the high gear.
Therefore, it shifts to a lower gear to amplify torque.
Conversely, when cruising at a steady speed, it uses a high gear to lower engine RPM.
In essence, the AT automatically selects the appropriate gear ratio for the situation.
Car speed is not determined solely by gear ratio.
Gear ratio is a very important factor that influences acceleration performance.
However,
it's not as simple as saying that a lower gear always makes it faster.
Actual acceleration involves many factors such as:
Maximum engine power.
Torque curve.
Vehicle weight.
Gear ratio.
Final drive ratio.
Tire diameter.
Tire grip.
Drive system.
Drivetrain losses.
Air resistance.
Shift time.
It's easiest to understand gear ratio as a crucial factor that determines:
how the engine's power is used at the wheels.
Reasons why power-to-weight ratio alone isn't enough
The same applies to the power-to-weight ratio discussed in the previous article.
For example,
1,000kg・100PS.
1,500kg・150PS.
Both are 10kg/PS.
However, if the gear ratios are different, the driving force transmitted to the wheels at each speed will also change.
Therefore,
the same power-to-weight ratio does not necessarily mean the same acceleration.
Power-to-weight ratio is an indicator of:
the balance between power and weight.
Gear ratio is an indicator of:
how that power is converted to the wheels.
By combining these two, you can gain a deeper understanding of a car's acceleration performance.
Key points when looking at gear ratios in catalogs
When gear ratios are listed in a car's specifications, it's helpful to look at them as follows:
First, look at the 1st gear ratio.
A larger number indicates a setting that can easily achieve significant torque amplification during starting.
Next, look at the spacing between each gear.
By observing how much the ratio changes from 1st to 2nd, and 2nd to 3rd, you can get a rough idea of whether it's closer to a close ratio or a wide ratio.
Then, look at the highest gear.
Gears below 1.0, such as 0.8 or 0.7, are sometimes called overdrive.
Finally, check the final drive ratio.
It's important to look not just at the transmission ratios, but at the product of:
Gear ratio × final drive ratio
Gear ratios reveal a car's design philosophy.
A gear ratio is not just a number.
It reflects part of the manufacturer's design philosophy, such as whether they prioritized:
Starting acceleration.
Mid-range acceleration.
High-speed cruising.
Fuel efficiency.
Sporty driving.
Honda's example with the CIVIC TYPE R, where lower gears were set to the low side for acceleration and higher gears to the high side for high-speed performance, is a clear illustration of how car characteristics are differentiated by gear ratios.
Next Science's perspective on "power transmission"
When considering a car's driving performance,
how much horsepower the engine has.
how much maximum torque in N・m it has.
These numbers alone are not enough.
The power generated by the engine is transmitted to the road surface only through:
Transmission.
Final gear.
Drivetrain.
Tires.
In other words, what's important is not just:
how much power is produced
but also:
how that power is transmitted.
Gear ratio significantly influences how this power is transmitted.
Furthermore, actual driving feel is also affected by:
Throttle response.
Friction and drivetrain losses.
Tires.
Vehicle weight.
Understanding the flow of energy throughout the entire car, rather than just a single maximum value, is crucial for comprehending its driving dynamics.
Summary: Gear Ratio - A Mechanism for Utilizing "Force" and "Speed"
Gear ratio is a crucial element for altering the relationship between engine rotation and wheel rotation.
Generally,
A lower gear with a numerically larger ratio is advantageous for acceleration because it can greatly amplify torque.
A higher gear with a numerically smaller ratio allows for higher vehicle speeds at the same engine RPM.
However,
it's not as simple as a low gear always being faster.
or a high gear always increasing top speed.
In a real car, the balance with factors such as:
Engine power output.
Torque curve.
Final drive ratio.
Vehicle weight.
Tires.
Air resistance.
Drivetrain losses.
is important.
It's easiest to understand gear ratio as:
a mechanism that converts the engine's power into "force" and "speed" according to driving conditions.
The next time you look at a car catalog, pay attention not only to horsepower and maximum torque, but also to:
1st gear ratio.
Spacing between each gear.
Highest gear ratio.
Final drive ratio.
You'll gain a more concrete understanding of why cars with the same engine can have different driving characteristics.