# Calculate Top Speed of Car Using Gear Ratio, Torque, Power & RPM

• Karan Dua
In summary: The tires which we are using will be 25x8-R12. And the r will be effective rolling radius if i am not wrong? And mass of the car is 400 kgs. And what aabout acceleration on slope? How to calculate that? And do we calculate torque at each gear @ different rpm?...For calculating acceleration on slope, you would need to use the following equation:Acceleration on slope = (G*v^2)/(F*r)Where G is the acceleration due to gravity, v is the speed of the car, and r is the rolling radius of the tire. You would need to calculate the acceleration at each gear at different engine speeds, and then put
Karan Dua
Hey guys,

I am a bit confused while calculating top speed of the car using gear ratio, Torque, Power and Engine Speed(RPM). Can someone help me out with this?

Specifications are given as follows:

Maximum Torque : 19.2 N-m @ 3800 RPM
Maximum Power : 10 Hp @ 2800 RPM

Tire Specifications: 25x8 - R12 or 254/65 - R12

Gear Ratios:
Initial Input Reduction - 2.869
First Gear Ratio - 4.6
Second Gear Ratio - 2.733
Third Gear Ratio - 1.667
Fourth Gear Ratio - 1.115
Final Forward Reduction - 2.38
Final Backward Reduction - 4.17

I am looking for top speed at each gear at different RPM, as well as acceleration, Torque and Tractive Effort.

Thanks and regards

Karan Dua!

Wheel diameter is 12 inches? Tire width = 254mm = 10 inches. Tread wall height = 65% x 10 inches = 6.5 inches. Tire diameter = 12 + 6.5 + 6.5 = 25 inches. Multiply initial and final gear ratios (6.82822) by each gear ratio: 1st = 31.41, 2nd = 18.66, 3rd = 11.38, 4th = 7.613 . See if you can continue from here.

Karan Dua said:
Hey guys,

I am a bit confused while calculating top speed of the car using gear ratio, Torque, Power and Engine Speed(RPM). Can someone help me out with this?

Specifications are given as follows:

Maximum Torque : 19.2 N-m @ 3800 RPM
Maximum Power : 10 Hp @ 2800 RPM

Tire Specifications: 25x8 - R12 or 254/65 - R12

Gear Ratios:
Initial Input Reduction - 2.869
First Gear Ratio - 4.6
Second Gear Ratio - 2.733
Third Gear Ratio - 1.667
Fourth Gear Ratio - 1.115
Final Forward Reduction - 2.38
Final Backward Reduction - 4.17

I am looking for top speed at each gear at different RPM, as well as acceleration, Torque and Tractive Effort.

Thanks and regards

Karan Dua!

Welcome to the PF.

Can you give us the context of the question? Is is a homework problem, or a schoolwork project, or a personal project? What kind of car is this that is running on only 10hp? Some kind of a go-kart?

berkeman said:
Welcome to the PF.

Can you give us the context of the question? Is is a homework problem, or a schoolwork project, or a personal project? What kind of car is this that is running on only 10hp? Some kind of a go-kart?
Just a project!

Karan, my first comment is that your horsepower and torque peak speeds can't be right. The power peak is (necessarily) always at a higher speed than the torque peak. You may have a typo, or got bad data.
As for top speeds in each gear, it depends on the aerodynamic drag Fd and rolling resistance Fr of the car. At each vehicle speed, calculate the amount of power it takes to propel it: (Fd+Fr)*v. Then pick a gear and calculate the engine speed omega at that vehicle speed. If the engine can make more than (Fd+Fr)*v, then the car can go faster. Keep trying higher vehicle speeds until the engine can't produce enough power to propel it, or until it hits redline.

Randy Beikmann said:
Karan, my first comment is that your horsepower and torque peak speeds can't be right. The power peak is (necessarily) always at a higher speed than the torque peak. You may have a typo, or got bad data.
As for top speeds in each gear, it depends on the aerodynamic drag Fd and rolling resistance Fr of the car. At each vehicle speed, calculate the amount of power it takes to propel it: (Fd+Fr)*v. Then pick a gear and calculate the engine speed omega at that vehicle speed. If the engine can make more than (Fd+Fr)*v, then the car can go faster. Keep trying higher vehicle speeds until the engine can't produce enough power to propel it, or until it hits redline.

Hey Randy,

I just want to correct myself.

Power : 10 Hp @ 3800 RPM
Torque: 19.2 N-m @ 2800 RPM

How do I find acceleration at each gear @ different RPM of engine.

For example: How do i find acceleration @ 1st gear with gear ratio of 31.45:1 @ 2800 RPM?

Karan, I couldn't tell what the tire radius rTire was from your tire spec's above, and didn't see the car's mass.
You'd need to calculate the tractive force F, which is (19.2 N-m)*(31.45)/rTire. Then it's just a=F/m.

Karan Dua
Randy Beikmann said:
Karan, I couldn't tell what the tire radius rTire was from your tire spec's above, and didn't see the car's mass.
You'd need to calculate the tractive force F, which is (19.2 N-m)*(31.45)/rTire. Then it's just a=F/m.

The tires which we are using will be 25x8-R12. And the r will be effective rolling radius if i am not wrong? And mass of the car is 400 kgs. And what aabout acceleration on slope? How to calculate that? And do we calculate torque at each gear @ different rpm? If so, how?

Karan Dua said:
How do i find acceleration @ 1st gear with gear ratio of 31.45:1 @ 2800 RPM?

The acceleration is found with equation 1b on this page. The force Ft in that equation can be found with http://blog.mechguru.com/vehicle-design/tractive-force-calculation-for-a-vehicle/ (TE = Ft).

Last edited by a moderator:

## 1. How do you calculate the top speed of a car?

To calculate the top speed of a car, you need to know its gear ratio, torque, power, and RPM. These values can be found in the car's specifications or can be measured through testing. Once you have these values, you can use the following formula: top speed = (power * gear ratio * 336) / (torque * RPM). This will give you the top speed in miles per hour.

## 2. What is gear ratio and how does it affect top speed?

Gear ratio refers to the ratio between the number of teeth on two gears that are meshed together. In a car, the gear ratio determines how much power is transferred from the engine to the wheels. A higher gear ratio means that more power is transferred, resulting in a higher top speed. However, a higher gear ratio also means that the car will have less torque, making it slower to accelerate.

## 3. Can a car's top speed be increased by changing the gear ratio?

Yes, a car's top speed can be increased by changing the gear ratio. As mentioned before, a higher gear ratio will result in a higher top speed. However, changing the gear ratio can also affect the car's acceleration and fuel efficiency. It is important to find a balance between top speed and other factors when considering changing the gear ratio.

## 4. How does torque affect a car's top speed?

Torque is a measure of the rotational force of an engine. In a car, torque is what enables the wheels to turn and move the car forward. The higher the torque, the more force the engine can exert on the wheels. This results in a higher top speed. However, as mentioned before, a higher gear ratio can also decrease torque, making it important to find a balance between the two when trying to increase top speed.

## 5. Can a car's top speed be limited by its power or RPM?

Yes, a car's top speed can be limited by its power or RPM. If a car's engine does not have enough power to overcome air resistance and reach a higher speed, the top speed will be limited. Similarly, if the RPM is limited, the engine will not be able to reach its maximum power output, resulting in a lower top speed. This is why it is important to consider all factors, including power and RPM, when calculating the top speed of a car.

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