Calculating Engine RPM From Motorcycle Speed

No Tachometer? This is how to Calculate RPM from Motorcycle Speed

In this article, we shall calculate Speed Constant for your Motorcycle, with the help of which, if multiplied with speed gives RPM, or if divided from RPM gives current vehicle speed. Hence it makes possible to find RPM at any given speed. This article is easy to understand and is not too technical.

Why you need to calculate Motorcycle Speed Constant?

1. To know the exact kmph/ mph. Generally motorcycle manufacturers modify their speedometer such that, even if the actual speed is less, the speedometer displays more. This is to convince customers about the Mileage of the motorcycle. I don’t want to name any brands here. I have compared two separate motorcycles side by side at ~30 kmph. One showed 28 kmph and the other 30 kmph.

2. Bikes without Tachometer have a greater advantage. Example: 100 cc Hero splendor, Bajaj Platina, Chethak, RX100, Bullet etc

3. Most useful before purchasing any Motorcycle, it is possible to estimate at what speed the bike will be at the Red line (10000>RPM). Example: Duke 390 will go to a max of 171.3 kmph at 10000 RPM. This doesn’t mean that it will be the top speed. But it will be harder to reach high RPMs at higher speeds. What I mean to say here is, if the BS3 Duke 390 is at 10K RPM in 6th gear, that means it is exactly at 171.3 kmph. If it manages to reach 12000 RPM in 6th gear, it should be flying at 206 kmph.

4. True speed of the vehicle can be known. Digital Displays always shows a value which is 1-5% greater than the actual value. Example, Bajaj Pulsar 180 displays 100 kmph, but the GPS indicates a max of 95 kmph which holds good with the Calculated Speed Constant.

5. While checking for the top speed of a motorbike, it is enough if we record the maximum possible RPM at any gear. So, by Dividing the Speed Constant with the RPM gives true speed. Example: At 9500 RPM pulsar 180 will be at 125 kmph. But won’t reach 9500 in 5th gear. Point to be considered.   

Assumptions

1. The Rear Tire is loaded with the optimum Air/ Nitrogen pressure.

2. No modification is done on the Sprockets. Everything should be stock. If it is not stock, your values will differ from other motorcycles. If have modified your Chain Sprocket, just remember to consider newer values for calculations.

3. The RPMs are usually considered full figures like( 666.666666=667). But in this calculation they are written as obtained like (666.6666666=666.66667)

4. The manual used is of the same motorcycle, who’s Speed Constant to be calculated. The versions like UG3 and UG4, V1.0 and V2.0 are different.

5. Top gear is the highest gear in the vehicle. And the vehicle is moving with the top gear engaged. Top gear for Bajaj Pulsar 220 is 5th gear, for KTM Duke is 6th gear for Honda Shine is the 4th gear. You can still calculate individual values for every gear, once you get hold on the calculations.

Note

All the data required is mentioned in the Manual. If not, google yourself with vehicle model and production year. We have managed to get most of the data for Motorcycles manufactured in India. You can just use these values readily. Any missing information/ wrong info, please contact us through contact form at the bottom of the page. Your contributions will be appreciated. (Currently it is very difficult for me to find technical specs of any motorcycle. Most of the manufacturers hide them)

Click here to know the Engine Specifications of most motorcycles in India. You can also contribute to this Article by providing us the core specifications of motorcycles along with manufactured year that are missing. We will give you credits by mentioning your name in the article itself. (We can also redirect users to their Facebook page). Remember to mention your name and country. Motorcycle models vary in different countries.

Basic Calculations

Final Reduction: No of teeth in Rear Sprocket/ No of teeth in Front sprocket. The value obtained is written up to 4 decimals if available. Varies from 2.0 to 3.5. (In Manual)

Primary Reduction: No of teeth in the Clutch/ No of teeth in the Crank gear. Varies from 3.0 to 4.0 (In Manual)

Gear Ratio: Better to have Gear Ratios of all the Gears. Top gear Ratio is mandatory. It varies around 0.7 to 1.2. (Manual/Internet)

The only tricky job is to find the Tire Diameter and Circumference. It can be measured using a tailoring tape wound around the Rear Tire or by the method below. Easiest way is to buy a tailoring tape.

Calculating the Tire Diameter:

Basically all tires have something like this printed on it,

P315/70R17

This is, Width / Ratio Radial Rim
Width=315, Ratio=70%, Rim Diameter=17”

(There is a separate article on How to Calculate Tire Diameter. This article will make it very easy to understand)

The Ratio after slash is the height of the Tyre. This means, 70% of the width is the height.
Tire Diameter Formula is as follows:

Metric Tire Diameter= [(Width x Ratio)/ 1270] + Rim

The value obtained will be in inches and is to be converted to metre. It varies from 0.5 to 0.7.

Note: For Imperial Tire markings, just use below Formula. Keep everything in inches. For any confusions, please read this article on Calculating Tire Diameter.

Imperial Tire Diameter= [ 2 x Height ] + Rim

Again, convert obtained value to meter from inches.

Calculating the circumference & RPM @ 1 kmph

The obtained value is multiplied with π ( 22/7). Now we get the circumference in meter. It will be around 1.4 to 2.2 m.

Assume that you are riding your motorcycle at 1 kmph. In order to complete 1 km, your Tire has to rotate some revolutions. Let’s calculate that itself.

Tire RPM= (1000/ 60)/ Circumference = 16.6667 / Circumference

The obtained value is itself the RPM of the Tire at 1 kmph. The value will be ≈ 8 RPM

Now the Final Formula to find Speed Constant goes like this.

Speed Constant= Primary Reduction x Final Reduction x Gear Ratio x Tire RPM

Below is an Example of Bajaj Pulsar 220.

Tire Size= 120/80-17. Here 120 is the width, 80 is the Ratio, and 17 is the Rim.

Tire Diameter = (120 x 80/1270)+17=24.559 inches.
Converting 24.559 inch in meter becomes 0.6238 m
Hence Circumference = (22/7) x 0.6238= 1.96 m.

For a speed of 1 kmph, the Tire RPM = 16.667 / 1.96 = 8.5034 RPM.
Speed Constant = 8.5034*3.4736*2.5714*0.8888 = 67.5

Now Multiplying SC with Speed gives RPM,
Engine RPM @ 100 kmph = 67.5 x 100 = 6750 RPM
OR if assume, the motorcycle is at 8000 RPM in its Top Gear.
Then the Speed will be = 8000 / 67.5 = 118.5 kmph

Calculating minimum Top Speed for your motorcycle

It is possible to calculate minimum possible top speed of your motorcycle even before testing it. First, calculate the speed constant. Divide the value with the RPM mentioned in the manual for maximum power. The value you get is the minimum top speed.

As an example, lets consider Pulsar 220.
RPM at maximum power output, [email protected]

Since the Speed Constant we just calculated is 67.5, dividing with max RPM gives,
8500/67.5= 126 kmph.

This means, according to specifications, it should reach 126 kmph. For maximum speed, it depends on the actual power output. If the engine is at 8500 RPM, it does not mean that you are getting 21HP. It depends on the type of Fuel, condition of Air Filters, Spark Plugs etc. And moreover, how can you trust in the manufacturer that their engine produces 21HP? During the time of testing a new Motorcycle, manufacturers somehow clear the ARAI test with some Jugaad compiling the latest emission norms. Who is there to regulate? Remember the Volkswagen Emission Scandal. As the Engine ages, Tolerances increase between Bore and Piston. Thereby reducing power output.

Once the Engine reaches 8500RPM, it wont stay there. It will still increase. But the Power Output will decrease. This makes the motorcycle take more time to reach its maximum speed with constant power output. Also, it has to bear Rider Weight along with road conditions. So, maximum speed calculations have more factors.

Kawasaki Ninja 300 produces maximum power at higher RPMs about 11000RPM. Because, if it makes that exact power at lesser RPM, then it will struggle to reach its top speed because there will be no required energy. And that is the reason Ninja 300 reaches 190 kmph with 38 HP and Duke only reaches 170 kmph with 43 HP. In the case of Duke, the maximum power is produced at 9000 RPM.

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By the way, I already have Speed Constant Values of most Motorcycles in India. You can find it here, Speed Constant Values of Motorcycles

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