Did you know that the engine’s RPM gauge, also known as the tachometer, was not present in cars in the past? In fact, it was added later when car manufacturers started incorporating advanced transmissions into their vehicles. The primary reason was to help drivers make the correct gear selection, especially in cases where the driver cannot rely solely on the sound. An example of this is well-insulated cars where the engine noise is not audible while driving.
To better explain what RPM is, let’s start by understanding its meaning. RPM stands for “Rounds Per Minute” and means the number of engine revolutions per minute. Specifically, it indicates the number of rotations the car’s crankshaft makes in a minute.
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How Does a Tachometer Work?
In older cars, the tachometer used to rely on a small gear connected to the crankshaft, providing the gauge on the dashboard with the rotation speed in a mechanical way. With modern technologies, the gear has been replaced by an electronic sensor called the crankshaft sensor, which provides an accurate reading of the engine’s RPM to the vehicle’s computer.
Gasoline car tachometers typically range from 1 to 7, or up to 10 on higher-performance models. Diesel vehicles, on the other hand, usually have tachometers that go up to 6. In the center of the tachometer, there’s a number (usually 1000) with an “x” next to it. This indicates that the engine RPM is calculated by multiplying the tachometer reading by 1000. For example, a reading of 2 on the gauge equates to 2000 RPM.
The Connection Between RPM and Speed
While many might think that a greater RPM means higher speed, that is not always the case. There is no fixed relationship between the speed of the car and the engine’s RPM. For example, when launching the car, RPM could easily reach 3000 while the speed doesn’t exceed 10 km/h. In other situations, a vehicle could be doing 120km/h on highways while the engine’s RPM isn’t exceeding 2000.
Many factors can affect the numbers you see on your dash and below we list some of them:
Gear Design and Number
The gear design varies from one company to another. Each company has a different vision regarding the torque that should be produced by their engines and at which RPM should it reach the maximum torque. This varies from one car to another depending on the type and the manufacturer. The number of gears in the car’s gearbox also affects RPM. Some vehicles have only 3 gears while others have 4, 5, 6, or sometimes 10 gears.
Engine Type
Engine type also influences the relationship between speed and RPM. Diesel engines typically operate at lower RPMs, which is reflected in their tachometers, usually only being scaled to 6.
Car Load
The amount of load that the car is carrying affects the ratio of speed to RPM. Two similar cars of the same type running together on the same road and under the same conditions will certainly have different RPM per speed ratios if one has a heavier load onboard.
Air Conditioner
An air conditioner is another factor that leads to an increase in RPM. although its effect on high speeds is not very noticeable, launching a car with the AC on even feels different and the RPM is usually noticeably higher.
Road Type
Even the same car may have a different RPM-to-speed ratio on different roads. Uphill roads increase the RPM much more compared to speed than downhill roads do. Even slightly pitched roads that we think are flat can have a major effect. You could be traveling and wonder why the car’s torque is weak without knowing that the road is going uphill.
Aerodynamics
The aerodynamic efficiency is the car’s ability to cut through the air with the least possible resistance. Automakers have extensively studied this subject to improve their vehicle’s performance and fuel economy. Even driving against the wind can affect the RPM-to-speed ratio.
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