Many older cars are not structurally equipped with engine speed indication devices - tachometers. Experienced drivers determine them approximately by ear. But there is a way to more accurately determine the crankshaft speed based on the speedometer readings.
Necessary
- - vehicle operation manual;
- - reference and technical literature;
- - micro calculator.
Instructions
Step 1
To ensure the required accuracy in determining the rpm, first perform some calculations based on the data that can be found in the vehicle's operating instructions or in the technical literature. It is necessary to know exactly the gear ratios of the gearbox and final drive of the vehicle. You also need to calculate the diameter of the tires based on their dimensions.
Step 2
To calculate the outer diameter of radial tires, multiply the width of the profile by its height, then by 2 and divide by 100. To this result, add the rim diameter converted from inches to millimeters. In the designation of the size of such tires, the section width is indicated by the first number. Then, after the sign of the fraction, the height of the profile as a percentage of its width. After the letter R, the bore diameter in inches.
Step 3
To calculate the outer diameter of bias tires, keep in mind that the first digit in their dimension is the section width in inches. The percentage height of the profile is always 80. The second figure is the bore diameter, also expressed in inches. One inch equals 25.4 mm.
Step 4
Then calculate the revolutions of the drive wheels at a certain speed. Before that, do not forget to convert the speed value from km / h to m / s, and the outer diameter of the wheels in meters, dividing the first by the second. The result is the wheel speed r / sec at a certain speed.
Step 5
Calculate the total transmission ratio. To do this, multiply the gear ratio of the gear in which the movement takes place with the speed taken for the calculations by the gear ratio of the main gear.
Step 6
To calculate the exact number of revolutions of the engine at the speed taken for the calculation, multiply the results obtained in the 4th and 5th steps among themselves. Make calculations for different speeds so that you can quickly determine the crankshaft rpm while driving. In this case, do not forget to take into account the gear ratio of the gear used in the calculations.
