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ABCD ABCD • May 19, 2011

# Speedo and Tacho

Speedometer reads speed of the vehicle, and Tachometer reads RPM of the vehicle. Why do we require two meters, though i feel both seems to be like same? Both tell speed of the vehicle. 1. in Mi/Hr or KM/Hr and others in rpm(revolutions per minute).

If you know one, you can calculate the other. Whats the exact use of using tachometer?
Al-Otaiby • May 19, 2011
Good afternoon,

The speedometer (right in this photo) is a gauge that measures and displays the instantaneous speed of a land vehicle (ground speed, or how fast you are going)

A tachometer (revolution-counter, Tach, rev-counter, RPM gauge) (left in this photo) is an instrument measuring the rotation speed of a shaft or disk, as in a motor or other machine. In cars it measures Revolutions Per Minute for the crank shaft.

A steady, fully stopped, engine running car might count 750-1500 RPM, on the other hand, in a fully stopped car when you hit the gas pedal to the metal you can drive the RPM gauge to its max.
ABCD ABCD • May 20, 2011
A steady, fully stopped, engine running car might count 750-1500 RPM, on the other hand, in a fully stopped car when you hit the gas pedal to the metal you can drive the RPM gauge to its max.
I didnt understand this. If i press the gas pedal the car should start moving(provided car in neutral). Whats the basic use of knowing RPM of the vehicle?
Al-Otaiby • May 20, 2011
If the gear is in neutral position, and the ground is flat, no matter how much you press because it won't move (speedometer is ZERO)!
however, the RPM will move and indicate and show the rate of rotation of the engine's crankshaft, and typically have markings indicating a safe range of rotation speeds. This can assist the driver in selecting appropriate throttle and gear settings for the driving conditions. Prolonged use at high speeds may cause inadequate lubrication, overheating (exceeding capability of the cooling system), exceeding speed capability of sub-parts of the engine (for example spring retracted valves) thus causing excessive wear or permanent damage or failure of engines more applicable for manual transmission.

zaveri • Jul 22, 2012
speedometer : measures vehicle speed in mph or kmph.

tachometer : measures engine speed in rpm.

it should be noted that while speedometer measures linear speed, the tachometer measures rotational speed.
white_hat • Jul 23, 2012
mreccentric
Speedometer reads speed of the vehicle, and Tachometer reads RPM of the vehicle. Why do we require two meters, though i feel both seems to be like same? Both tell speed of the vehicle. 1. in Mi/Hr or KM/Hr and others in rpm(revolutions per minute).

If you know one, you can calculate the other. Whats the exact use of using tachometer?
You need to know the overall gear-ratio to calculate engine rpm from linear speed.
I think tachometer is provided to optimize the gearshifts, better fuel economy can be achieved by shifting at the correct rpm.
veera s kamesh • Aug 2, 2012
white_hat can i know at which rpm i hav to shift the gear. is it depends on cc of a engine
white_hat • Aug 2, 2012
veera s kamesh
white_hat can i know at which rpm i hav to shift the gear. is it depends on cc of a engine
Depends on what rpm does the engine produces the maximum torque.But you don't have to wait to reach a higher rpm, it depends on engine power,whether it is diesel or petrol engine, load on engine, road conditions etc.
Once you become an experienced driver you will get a hang of it, you will start shifting depending on the driving conditions.
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Some vehicles have got gear shift indicator to help you shift at the correct speed.