How does a TH400 speedometer work?

How Does a TH400 Speedometer Work?

The TH400 speedometer is one of the most commonly used and reliable speedometers in modern vehicles. Accurate speed readings are critical to safe driving, and the TH400 provides a dependable and accurate way to measure and display vehicle speed. In this article, we will discuss the mechanics behind the TH400 speedometer and how it works.

What is a TH400 Speedometer?

A TH400 speedometer is a type of mechanical, analog speedometer typically found on older cars. This design uses a combination of gears, an electronic circuit board, and a traditional needle-style speedometer gauge to accurately measure vehicle speed from 0 to 120 mph. The device also features a trip odometer that measures the total distance traveled and can be reset manually.

How does a TH400 Speedometer Work?

To understand how a TH400 speedometer works, you must first understand the components that make up the system. There are two main parts of the system: the cable drive unit and the indicator head.

The Cable Drive Unit

The cable drive unit is typically located at the transmission side of the automobile and consists of a drive gear and cable assembly. The drive gear attaches to the transmission output shaft spline and connects to the speedometer cable via a groove in the gear’s periphery. When the transmission shaft rotates, it drives the gear, which in turn causes the speedometer cable to rotate as well. This rotation creates tension on the cable which is then transmitted to the indicator head.

The Indicator Head

The indicator head (or simply “head”) is composed of several components including an electric circuit board, a rotatable needle, and calibrated gears. As mentioned above, the tension generated by the rotating cable carries torque to the head. This torque is then transferred to a series of small gears within the head where it is multiplied before being applied to the needle. The needle then rotates accordingly, providing an accurate reading of speed relative to its calibration marks.

In addition to measuring speed, the heads also contain a mechanism for resetting the trip odometer. This feature allows drivers to track their total mileage over time and reset it when necessary.


Understanding how a TH400 speedometer works is an important part of maintaining your vehicle’s performance. It is important to note that while other types of digital or electronic speedometers may exist, the mechanical elements of the TH400 remain largely unchanged. If you find yourself in need of replacing a damaged or malfunctioning TH400 speedometer, contact your local automotive parts store for assistance. With proper care and maintenance, you can ensure many years of accurate and reliable performance from your TH400 speedometer.

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