What causes a car speedometer to go wrong?

Changing a rear differential gear ratio or the wheel and tire size are two reasons for the speedometer to provide incorrect readings.

What do you need to know about speedometer calibration?

This calibration must take into account several factors, including the ratios of the gears in the drive cable, the final drive ratio in the differential and the diameter of the tires. All of these factors affect the overall speed of the vehicle. Take tire size, for example.

Is it possible to make your Speedo and odometer inaccurate?

You just made your speedo and odometer inaccurate. Here’s how to fix them both. If you’ve ever modified a car with smaller or larger tires or, perhaps, a different final-drive gear, you’ve immediately screwed up your speedometer. This is, of course, annoying, but for modern cars with a digital speed sensor, there’s a relatively easy way to fix it.

Where is the speedometer cable located on a car?

The speedometer cable gear (driven gear) is mounted within a sleeve/housing that goes into the side of the transmission’s tailshaft housing. This is where the speedometer cable attaches to the transmission. The driven gear’s teeth contact another gear (drive gear) that is attached to the transmission’s output shaft.

What happens if you don’t change gears on speedometer?

If you didn’t change speedometer gears, actual vehicle speed would 73.1 percent of indicated speed (2.73/3.73 = .731). That being the case, what’s required is speedometer gearing that will result in the driven gear spinning at 73.1 percent of its current speed.

Where does the speedometer go in the transmission?

There are two gears in the transmission that make the speedometer work. The speedometer cable gear (driven gear) is mounted within a sleeve/housing that goes into the side of the transmission’s tailshaft housing. This is where the speedometer cable attaches to the transmission.

What should I do if my speedometer says 52?

“If you need to slow down the speedometer, you need more teeth on the driven gear.” For example, if your speedometer is showing 60 mph and it should be showing 52 mph, you will need to use a driven gear with more teeth to spin the speedometer cable slower.

How many teeth are in a GM speedometer?

The Turbo 350, Powerglide, and GM four-speeds have driven gears that have anywhere from 17 to 25 teeth. Speedometer driven gears in the Turbo 400 and 700R4 have between 34 and 45 teeth. In these transmissions, each tooth added or deleted changes your speedometer reading.

Can a speedometer read slower than the actual speed?

It’s a lot of math, but the simple version is that no speedometer can read slower than the actual speed. Ever. On the high side, it’s allowed to read up to 10% above the actual speed plus four or six kilometers per hour, depending on the type of vehicle. Compare this chart to the first one.

Is there such thing as margin of error on speedometers?

This can have a big impact on the speedometer’s readout, which means in order to be effective, it has to allow for a margin of error. For ordinary passenger vehicles, there’s no law in the U.S. that regulates speedometers, but U.S. manufacturers (and most Japanese as well) subscribe to a standard called SAE J1226.

Why does the speedometer read higher when the tail shaft spins?

The faster the tail shaft spins the higher the reading on the dashboard. This type of implementation provides car makers with some flexibility to change calibration by changing the size of the gear that mounts on the transmission.

How can I tell if my speedometer is not working?

Whether you have a digital or manual speedometer, if it’s not displaying accurate information, it doesn’t really help you. One of the biggest signs your speedometer sensor is not working properly is the speedometer will display inaccurate readings or the needle or digital numbers will not move at all on the speedometer display.

What’s the difference between a speedometer and speed sensor?

A speedometer is an instrument which provides the driver with instantaneous readings of speed. Traditional speedometers used gears and wires to determine speed, while most modern vehicles use speed sensors for the same.

What should the bias be on a speedometer?

Your speedometer reading must be within a range of plus or minus four percent off, but that’s four percent over the entire range of the speedometer, and that range can then be shifted in what’s called a bias. Look at the above chart based on a Scion’s technical guide.

How many miles per hour can a speedometer read?

On the high side, it’s allowed to read up to 10% above the actual speed plus four or six kilometers per hour, depending on the type of vehicle. Compare this chart to the first one. The most a speedometer can be off at that same 100 mph (160 kph) is nearly 13 mph.

Who is the best person to repair a car speedometer?

Mark Gittelman is an ASE-certified master technician with over three decades of experience in the auto repair field. Have you ever taken a look at the speedometer on your classic car and the needle was shaking in rhythm with a horrendous high-pitched sound?

Where does the signal come from on a speedometer?

A signal originates from a driven gear that meshes with the transmission tail shaft. This set up rotates a flexible metal core inside of a speedometer cable, which in turn, connects to the back of the speedometer head mounted in the instrument cluster . The faster the tail shaft spins the higher the reading on the dashboard.

Why is my speedometer sticking?

The speedometer cable may be damaged or broken. If the vehicle has cruise control, the cruise control transducer could be faulty and a new one should put in its place. Scenario: you are driving and suddenly the speedometer needle becomes stuck. Even when you slow down or speed up, the needle stays in the same place.

What causes a speedometer to stick?

Older analog speedometers often stick due to problems with their speedometer cable or wear and tear damage to the speedometer itself. Digital speedometers and gauges in newer cars that “stick” or remain at a specific speed likely necessitate replacement by a dealership.

What causes my speedometer needle to bounce?

“Bounce” in this context means that although the car is moving at a constant speed, the speedometer’s needle seems to jump around at +/- 5 MPH or so of the current speed, never resting at any particular value. Common causes for this problem are a binding speedometer cable, a failing angle drive, or a slipping dust cap.

Why does my speedometer bounce?

Bouncing or jerking of speedometer. In cases where the speedometer does not settle on any particular reading but keeps moving between speeds, it is almost always due to bad wiring, in case of a cable system, or a faulty speed sensor.