23

There are pros and cons of each, remember you can easily spin or lock up the rear wheel, and in fact the rear wheel may be far less often at the correct speed. So the decision on this stems from physical connectivity it's much easier to connect from the front wheel to the odometer which is on the handlebars than to route from the rear wheel, up under the ...


13

You can calculate the size of the effect from simple geometry. Tire wear reduces the diameter of the tire, which reduces the circumference of the tire. New passenger car tires typically come with 10/32" to 11/32" of tread depth (source). Tires are considered fully worn-out when only 2/32" of tread remains. So the tire has lost 8/32"--9/32" on the radius, or ...


12

Its not a rule of thumb that the Speedometer is connected to the front wheel. There are many Suzukies and hondas in my country that have the speedometer connected to the rear wheel. As Rory stated, the only reason I guess is due to the convenience factor since the front wheel is closer to the cockpit.


12

The front wheel is used in most cases with mechanical speedometers (though there are exceptions) because it's just easier (and probably cheaper) to couple the front wheel to the mechanism. Additionally, a mechanical speedometer uses a cable. The longer it is, the less reliable and accurate it becomes, thus the speedometer takes the speed from the nearest ...


9

The speedo will be a standard linear voltmeter. Vehicle speed sensors are usually attached to either the gearbox output or the differential and produce a pulsed output at a frequency proportional to the vehicle speed. On a 'normal' speedo, the ECU will read the pulses from the speed sensor and produce a linear voltage proportional to the vehicle speed to ...


9

The only thing your car can sense is the number of turns of some part of the drive train. In the old days it was a gear right off the transmission, and today it's an electronic sensor in the transmission or somewhere in the drive train near the wheels. It's going to be a sensor that can determine how quickly the wheels are rotating. The way to calculate ...


7

Generally a 30 zone implies you are in town, where you are subject to children, pets, footballs, stopping traffic etc. Cruise control is for long distances at a constant speed where you are not subject to rapidly changing risks - not for stop-go situations. In town you should be concentrating on what is going on and adjusting your speed constantly based on ...


7

That's the check engine light, or CEL - if it lights up, you have a problem with the engine and should stop and investigate it. If it's blinking, count the number of flashes, as this will indicate a particular error code which can help you to determine what is wrong.


6

Most speedos in the dash for modern electronic based cars, have a sensor in the output shaft of the transmission. It is a simple reluctor wheel and Hall effect sensor, or a gear driven motor which can then produce a signal for the PCM to interpret. Cars can also use the same method off of the anti-lock wheel sensor to get a "speed" indication. The more ticks ...


6

All of my competition cars have had speedos, some of which didn't work at all. I have never once been in a race and had time to take my eyes off what was happening around me to look at the dials. This is why competition cars tend to equip their dashboards with bright lights which come on when attention is required. The times a speedo is useful is during ...


5

I would bet the electronic speed sensor in the transmission is malfunctioning. It should look something like this: It suggests it is located at the top of the transmission, so should be fairly easy to get to. One plug, one bolt.


5

Manufacturers deliberately calibrate their speedos incorrectly. They make sure the speedo always reads higher than the actual road speed, and there is a very good legal reason for this. Speedometers must never read lower than the actual speed (European law (ECE-R39) says speedometers cannot show speeds less than the actual speed. Other countries have ...


5

Follow up 10/4/2017 Since the incident reported below, I had the car diagnosed by Toyota and, as expected, they recommend replacing the instrument display unit (5 weeks to get the part and ~$900 in costs). In the last month we have now experienced 3 more "no display" events and I have been able to get the display back using the following "improved" ...


5

Yes, I'd take it back to the shop to see if there is something they missed. While 300 rpm at 60mph is not a huge difference, to me this is about the difference of when a lock up torque converter isn't locking up. This could be a programming issue which wasn't taken into account with the new CEM. While it isn't a huge issue, it will cause greater wear of ...


5

If the ECU would calculate speed from RPM, how would it calculate speed when the car is in neutral or during shifting? The speed usually is derived from the RPM of the output shaft of the transmission (to the differential). This way, one gets an average of the speed of the left and right wheels. The key to your observation is the differential. It ...


4

The symptoms point to a failed Combination Meter. Besides the speedometer, it also controls the power off button and the dashboard lights (except the check engine light). Apparently it can also be rebuilt, but I could not find how is this repair done. There are some eBay ads of people who do it for about $150. It may resume working randomly. To stop the ...


4

UK type approval states that a speedometer may read with upto a 10% error provided that the speed shown is either true or over. A speedometer may not under read when a car is new. Most manufacturers therefore calibrate their speedos for a slight overread. You must also remember that the difference between tyres with new tread and those worn to the UK ...


4

There are two basic approaches, you can tap a speed sensor directly before it reaches the computer, or use OBD II signals (generated by the computer). Sensors usually generate a voltage, so you have to find the wire you are interested in and then install an analog-to-digital converter. This then has to either go directly to a COM port (if your computer has ...


4

The speed is very simply calculated from number of rotations of the axle multiplied by the circumference of the tyre. All that onboard computer then does is then divide by time to get average speed.


4

Depending on the age of your Jeep the recalibration involves either swapping speedo drive gears in the tranny or reprograming the computer. The reasons for having this done are that modern vehicles use many data points for different systems and they all interconnect at some point. ABS, transmission shiftpoints,traction control, cruise control are all looking ...


4

Often times the speedo cable just needs lubrication. You can do this by disconnecting the speedo cable at the back of the speedometer and dropping some oil down the cable. Happens on Miata's all the time.


4

Somewhere there is a signal that measures rotating speed on the driveline, downstream of the torque converter. It is typically somewhere inside the transmission housing. Because of how a torque converter operates, engine RPM would not be a totally great indicator for vehicle speed. Typically auto manufacturers use a hall effect sensor to determine ...


3

There are many variations, and no consistent method across manufacturers. That said, your theory is not correct - you are more likely to get the legal highway speed somewhere near the top because that is an area where the eye can easily see variation. Even within manufacturers there seems to be a lack of consistency. Where they use the same clocks across ...


3

The older mechanical speedometers were cable driven. The cable is more like a very tight coil spring with a small square driver crimped on each end. One end was driven by the transmission the other end drove the speedo head. The speedometer was calibrated to the vehicles tire size and final gear ratio by installing specific speedometer drive gears in the ...


3

The speedometer is generally connected somewhere after the gearbox eg the driveshaft. So in normal circumstances this is directly proportional to the angular velocity of the wheels, depending on the differential ratio and assuming no wheel slip. This needs to be somewhere between the clutch and the differential otherwise it would jump around whenever the ...


3

Firstly you need to determine whether your speedometer is accurate or not. Ask a friend to drive and hold different speeds in a car with an accurate speedometer and then pace behind that car to see how yours compares. If your car's speedo reading is different then you know you have an issue. Speedometers work by measuring the rotation speed of something ...


2

Size: YES. Wear: Yes... but it's negligible. If the bigger the tire the slower the speed on the speedometer. The easy fix is to just use a GPS in addition to the speedometer. Since you are monkeying with ratios the speedometer will be off by some percentage. So the faster you go the more the meter is off. I put a smaller wheel on my motorcycle. Now the ...


2

Considering that all cars after 96 I believe have OBD installed, it is possible to make a device that records your every move (I am actually making one as we speak). ODB may do this already (the car needs this info for other tasks). However, this is not necessarily court approved and depending on the offense and the level of court it may not be admissible.


2

Having owned a 97 Taurus which is a Sables cousin a common problem is the vehicle speed sensor. If the sensor completely fails it is easy to diagnose. Mine slowly failed over a two week period during which time it would shift into nuetral, have eratic speedometer readings,and would shift up or down at odd times or work perfectly fine. The coolant resevoirs ...


2

I assume you're using a factory speedometer. If so, are your wheels factory size? Wheels being the wrong size is a sure-fire way to throw off your speedometer.


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