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12

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 ...


10

At risk of sounding facetious, yes it is normal. It's due to gravity. The steeper the slope the greater the acceleration. Exceptions include: some off road vehicles which use intelligent engine braking for downhill, but even they will only cope up to a certain slope angle, loss of grip.


8

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 ...


8

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

It could be the gear ratios in your car. Vehicles that are geared lower will hold their speed going down hills using the concepts of engine braking, while vehicles geared really high will be more likely to gain speed as a result of little to no engine braking. The other cause could be your idle setting. If your car is set to idle really high, it's going ...


5

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 ...


5

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 ...


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

There are four possible culprits: The vehicle speed sensor which plugs into the back of the differential The wiring from the speed sensor to the back of the instrument panel The printed circuit board on the back of the instrument cluster The speedometer itself As always with electrical problems, test the simplest/cheapest component first. In this case, ...


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 ...


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 ...


3

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 ...


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 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.


3

If the vehicle has a speedometer cable it may be frayed causing the fluctuating of the needle. It's also possible that the speedometer needs to be rebuilt.


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

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 ...


2

Some of them have a GPS reciever, accelerometers and a phone in them, and regularly upload data back to the insurance companies - some of the ones that have been featured on the TV in the UK come with online accounts so you can log in and see the performance data. I've never quite worked out how they are supposed to cope with multiple drivers in the same ...


2

Your behaviour could affect her premium then - these black boxes typically track speed, style of driving, how harsh you are on acceleration and braking, the times of day you drive etc. They collect this data all the time - some insurers collect data from them regularly, others only in the event of an accident - either way, if you do not stick to the driving ...


2

Yes. That is a significant difference so your speedo will be out by a lot. In some regions it is required by law to have an accurate speedo!


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.


2

This very much depends on the incline. If it is a slight incline, I would expect that the vehicle would still accelerate, but probably not if you are going fast (Over 30 mph, say). If it's more than a 5 degree incline, then you could probably go highway speeds without touching the gas. If you find yourself riding the break, as has been suggested, try ...


2

The 92's are known for their speedometer failures. I bought a replacement speedometer from www.techbargains.com. Kinda. It's made by Garmin, and when I put it on the dashboard it tells me how fast I'm going. And where I'm at, and what direction I'm going, as well as my elevation from sea level. AND I can enter an address and it tells me how to get there. ...


2

The Vehicle Speed Sensor sends signal to the Instrument Cluster. You can check pin 2 (blue/white) on the B connector for signal from the VSS. If there is signal you need a new cluster. If not you have to check continuity between the cluster and VSS


2

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 ...


1

There are three things which come to mind with your issue: The wiring which is going to the speedo sensor on the transmission is somehow messed up. This could be a break in the wiring, or it could be corrosion in the socket where it connects to the wiring harness. You can do a continuity check between the wiring harness at the ECU (or TCU if so equipped) ...


1

I think you may be correct about the VSS. It certainly would confuse the automatic transmission and would cause problems on the speedo. Why it would spring into life when the transmission changes gear is a bit puzzling. The VSS could also be involved in comparisons of speed by the ABS system, so could be affecting the ABS. If you are not able to get ...



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