Motor Vehicle Maintenance & Repair Asked by BillDOe on May 14, 2021
My 2014 Honda Accord has GPS and uses the GPS clock signal to set the vehicle’s clock. This made me wonder if there are any vehicles that use the GPS signal to adjust the vehicle’s speedometer. Of course, using the GPS signal alone to display the vehicle’s speed wouldn’t be a good idea, as a GPS signal may not always be available (deep canyon, tunnel, etc.), but it could check the vehicle’s speed using GPS data and adjust the speedometer’s input accordingly. For instance, if the speedometer is supposed to use a current input of, say, 6 milliamps to display a speed of 60 mph, but input from GPS indicates that 60 mph is actually 5.9 milliamps, then the speedometer could be calibrated to use 5.9 milliamps to display a speed of 60 mph and, assuming the current output is linear, adjust all other speeds accordingly.
that is a good idea, but in my experience no. the speedometer is usually driven by wheel speed sensors (newer vehicles) or mechanical cable driven by the transmission (older vehicles). depending on the model, there can be one sensor per wheel. the wheel speed sensor provides a squared wave signal produced by a magnetic pickup style sensor to the vehicle's on board computers. based on the frequency of the square wave a speed figure is deduced by the computer and displayed on the dash. the computer also compares all wheel speed sensor inputs for any differences above a preset threshold to monitor for wheel slippage. this is how most modern day traction control systems function. that is also why a larger(new tire) or smaller(worn) tire compared to other three can cause traction control lights to illuminate on the dash or the speedometer to read incorrectly if 4 larger/smaller tires installed. i am guessing auto manufactures use the most cost effective means when building vehicles and choose components accordingly. as for gps driven speedometers, they do exist. you can buy them aftermarket for say a project car or boat. hope this helps! have a great day
Correct answer by Max on May 14, 2021
in my experience you are correct, no manufacture that i have seen so far updates speedometer accuracy via GPS. it would be a good idea however, wheel speed sensors are fairly accurate but with tire wear it does loose accuracy
Answered by Max on May 14, 2021
I have a vague recollection of watching Clarkson driving a concept car, a 4x4, in the desert somewhere on an episode of Top Gear many years ago. I can't remember the manufacturer but have a sneaking suspicion that it was Nissan. I distinctly remember him commenting that the speedometer used GPS, not wheel speed.
Sadly Google hasn't helped me to remember what car this was and I don't think it ever made it to production.
Answered by Steve Matthews on May 14, 2021
This sounds like something that would be used in a fighter jet to correct the drift of the onboard INS, but there's no need for such an accuracy in a land vehicle; also, the speedometer doesn't accumulate a drift over time like an INS does. In fact, it's the other way round: the speedometer's reading is used to correct the accuracy of the GPS receiver and improve the position update frequency. A gyrocompass improves accuracy and update frequency further since it can also sense where the vehicle is actually pointing long before the GPS data is interpolated. Add some accelerometers to all of this and you have a full blown GPS augmented INS system too, pretty useless in a land vehicle except for military or research purposes. Google for "Kalman filter" if you want to know the math behind this too.
Answered by Al_ on May 14, 2021
No. The reason the speedometer is slightly out on just about every car is that they could get sued if the speedo says 60 and you were clocked doing 61 or something. So, what manufacturers do is make the speedo read between 5% and 10% faster than you're actually going to give you a margin of safety and also to provide for changes in tire circumference between brands (something they have no control over). So the likelihood of someone doing this in a car is very low.
Answered by Captain Kenpachi on May 14, 2021
The Polar FIS (https://www.autopolar.net/product/polar-fis-mk5-mk6-8p-8j/) for VAG cars has this feature (called Vehicle Real speed). Though on a separate speed indicator in the FIS/MFD cluster. It uses the wheel speed upon correction data calculated when stable GPS speed is received from the navigation system (usually within a kilometer after startup). The benefit above pure GPS displays is when running in confined spaces (tunnels or mid-city roads), the speed is still visible and corrected.
Answered by Johannes Bø on May 14, 2021
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