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Discussion Starter #1 (Edited)
EDIT: This situation was user error with calibrating the HUD....not an issue with the OBD2 calculated speed.
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I have a situation and I’m not quite sure where to research. I was having trouble reading the factory speedometer, especially at night on my 2014 Forester. I purchased an aftermarket OBD2 Heads up display from amazon. This HUD works great – except I’ve discovered a discrepancy. The factory speedo and Waze GPS measured speed are dead on. However, the HUD displays 5 MPH faster than GPS and factory speedo @ 65MPH. The HUD unit is a simple display of what the car computer reads as the speed. This means that the car computer says the car is going faster than the factory speedometer. I’m shocked due to the implications of how the car calculates all the other metrics (MPG, miles until empty, odometer). Also, what if you got one of those gadgets from your insurance company to reduce your rate – it would report you drive faster than you do.

Is this surprising to anyone else? Any suggestions on how I might further diagnosis this situation? Thanks all.



 

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2019 Forester Sport Auto
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132 Posts
My former 2007 Mazda 3 and my wife’s 2012 Toyota Highlander speedometers are both 3mph faster than what Waze, a Garmin GPS and Magellan GPS say. Example: speedometer says 65mph but Waze and external GPSes say 62mph. I think it’s the car speedometer that’s wrong and most likely the car manufacturers being safe to avoid any lawsuits over getting speeding tickets.


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2018 Forester 2.5i Base CVT
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Research the manual first ;) See if there is a speed offset/adjustment setting. Typically, devices and apps reading speed from OBD allow for speed adjustment as discrepancies are common.

Thank you,
 

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2019 ForesterTouring 2.5
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The OBDII indicating a faster groundspeed than an independent GPS groundspeed is certainly consistent with the vehicle indicated fuel efficiency mpg being consistently optimistic by about 2 mpg, isn’t it?
 

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Discussion Starter #5
My former 2007 Mazda 3 and my wife’s 2012 Toyota Highlander speedometers are both 3mph faster than what Waze, a Garmin GPS and Magellan GPS say. Example: speedometer says 65mph but Waze and external GPSes say 62mph. I think it’s the car speedometer that’s wrong and most likely the car manufacturers being safe to avoid any lawsuits over getting speeding tickets.


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The speedo and GPS are in synch. It is the values generated by the OBD2 that are faster.
 

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Discussion Starter #7
Research the manual first ;) See if there is a speed offset/adjustment setting. Typically, devices and apps reading speed from OBD allow for speed adjustment as discrepancies are common.

Thank you,
Solid advice. Thanks. I guess no one else is as shocked that the computer reads too fast -- and that all metrics that rely on speed and miles are likely incorrect.
 

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2018 Forester 2.5i Base CVT
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The most accurate speed reading is from a fast refresh GPS device receiving strong signal from as many satellites as possible. Vehicle odometer may be less accurate than GPS and typically shows higher than actual speed (IMO liability and safety reasons). Change of perspective - your "computer reads too fast" statement is inadequate. Computer does not read speed as there is no speed sensor (despite VSS Vehicle Speed Sensor acronym used formally) - VSS refers to RPM sensor and computer just calculates speed. It may be the transmission output shaft sensor or wheel ABS sensor (BTW, sometimes data from those sensors are cross checked to detect malfunctions). So, computer uses wheel size factor to calculate vehicle data, this data may or may not be corrected within the odometer cluster. I do not know what Subaru ECU sends to OBD port, but I would just try to correct it within the HUD unit and drive away,as it is your only option.
 

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Discussion Starter #9
You are right...it is more accurate to say that the computer calculates the speed to be faster than the speedometer and GPS. Given no changes to original tire size, I would have expected the ECU calculated speed to be closer to that generated by the speedometer. It appears that the specific HUD unit I have cannot be adjusted to accommodate for the incorrect ECU calculation of MPH. I'll need to adjust in my head to accommodate for difference. Thanks for your reply.
 

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@Beavz One more time. Attached is a manual for the unit you purchased (at least one you have provided Amazon link to). It shows the vehicle speed calibration factor is set to default value of 108, but it can be changed no problem - I'm surprised you cannot. Now, 65 MPH * 1.08 = ~70 MPH, quite close to 5 MPH positive offset you are experiencing. It proves your ECU provides accurate vehicle speed data and HUD unit just factors it up - set the factor to 100 and everything should be tip-top.

Cheers.
 

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