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Author Topic: OEM Speedometer reads 2 mph higher than GPS unit is reporting  (Read 550 times)

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Online DrM

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OEM Speedometer reads 2 mph higher than GPS unit is reporting
« on: September 05, 2020, 10:58:15 pm »
Not looking for a fix, just wanted to note that My OEM speedometer consistently shows a speed 2+ mph higher than the speed displayed by my GPS unit (Garmn Zumo XT.)  Anybody else running a GPS unit show similar differences?  For what it is worth, I tend to believe the GPS unit is more accurate.  Also, I run a TPMS unit that pretty much reports the same air pressures as the gage on my air compressor (stock OEM front tire at 34psi, stock OEM rear tire at 38psi.)



Offline Pir8

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Re: OEM Speedometer reads 2 mph higher than GPS unit is reporting
« Reply #1 on: September 05, 2020, 11:29:42 pm »
This is very common. Speedos are only required to be so accurate, around 5-10% variance. Also, tire diameter makes a big difference.  (https://tiresize.com/comparison/)  This means your speedo will lose accuracy based on tire wear and temp.  GPS is definitely more accurate.

Offline lunkhead

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Re: OEM Speedometer reads 2 mph higher than GPS unit is reporting
« Reply #2 on: September 07, 2020, 04:56:44 pm »
They always display a higher than actual speed so you can't blame your speeding on the speedo error. Mine reads 1 mph over at all speeds but that'll change as the tire wears. A 160/80 or taller tire could make it exact at low speeds but read under at high speeds.
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Offline ShakerNorm

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Re: OEM Speedometer reads 2 mph higher than GPS unit is reporting
« Reply #3 on: June 25, 2021, 12:01:28 am »
When I first bought my Bolt, I noticed that at the speed limit, other vehicles were going MUCH faster than I was used to, so I checked mine with a GPS, too.  Found out that it was reading about 7-8% faster than I was actually going.  This means that at the speed limit, Semi's were riding my ass at 120 km/h, when I was only doing 110.....  NOT A GOOD FEELING!

I ended up buying a SpeedoDRD (http://shop.12oclocklabs.com/index.php?route=product/product&path=31&product_id=59) and recalibrating my speedo. It also has a few other features that I don't use, like converting Km/H to MPH, and recording maximum speed, etc  MUCH better! 

Apparently you can calibrate your speed internally with the Koso Speedo.
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Offline DelawarePete

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Re: OEM Speedometer reads 2 mph higher than GPS unit is reporting
« Reply #4 on: June 30, 2021, 12:04:36 pm »
I calibrated my KOSO with GPS.  When I match it up to my speed on WAZE it's only 1-3 mph off at times. 
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Offline Antonio9M

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Re: OEM Speedometer reads 2 mph higher than GPS unit is reporting
« Reply #5 on: July 21, 2021, 08:58:05 pm »
this is common. GPS are the most accurate since they find ur speed via satellites, while on your gauge is via in house equipment. Same thing goes for cars and etc, 2mph-5ish mph is the normal difference I would say