Checking speedometer accuracy.

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Old 08-24-16, 07:04 PM
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Checking speedometer accuracy.

Is using a GPS and/or cell phone app good enough to check the accuracy on a speedometer? I always thought my van was about 3 to 5 mph reading over the actual speed. And seemed to check with an old GPS I used. However, I now started using a smart phone app and it reads almost dead on (although there is a time delay of a 1/2 second or so).

On my car I have a HUD display attached to the OBD2 connection and it corresponds exactly to the speedo needle on the dash. It also checks out with the smart phone app (DigiHUd Speedometer).
 
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Old 08-25-16, 03:24 AM
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I find my GPS to be pretty accurate. I have never used Maps to gauge speed, but I guess with any satellite oriented receiver, there is a lag time. I know this from watching altitude markings. Here in the mountains it will read ascending altitude even after I top a gap and begin downhill.
 
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Old 08-25-16, 04:08 AM
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Before GPS I always checked the odometer with the mile markers on the highway. Might not be as accurate and requires some math but always gave me a good idea of how far off the speedo was after a gear or tire size change.
 
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Old 08-25-16, 04:40 AM
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Wife's Camry indicates about 1-2 mph below what the gps does when we have it on. Also most times when we go by a portable radar speed sign, they also show the same error, so I'm assuming the car is actually off. Kind of surprising really.

:WRECKER:
 
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Old 08-25-16, 04:56 AM
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I get about a 2 mp difference between GPS and speedo.
 
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Old 08-25-16, 05:28 AM
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Mine is +/- 2 mph. If my cruise is set on 70, gps speed varies from 68 to 72.
 
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Old 08-25-16, 05:40 AM
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There are many different tires out there to fit your vehicle from numerous manufacturers in various quality levels and design (all-season, performance. M/S etc)

You would have to check the circumference of all those tires and verify that they are all identical to expect a speedometer to be accurate in all cases.

And don't forget tire pressure - a couple of pounds either way will affect speedometer readings.


So many variables you can't expect speedometer readings to be "right on"
 
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