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I've been driving this to work over two months with GPS app running on the phone. And the speedometer is off showing faster than it's actually doing. At 70mph it's 4.5 mph slower than the speedo is displaying. It's even worse at 90+ but that's because it's a linear progression. Suspect that the deviation is from tire/rim. However they are factory. Could the factory set the calculations with another tire/rim configuration? And can this be adjusted? Newer electronic speedos have a adjustment just for this situation.
 

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It is a requirement in the motor vehicle standards that the speedometer not read less than the real speed with any approved tire on the vehicle (and there are tolerances on the tire size).

It has to be this way ... otherwise people would be blaming the vehicle manufacturers for their speeding tickets.

Therefore the speedometer always reads higher than the real speed ... the only question is by how much. There are allowable tolerances for how much it can read high.

From the theoretical motor vehicle safety standpoint, it's better if the driver isn't going as fast as he thinks.
 

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I have four cars that are spot on at 70 mph. Both the 451 and 453 was/is optimistic as indicated by others here.
 

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I have four cars that are spot on at 70 mph. Both the 451 and 453 was/is optimistic as indicated by others here.

In 1994 my father in law bought me my first GPS upper right the Lowrance
As you see, I have aquired a few more.
I have used these GPS units to check the speed on every car my wife and I have owned since 1994. I have never found any of our cars to off more than 1 MPH at 60 MPH, as a matter of fact, our 20 year old 1996 Chrysler minivans speedometer is and always has been DEAD ON, regardless of tire wear or which GPS unit I'm using and yes I have had ALL of them running at the same time and all displayed the same speed.
Our 2016 Ford Escape displays 61 MPH fast at a true 60 MPH. My 2009 Toyota is dead on.
On the other hand my 2009 Smart Fortwo cabro is 5 mph optimistic at 70 mph, means I'm only doing 65 mph.
I believe as consumers we have a right to properly functioning equipment.
A gauge that doesn't display the correct information and can't be adjusted to display the correct information is malfunctioning and useless.
To be set intentionally any percent off is ridiculous.
We the consumers need to demand accurate and correct gauges.
 

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my '08 with stock tires read 60mph while doing 58mph, the snow tires read 60mph while doing 57mph, my new 185/60-15's 60mph while doing 61mph.
 

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Pretty common, on all vehicles. Was driving side by side Honda Civic. It had a digital readout. Completely different than the smarts speedo read out. Have 2 speedo on the 500. Digital and analog. Both read different. Add the navigation. Another numbers. Add a different size tire and wheel. Your in the Twilight zone.
 

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Well let see 4 GPS units used in 3 cars, 2 motorcycles with equal results. Close enough for me to say the smart speedometer suc## for accuracy.
The GPS units may agree, but they are all using information that is slightly degraded in accuracy from the military GPS system...:)
 

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Even using police radar guns will show the stock smart is about 2-4 mph slower than the speedometer reads at 65 mph. They have been like that since 2008.
 
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