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Has anyone had a problem with the odometer on the 2010 Smart Fortwo reading 60 km per hour when the car is actually going 55 up to 100 km per hour when the car is actually going 90 km per hour? Is this is common thing?
 

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Many speedometers are calibrated on the low side, indicating a speed of a few mph or kph above their actual speed. It’s very common. Both of my Smarts indicate 2-3 MPH above their speed based on feedback from several differed GPS receivers I use.

Better to error indicating a speed that’s higher than actual than lower and getting a speeding ticket.
 

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How do you know?

If you think a GPS speed is more accurate then you are in for a bit of a shock. :)
The normally read about 10% higher than actual speed. They rarely read under and are supposed to be better than 10% over.

It is affected by tyre size obviously.
 

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Change tyre size?
To give a bit more detail...
Cars are designed for a specific alloy wheel size and tyre size so the MPH is reasonably accurate. This is called the rolling radius, you can find rolling radius calculators on the internet.

If you have changed wheel tyres combination, it may now be a different rolling radius and throws the MPH reading out.
 

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Federal regulations (United States) state that speedometers must be accurate within 5 mph at 50 mph.

Fun fact about that regulation: it's 5 mph plus or minus. So a speedometer reading 50 mph when the car is going 53 mph is still federally legal. Of course, that's not stopping you from getting pulled over.

Thankfully, no manufacturer is that bad with speedometer calibration. They program to be dead on or slower than actual.
 
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Speedometers on all cars, are off. One day going to work, on the highway. I was doing 62 mph. There was a Honda Civic next to me. He’s large digital speedometer reading was 67 mpg. My Tom Tom, was reading. 65 mpg.
 

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Are you meaning odometer or speedometer?
Speedometer, though I just realized that I'm quoting the law for commercial vehicles.


Interestingly, there are conflicting answers for speedometer accuracy for private motor vehicles. Some, like Edmunds, say that the accuracy has to be +/- 5% at 60 mph while others, like Road & Track, say that there is no law at all. I'm still scanning U.S. legislation archives and can't find where Edmunds is getting its information from.

The gist of it is that no matter how you look at it, the U.S. does allow speedometers to read too slow at least to a tiny degree.
 

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UK is + 0, -10% +- 2 MPH I believe.

Certainly UK speed cameras are 10% +2 so minimum speed for a 30MPH zone for a successful prosecution is 35 MPH.
 

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We own 5 different brand cars The only one that I feel is close to being accurat, is our 2018 Honda Accord Touring.
That car, has the ability to read speed limit signs. Which automatically adjust the speed to the car.

That feature, is becoming more available on newer car.
 

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We own 5 different brand cars The only one that I feel is close to being accurat, is our 2018 Honda Accord Touring.
That car, has the ability to read speed limit signs. Which automatically adjust the speed to the car.

That feature, is becoming more available on newer car.
We have a 2006 Honda Civic that has a digital speedometer. It seems to be the most accurate and agrees with my Garmin GPS.
 

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I have a 2016 Smart and if I want to go 55 I have to push the speedometer to 58...so mine always registers less then what I think I am going.
 

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To give a bit more detail...
Cars are designed for a specific alloy wheel size and tyre size so the MPH is reasonably accurate. This is called the rolling radius, you can find rolling radius calculators on the internet.

If you have changed wheel tyres combination, it may now be a different rolling radius and throws the MPH reading out.
This was my experience when I changed the stock tires to 195 rear and 175 front. I noticed when I passed those “your speed” signs on the road that my speedometer was consistently 5mph greater than what the sign read.
 
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