Speedometer Inaccurate? +5 mph?

[bryanackermann]

I live in an area with a *lot* of "Your Speed = X mph" signs (which is good and bad ), but I notice that they are always showing about 4-5 mph LESS than my GT's speedo. My other cars seem much closer to the sign's mph. Any ideas on what the cause may be before I call service? I have the 19inch wheels. Thank you in advance!

 

[Canyonero!]

Well I can report, based only on my limited experience using those street side, traffic calming, radar devices, .

I wouldn't trust the calibration on those devices.

 

[hmp10]

According to the GPS in our radar detectors as well as roadside readouts:

Our Honda Odyssey is spot on.

Our Lucid has a variable variance that reaches 2mph low around 60-65mph but can get as high as 3mph off as speed climbs.

Our Tesla Model S Paid . . . same variance as the Lucid.

I've sometimes wondered if Tesla and Lucid don't do this to fudge the range numbers, as the car thinks it's covering more distance in a given time than it actually is, thus making range look higher.

 

[Edward]

Mine exactly 2 over also. Glad it's not anymore as that would be harder to figure out.

 

[hmp10]

I checked a Porsche Taycan forum and found the same comments about its speedometer reading 2-3 mph slower than satellite tracking indicated. I then dug and little further and came up with this:

How accurate is your car speedometer? #AskTCE

Have you noticed that your car speedometer reads faster than your satnav says? Find out why this happens in this week's #AskTCE at TheCarExpert.co.uk

www.thecarexpert.co.uk

At least in Europe, it is illegal for a speedometer to read slower than actual speed, but they are allowed to read up to 10% higher. The article also explains how speedometers work off wheel rotation counts, whereas GPD systems work off actual ground traversed, and why differences in tire wear, brand, and pressure can skew readings. Thus many manufacturers design the speedometers to read a bit fast so that the speeds don't read illegally low as tire factors come into play.

 

[Steveinarizona]

Ditto, thankfully it's +2mph rather than a percentage, so it's easy to compensate for.

That is what I have always done. My BMWs always ran 2-3 miles faster than I was actually going. I believe in Germany if someone is caught speeding and the speedometer is at fault, there are penalties for the car manufacturer and no car company wants a customer cussing it out because he got a speeding ticket when the speedometer told him he was driving within the speed limit,

Why you may not be driving as fast as you think

Speedometers on German cars are designed to err on the fast side, so that you will never speed accidentally

www.theglobeandmail.com

 

[xponents]

Minimum speed limits right now: