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!

 

[Unmutual]

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!

I've noticed the same thing, but my speedo is 2 mph over. I've asked service to look into it.

 

[borski]

It is 2mph over. Most auto manufacturers in the US do this, for liability reasons.

 

[bryanackermann]

It is 2mph over. Most auto manufacturers in the US do this, for liability reasons.

Huh - it seems like more than 2 mph over for me, but maybe because my other car is German I just am not used to it... Thanks!

 

[Unmutual]

Yeah, my Merc and US-made VW Atlas are dead on, as were my Explorers. I would think underreporting speed might also affect the odometer, but I guess 5% is an acceptable margin of error in the eyes of the government.

 

[MHDave]

I have the same +2 on my BMWs and other vehicles. The radar units installed on school and traffic signs are not required to be calibrated for legal-term accuracy and are not designed to be able to track across angular changes or multiple targets, etc. -- I've frequently seen differences of +5 to -8 when presenting straight on, and you can maintain a steady speed but see your MPH readout change as the angle from straight on changes. It's a geometry thing.
You used to see "speedometer check" signs on interstates, marking off "Mile 0" ... "Mile 10", inviting you to drive 60 MPH indicated, start your stopwatch as you cross mile 0 and stop at mile 10 and see how close to 10 minutes that was. I'm not sure that many people besides me took on that invitation! Then more recently you could get a good idea by comparing your GPS-reported speed against your speedometer, but many of us may have put away our Garmin units in a box in the garage.

 

[EricM]

Waze will show your GPS calculated speed as well

 

[bryanackermann]

I've downloaded an app on my iPhone - I'll check it to see how far off it really is - appreciate everyone's thoughts!

 

[GEWC]

All 3 of our cars - Lucid, Tesla & BMW - show a 2mph difference. It is the norm.

 

[Steveinarizona]

Yeah, my Merc and US-made VW Atlas are dead on, as were my Explorers. I would think underreporting speed might also affect the odometer, but I guess 5% is an acceptable margin of error in the eyes of the government.

It seems weird to me that your Merc and VW are dead on. IT is my understanding that the reason that many cars, especially German cars, read faster than they actually are going is that European regulations require that they never go wrong in the opposite direction:

Why Your Car's Speedometer Is Wrong

You don't speed nearly as much as you think.

www.thrillist.com

My BMW tends to run 2-3 mph too fast.

 

[bryanackermann]

UPDATE: I downloaded a speedometer app for my iPhone and went for a drive - the car is +2 mph... exactly as all of you said . Many thanks!!

 

[PAbuilt]

I agree that mine is also exactly 2 mph over (car always says it is going 2 mph faster than it actually is). I have never had any other cars that were set up that way. I have gotten used to it with this one but wish it were a bit closer to reality. 21” tires; AGT here.

 

[borski]

I agree that mine is also exactly 2 mph over (car always says it is going 2 mph faster than it actually is). I have never had any other cars that were set up that way. I have gotten used to it with this one but wish it were a bit closer to reality. 21” tires; AGT here.

Literally every car I’ve ever driven, including rentals, were 1-2mph off. It’s intentional, so you can’t blame the speedometer for a speeding ticket

 

[Steveinarizona]

Yeah, my Merc and US-made VW Atlas are dead on, as were my Explorers. I would think underreporting speed might also affect the odometer, but I guess 5% is an acceptable margin of error in the eyes of the government.

Unless you had the Merc coded I would be surprised. The law in Germany is very strict on having a speedometer read erroneously low. My prior BMWs ran 2-3 mph over the actual speed. Car companies do not want to be blamed for speeding tickets and accidents (I don't blame them). ("Your Honor, I swear I wasn't speeding. I was just under the speed limit according to my car's speedometer. If I was going too fast, it was the car company's fault")

2 mph over seems to be the industry standard.

 

[PAbuilt]

Well I can report, based only on my limited experience using those street side, traffic calming, radar devices, that my two past cars (merc s550 and a honda civic) were dead on with their speedometers.

 

[hmp10]

We have radar detectors with GPS speed readouts in our three cars: Honda, Tesla, Lucid.

The Tesla and Lucid speedometers read 2 miles faster than the GPS readouts up to about 60 mph, at which point the cars read about 3 miles faster.

The Honda does the opposite, with its speedo showing 1-2 miles below the GPS readout.

When hitting a police K-band speed billboard along the roadside, the GPS readouts are the ones that are always spot on.

 

[Jeanwoodie]

My 2019 Subaru Ascent speedo is ‘dead’ on. However my GT is consistently 2MPH optimistic.
I believe this offset is purposeful for whatever reason and could be corrected by SW update.
To correct and to better match traffic flow, I set cruise at 2MPH over, but then get traffic speed change alerts, which are somewhat a nuisance.

 

[Keith_SF]

Well I can report, based only on my limited experience using those street side, traffic calming, radar devices, that my two past cars (merc s550 and a honda civic) were dead on with their speedometers.

I see the same. My '22 Benz GLS450 is dead on when comparing to gps speed on my phone. My Lucid is always +2mph.

 

[HariK]

Whenever I drive past a speed detector display I too noticed exactly Lucid showing +2mph. So, I started using HA at 72mph when I wanted 70mph.

 

[Keith_SF]

Whenever I drive past a speed detector display I too noticed exactly Lucid showing +2mph. So, I started using HA at 72mph when I wanted 70mph.

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