blizzard12

I get the impression that my speedometer (2017 C300) shows a bit (5%-10%) more than the actual speed of the car.
I haven't got a chance to measure it yet, but based on my driving habits something seems to be off.

Did anyone else notice this?

Cbc200

Yeah I have noticed this as well, near me there are road signs that show your speed as you drive past then, I've tried going past at various speeds and at 33 on my digital speedo the sign shows 30-31.

clifish

maybe that's the reason when I picked up my car and looked down I was going 110 MPH?
seemed more like 60

blizzard12

KDaFrank

I think this is pretty standard -- something about it being illegal for the speedometer to under report the current speed.


My prior car (99 3 series) did this too.

blizzard12

Volvo doesn't do it. Volkswagen on the other hand does. Big time! 10%!

AAN

I have driven many rental cars in France where there are many road speed measuring signs. All cars show slightly higher speed than the measured one. Different brands, different years, so I assume this is normal.

GerritKw

In the Netherlands all cars standard indicate 1-3℅ more. This is a safety margin. Different wheels might have different diameters and that can change the deviation in a positive or negative way.

WhiteMerce

I experienced the same in BMW 3-series and Infiniti G37. Manufacturers do not want to be responsible for your speeding tickets.

AmrDXB

That's normal.

Mike5215

YES! I came home from work early one afternoon and caught my speedometer with my tachometer!! I always saw them together but I thought they were "just friends". I don't blame my tach though; that speedometer has always been a *****.

PR2000

I would say about 3-4% on mine based on road signs measuring speed in Canada

Bueller178

Why does everybody assume the road measuring signs are accurate?

pjw1967

There are a few speedo apps for iPhone. I used one to check my GTO speedo after the trans was rebuilt. Tranny has 2 gears to turn the cable. Gears are diff colors. I had to get the right match. Never did though. Speedo overreads by about 5 mph vs the speedo app.

BenzC400

A properly calibrated and accurate speed measuring device (RADAR or Laser), will only give an accurate speed when a car is coming directly toward it. As the angle between the RADAR or Laser unit and the car being measured increases, error is introduced. The error will always favor the driver (the RADAR unit will show a slower speed than the actual speed of the car). So, if the RADAR unit for a speed measuring sign is at the side of the road, you can expect to see a lower speed on the sign than the speed on your speedometer.

Speed signs that just show your speed and are not used for enforcement may or may not be routinely calibrated and accurate.

Lionel_Hutz

Very typical for German cars

Waveform2

Most speedos do to keep you out of jail.

Luton Driver

I would have thought that the accuracy of the speedo is dependent on the size of the tire fitted. The bigger the tire the faster you go for the same revolution.

gfmohn

Who says the road side speed measurement signs are more accurate than your speedometers? Your GPS (esp. a freestanding one) should be the most accurate measurement. Has anyone compared their speedometer with a GPS? (I have not done so yet.)

Kartergreg

I have a freestanding GPS that gives MPH readout and have checked it against both of my cars. My ML was dead on. My C300 speedo actually measured between one and two MPH low, so I was going faster than my speedo indicated. Measured at 70 MPH on the interstate. Seems to go against what most people are saying.

Mr. J

It's the car.

I passed a slowpoke on a country hwy today, glanced at the HUD and realized I was doing 87mph in 3rd gear The car felt like it was on greased rails.

gfmohn

You must have been dreaming that you were back in your old Nash Rambler:

Beep! Beep! (Beep! Beep!).
His horn went Beep! Beep! Beep! Beep! Beep!
Now we're doing a hundred and twenty, as fast as I could go,
the Rambler pulled along side of me as if I were going slow.
The fellow rolled down his window and yelled for me to hear,
"Hey, buddy, how can I get this car out of second gear?"
(Donald Claps & Carl Ciccheti, The Playmates, 1958)

pjw1967

Went on I-45 S to Houston this morning. Wifey driving her '14 ML350. I opened up Waze to get traffic info. Lower left on Waze shows your speed vs posted speed. I glanced over at the speedometer and it was right on. 65 on speedo, 65 on Waze. At no time did the speedometer vary from Waze by more than 1 mph.

grege

As part of making an electronic throttle (for disabled people) we found that the Canbus reports the right speed but the dial speed reads 4kph low right from the bottom. (01 W203)
I think pretty much all do this, not really any reason in a digital world to have speedos that are not right on the correct value, tire wear even has a pretty tiny impact.
I'm sure its a deliberate policy. I always add the extra 4kph to my speed plus whatever margin over I feel like doing!

Mjollnir1983

I read some where MB says odometer is calibrated 2km under the speedo shown.

Using my GPS dashcam, it is pretty much spot on.