You're right that some countries require analogue but I can't think where.. it can't be the UK as we had cars here just with digital dashes.

Also acceptable: That thing you cover up with a boost gauge.

Mine is def off my a mph or two. Heck even my digital one seems to read about .5 mph faster than any speed radar signs I pass on the road.

All speedometers can be off that much. It doesn't display fractions so a reading of 50MPH could actually be 50 or 50.9. If another source is off by .1 of a MPH that means it will show 49 or 60 MPH even though that isn't really what you are doing.
Now if there is a difference of 5 or 10 MPH then you have an issue but go with your car not the remote device. By the time the device reads and displays your speed it may already be different then what your display tells you at the instant that you look.

AFAIK speedmeters are always set to overestimate rather than underestimate. I mean, each measuring device has a certain amount error, both due to intrinsic inaccuracies and due to external influences (e.g., tire pressure).


The manufacturers set up speedometers in such way, that whatever the error comes up to be (within reasonable limits), the shown speed will be same or lower than the actual speed.


This is to protect the manufacturer from drivers suing them saying "the speedometer showed I was at speed limit and I got ticketed anyway". No one is going to (successfully) sue for damages because "the speedometer was shoing 70 mph all the time, yet it took me as much as 32 minutes to drive 35 miles"

As I recall it (and checked against the source of all knowledge, Wikipedia) the international standards for speedometers is they must 1) never display an indicated speed "slower" than actual speed and 2) must not show an indicated speed of greater than 110% + 4kph of actual speed at certain "check speeds".

Various nations may have various or additional standards but nearly all of them say the speedometer can never show less than actual speed, but can by off in the other direction by quite a bit.

The example given on Wikipedia for the UK says "For example, if the vehicle is actually travelling at 50 mph, the speedometer must not show more than 61.25 mph or less than 50 mph."

"Son, do you know how fast you were going?"
"About 0.5 bar."

See? It works!

Cops also allow for this, 10% error.

nether regions

Nein Nein Nein Nein

pretty sure they can only be off to a higher reported speed than actual speed (in other words, going 50 with speedo saying 51 can happen, going 51 with speedo saying 50 should not happen). Can't remember where I saw this or the exact reasoning behind it but I believe it's regarding legal liability (oem/manufacturer could be held liable if speedo reads lower than actual travel speed as it is unsafe... e.g someone going 60 in a 50 zone because speedo says 50.)


I may be wrong though

Until you guys change from 17 to 18" wheels with the same tire sidewall... or does it not work that way anymore?

Can't read that far off in any direction. When they are that far off it means something is broke or has been changed on the car.

N00b question but how do you fix a speedo after going up an inch in wheels and getting wider tires?

Beats me, that is way too new tech. Back in the day of mechanical cables driving the gauge (yes the speedo is a gauge) we had somebody pace us and tell us how fast wee were going. Then made the corrections in our head once we had that baseline. Now I bet you can change some numbers in the computer and bingo, it is right again.
I will let somebody that knows what they are talking about field the real answer to this.