They are always set slightly above actual speed not slightly below. Speedo indicating 50 would be around 48 in practise. It would be illegal for the speedo to under report.

There is no reason why a digital speedo would be any more or less accurate than an analogue one. The car measures the speed in the same way, it is just the way it displays what it is measuring that is different.

 

Some cars have the facility to remove the deliberate overreading, but the law requires them to be between 0 and 110% + 6.25 MPH from 30 to 70 MPH. My LEAF which has a digital speedometer overreads by about 12% consistently so is legal but annoying. The cynic in me says it's to extend the range by making me drive slower.

 

Easy to check and best not guess.

Install a GPS speedometer app on your phone, such as the popular and free Cool Niks one.

My Leaf was only 1-2mph optimistic and same for the Golf.

 

Google map is very accurate as well. When you put a destination and start the navigation on the top left corner you have the speed limit for the road you are traveling on and ot the right corner you have your actual speed.

 

I estimate my eNiro is 10% out….reads 33 doing 30

 

Mine also reads 33 when doing 30, but is not 10% out over all its range. At 60 mph it reads 63% which is a 5% error.

 

I think I'm going to miss my Speed Cheetah with the GPS speedometer and will probably end up fitting it!

 

A lot of dashcam are GPS enabled and can also display speed, ours is an old (6 years) Nextbase and can be configured to fade out the “road display picture” after a minute or so and display speed in its place.

 

What I find “interesting” is that my speedo is up to 10% out ( agree with @peterca that it is variable as speed rises) BUT the odometer range is accurate……how?

 

I had a Ford Focus a while ago. The speedo would read 6% over at 50 mph but if you started the average speed function it would start to read 50 mph and the SatNav would be showing 50. The computer adds a percentage and/or offset for the speedo to comply with the law but the car uses the correct speed for everything else including the odometer.

 

Exactly, car knows the correct speed, it can be slightly off due to non-standard rims or worn tyres, but still very close to the GPS speed.

If you use same app as Bjorn, it shows speed at diagnostic port (and GPS speed for comparison). And app shows battery temperature, which seems important when charging at CCS charger...

 

Just on this subject again, I'm just wondering if anyone who is like me in wanting an accurate gps speed indicator has found a way of having say a GPS app on an android through android auto to display on the head unit?

I know you can use google maps or waze, which both can display GPS speed, but if you want to have navigation directions on the HUD you presumably have to use the on board KIA navigation?

I don't really want to put my Speed Cheatah on the dash but might have to!

 

Once I tested the discrepancy vs GPS speed at the local speed limits, I know I can drive that extra 1, 2 or 3 mph over the displayed speed. YMMV.

 

This seems to be common across the manufacturers and it seems to always read faster than what you are actually travelling. I’m a surveyor and I laid out a test to check against gps’s as well and all my newer vehicle were over. If you think about it it’s a devious way for the auto manufacturers to get you out of factory and extended warranty sooner. It adds up to quite a bit and I don’t think anyone has challenged the auto giants on this matter yet. Think about this, if they really need such a large margin for calibration you think as long as tire sizes are of factory spec then there should be just as many lows as there are highs but it doesn’t seem to be the case. Suspicious.

 

This seems to be common across the manufacturers and it seems to always read faster than what you are actually travelling.

Yes... apparently this is even the law. More or less.

At least in Europe, the speedometer has a maximum permitted error from the true speed; it's allowed to read about 10% over but 0% under. From the law makers' point of view, this means that a speeding driver cannot reasonably claim in court that the speedometer didn't show they were speeding (for example, if the limit is 70 then a driver caught travelling at 75 can't expect to claim the dial was showing only 70). From the car makers' point of view, it means they can set all their cars to 8% over-read so as to look more impressive, and still be within the law if the technical accuracy is 2%.

The various computer modules in my car, and probably all modern cars, are variously programmed to use either the true speed or the speedometer speed as input. Both numbers are broadcast on the CAN bus so as to be available throughout. Example: ABS, motor control and odometer all use the true speed, so driving for one hour at an indicated exact 60 mph would show a trip meter difference of only about 56 miles.

 

There has to be an allowance as all tyres to not have same rolling circumference & they wear down. My EV6 over reads about 2% throughout the range, ODO is closer according to GPS figures.

 

My EV6 speedo is usually 2mph faster than Google maps speed.

 

Here are measurements done by a motor magazine.