How accurate is the Speedo

[AdamInKent]

Hi all. This is my last thread tonight, I promise The speedo in my last car (a mark VI Golf) over-read by about 10% (which seemed to be standard for European cars about ten years ago). I have a sense that the speed display in the A3 is more accurate. Has anyone cross compared with a GPS device?

 

[Billy Shears]

From 8V experience, it should be close than 10% but will still slightly over-read as they all do. Best way to check is to run Waze (best Nav app by far) on Andoid Auto/Apple Carplay and you'll get a speed reading on that as well as your speedo.

 

[Soapy]

I checked my 8Y at 70mph speedo reading against my Garmin Fenix 6 watch and the watch reading was 69mph.

Cruise was on.

 

[AdamInKent]

I cross compared against my TomTom SatNav this evening and found that my A3 pretty consistently over-read by 2mph in the range 30 - 70 mph.

 

[BahnStormer77]

it will vary from one car to the next.... tyre sizes, inflation level, tread depth, etc....

general rule of thumb is 5%-10% on factory tyre specs, so you don't sue the manufacturers if you lose your license.... only times I've seen it much below 5% has been with non-standard tyre sizes...

 

[AdamInKent]

it will vary from one car to the next.... tyre sizes, inflation level, tread depth, etc....

general rule of thumb is 5%-10% on factory tyre specs, so you don't sue the manufacturers if you lose your license.... only times I've seen it much below 5% has been with non-standard tyre sizes...

Two of us, here, have found the car not to follow the 5-10% rule. (Mine is on factory tyres.) I’m wondering if the 8Y is measuring speed differently? (It has a permanent connection to GPS so, assuming it’s legal to do so, it might make sense to use it to measure speed, as it is more accurate than measuring gearbox RPM, for the reasons BahnStormer mentions above.)

 

[B5NUT]

The other question is how accurate are the third party nav units & phones. They will also have a margin of error and not be 100% correct.

 

[KXL]

I haven't got the 8Y yet, but on previous cars I had with analogue speedos, it would usually over read by 2-3 mph compared to a standalone sat nav (Garmin) which has digital (assuming GPS data) speed. My previous car that has both analogue and digital (on Sat Nav screen) speedo, the Sat Nav one was exactly same speed as standalone Garmin GPS.

 

[sebtomato]

On the previous generation A3, car speedometer was usually 2 or 3mph higher than various sat-nav used, so the car is over estimating the speed slightly.