Speedo not accurate

[Neil Ives]

I've never believed the Speedo is accurate on any vehicle that I have driven. In my MK4 Jazz I use Google maps via Android Auto to navigate. When a route is set the AA display shows marked road speed and next to it actual vehicle speed. When the cars cruise speed is set to 73 mph the actual speed is 70. 

[jasonevans]

Yes that's correct.

Speedo's are meant to overead by default.
That way you technically cannot speed.
All manufactures set there car up this way.
 

[RuthieB]

For the Jazz EX on 16” wheels I generally set the cruise control at 52 to achieve a true 50mph as measured by GPS on our Nextbase Camera, it seems the Speedo is underreading by 2moh at speeds below 50mph and by 3mph as you stated at higher speeds.

[5thcivic]

Speedometers naturally have a tolerance range because absolute tolerance would require a manual setup which costs money and may drift over time anyway. So the tolerance range is not set in the middle of reading but at the high end so you are always lower than the readout and have no chance of having been caught speeding because the speedo says you are doing 70 when a camera will pick you up at a real 75.

[Neil Ives]

I understand why the Speedo lies but knowing that the Speedo is showing 73 you are actually not speeding may be useful to other people.

[Kremmen]

Because the latest cars have intelligent systems by EU mandate, the Speedo has to be a lot more accurate than the past

My 2007 Civic @ true 70 used to read 77
My 2009 Civic was slightly better at 75
My 2013 Civic was 73

But, my 2021 Jazz MK4 was only 1mph out at 70 and read 71

That true 70 was via a standalone Garmin and my phone GPS

Just looked up via the EU NCAP documents and the latest rule is :

"NOTE: For 2026, Euro NCAP will require a speedometer accuracy of -3/+0
km/h"

That was under the Safety Assist section here : https://www.euroncap.com/protocols/

[Nicksey]

My speedo always reads 2mph over what I am actually doing.

[Lord Voltermore]

Speedos normally read a few mph /Kph low.    Must admit I too  sometimes  set cruise control at perhaps 73 mph to achieve a genuine 70 mph.

  My standalone (Garmin)  satnav used to display an accurate speed and I tended to use this  rather than the car speedo.(except in tunnels)  . I miss this facility with the Honda Garmin and HERE systems.   

What I avoid is  also assuming police allow a tolerance before prosecution . Maybe they do, but taking advantage of BOTH  is pushing your luck.

I try to be a tolerant when driving  but get a bit  annoyed  when 'righteous'  drivers appear to be deliberately impeding 'speeding drivers' from overtaking them because their speedometer shows they would exceed  the legal speed limit. When in reality they may be imposing 66 mph in a 70 limit.    

[coldstart]

... When the cars cruise speed is set to 73 mph the actual speed is 70. 

Then you are "lucky" because the law on speedometer accuracy is abhorringly lax:
https://www.thecarexpert.co.uk/how-accurate-is-a-car-speedometer/

Long story short: A speedo must never show less than the actual speed, and must never show more than 110% of actual speed + 6.25mph.

My Jazz is off by about 4% with 16" summer rims and 6 to 7% off with the 15" winter rims.
(and I know this, because I still have my trusty old Garmin DriveSmart 50 on the dash, as it not only shows actual GPS speed but also doubles as dashcam)

[coldstart]

"NOTE: For 2026, Euro NCAP will require a speedometer accuracy of -3/+0
km/h"
That was under the Safety Assist section here : https://www.euroncap.com/protocols/

Hopefully, the manufacturers will adapt (because they want the 5-stars-rating)!
The legislation is a lot more sluggish to adapt!

[Lord Voltermore]

legislation is probably stuck in the days of mechanical speedometers. Digital speedometers able to read wheel revolutions can be much more accurate, although even these must make enough allowance  to remain within parameters for variations in wheel rotations ,due not only to different wheel and tyre size possibilities    but the difference in rolling circumference  on the same tyres, as tread depth drops from 7 - 8 mm when new  to maybe 1.6mm.  . 

[Neil Ives]

I try to be a tolerant when driving  but get a bit  annoyed  when 'righteous'  drivers appear to be deliberately impeding 'speeding drivers' from overtaking them because their speedometer shows they would exceed  the legal speed limit. When in reality they may be imposing 66 mph in a 70 limit.   

I now live at the western end of the A30 - 100 miles of two-lane dual carriageway! Driving this road and trying to maintain 70 mph is difficult - many drivers are pootling along, hogging lane 2. In addition, many drivers joing from slip roads make no effort to get up to the speed of passing traffic forcing lane 1 traffic to try to move over and if not possible to brake hard.

[Kremmen]

That's the same all over regardless of speed limits

Why some/most insist on staying in the middle lane beats me.

[CB72]

I have a Nextbase dashcam which displays the accurate MPH, the Jazz mph is 2mph under that.

[5thcivic]

Why some/most insist on staying in the middle lane beats me.

Exactly, it seems like being behind a lorry at the exact same speed was somehow banned in the highway code!