Speedo Accuracy

[irc]

As we know almost all speedos over read typically by around 4-8% compared to Sat Nav accuracy when checked on a straight level road.

So I was surprised to say the least when I checked my new 2025 Seat Ateca on the motorway at 40,50,60, and 70mph. Ever time the car only underread by 1mph. Now and then it would show 2 over the satnav, sometimes an exact match.

As this is with new tyres I assume at some point as the tyres wear I will reach the point where it is perfectly accurate. In fact a margin of around 1mph means I no longer need to think about true/satnav when going through average speed sections anywhere. This is on 215/55/17 tyres I guess models on different wheels/tyres may vary slightly?

[Pica-Pica]

irc said:

As we know almost all speedos over read typically by around 4-8% compared to Sat Nav accuracy when checked on a straight level road.

So I was surprised to say the least when I checked my new 2025 Seat Ateca on the motorway at 40,50,60, and 70mph. Ever time the car only underread by 1mph. Now and then it would show 2 over the satnav, sometimes an exact match.

As this is with new tyres I assume at some point as the tyres wear I will reach the point where it is perfectly accurate. In fact a margin of around 1mph means I no longer need to think about true/satnav when going through average speed sections anywhere. This is on 215/55/17 tyres I guess models on different wheels/tyres may vary slightly?

If the car speedometer under-reads, it is illegal. So I doubt it under-reads true road speed, if everything fitted is standard.

[Dave Hedgehog]

irc said:

As we know almost all speedos over read typically by around 4-8% compared to Sat Nav accuracy when checked on a straight level road.

I don’t know this, most cars i have owned, especially German cars have been bang on -1 mph, my Kia is -3mph at 30-140 according to a friend

[fooman]

My Mercedes reads 2-3mph higher at any speed, not a percentage

[OoopsVoss]

irc said:

As we know almost all speedos over read typically by around 4-8%

Cobblers.

It's an Internet meme, why would anyone actually believe this?

It does seem there are a few posters in here intent on spreading cobblers with an air of authority. Be it Finance, 2nd hand Vs new, cyclists and this.

[catso]

We have 2 modern Audis, both speedos read just 1mph over that shown by phone/app/speed signs etc.

I assume that speedos are more accurate than they once were but that the 1mph over-read is deliberately there for safety/liability reasons?

To add, I have a 1970's Moto Guzzi motorbike that, in true Italian style is very 'optimistic' and reads a good +10%, if not more.

[Triumph Man]

OoopsVoss said:

irc said:

As we know almost all speedos over read typically by around 4-8%

Cobblers.

It's an Internet meme, why would anyone actually believe this?

It does seem there are a few posters in here intent on spreading cobblers with an air of authority. Be it Finance, 2nd hand Vs new, cyclists and this.

They do generally overread though, not to that extent possibly, but they still do.

[Triumph Man]

It's a bane of my life that there is an average 30mph section near where I live, but most people sit at an indicated 28, which is a true 26 (in my car at least). I set my speed limiter to an indicated 33/34.

[RizzoTheRat]

OoopsVoss said:

irc said:

As we know almost all speedos over read typically by around 4-8%

Cobblers.

It's an Internet meme, why would anyone actually believe this?

It does seem there are a few posters in here intent on spreading cobblers with an air of authority. Be it Finance, 2nd hand Vs new, cyclists and this.

I'd guess because they probably did used to overread by around this much. My 2009 Octavia was around 5% over what the satnav said, and my 2008 motorbike is around the same. No idea how accurate my 2025 Lexus is as it has Android Auto which doesn't show the speed when using the phone as satnav.
It annoys me though that I can set an offset on the cruise control against what it thinks the speed limit is, but it's a fixed number of kph not a percentage.

[otolith]

My RX-8 consistently displayed a higher speed on the dashboard than was reported on the CANbus i.e. it was deliberately programmed to lie.

[irc]

Pica-Pica said:

irc said:

As we know almost all speedos over read typically by around 4-8% compared to Sat Nav accuracy when checked on a straight level road.

So I was surprised to say the least when I checked my new 2025 Seat Ateca on the motorway at 40,50,60, and 70mph. Ever time the car only underread by 1mph. Now and then it would show 2 over the satnav, sometimes an exact match.

As this is with new tyres I assume at some point as the tyres wear I will reach the point where it is perfectly accurate. In fact a margin of around 1mph means I no longer need to think about true/satnav when going through average speed sections anywhere. This is on 215/55/17 tyres I guess models on different wheels/tyres may vary slightly?

If the car speedometer under-reads, it is illegal. So I doubt it under-reads true road speed, if everything fitted is standard.

My mistake over reads by 1mph

[irc]

OoopsVoss said:

Cobblers.

It's an Internet meme, why would anyone actually believe this?

It does seem there are a few posters in here intent on spreading cobblers with an air of authority. Be it Finance, 2nd hand Vs new, cyclists and this.

Cobblers? Nope checked against satnav

[Tiglon]

Isn't it the case that analogue speedos read higher than actual speed, but digital speedos are generally accurate?

That's been my experience over the years.

Assumed it was due to perceived/predicted potential lack of accuracy in analogue speedos, hence the margin for error. No such worries with digital.

[John D.]

Triumph Man said:

OoopsVoss said:

irc said:

As we know almost all speedos over read typically by around 4-8%

Cobblers.

It's an Internet meme, why would anyone actually believe this?

It does seem there are a few posters in here intent on spreading cobblers with an air of authority. Be it Finance, 2nd hand Vs new, cyclists and this.

They do generally overread though, not to that extent possibly, but they still do.

Yep it's not cobblers for most cars.

My S1 Elise seems to be quite accurate against sat nav, so I have to remember to recalibrate my internal speedo in average speed zones.

Every other car I've ever owned has been 3-4mph out.

[Sporky]

OoopsVoss said:

Cobblers.

It's an Internet meme, why would anyone actually believe this?

It does seem there are a few posters in here intent on spreading cobblers with an air of authority. Be it Finance, 2nd hand Vs new, cyclists and this.

I think you need to read the bit you snipped in the context of the entire post.

[Dave Hedgehog]

Triumph Man said:

They do generally overread though, not to that extent possibly, but they still do.

So the manufacturer is not breaking the law, but generally they are very accurate.

I can see in the old days with sloppy mechanical devices attached via a cable to the speedo being inaccurate especially if worn or in need of maintenance but there s no reason for a digital system to be inaccurate

Edited by Dave Hedgehog on Monday 5th January 14:36

[Pica-Pica]

Triumph Man said:

OoopsVoss said:

irc said:

As we know almost all speedos over read typically by around 4-8%

Cobblers.

It's an Internet meme, why would anyone actually believe this?

It does seem there are a few posters in here intent on spreading cobblers with an air of authority. Be it Finance, 2nd hand Vs new, cyclists and this.

They do generally overread though, not to that extent possibly, but they still do.

The regulation in force (UNECE reg39) is speedometer must read between
+0 and +10%+4 kph of true speed, lthough modern speedometers are closer to +2mph.
Interestingly, the regulation states that the low verification speed is 40kph, which is around 25 mph. So a speeding charge of 24 in a 20 could may possibly be challenged.

[Earthdweller]

irc said:

OoopsVoss said:

Cobblers.

It's an Internet meme, why would anyone actually believe this?

It does seem there are a few posters in here intent on spreading cobblers with an air of authority. Be it Finance, 2nd hand Vs new, cyclists and this.

Cobblers? Nope checked against satnav

Every Speedo will overead, the amounts will vary vehicle to vehicle

They MUST either be 100% accurate or slightly over

The law dictates that and manufacturers will always er on the side of caution

I've driven hundreds of cars with 100% accurate speedometers ... everyone has been calibrated post manufacture and has a sealed unit normally from RS instruments, a calibration certificate and a calibration number on the instrument cluster

Every non calibrated car I've ever measured over a measured distance has over read to some degree .. every one

[otolith]

If your car reads 60mph at a true 60 with brand new 225/45/19 tyres with 9mm of tread, by the time they are worn down to 2mm it will be reading 61.25 at 60. The error decreases the larger the tyre is (because 7mm difference in a larger radius is a smaller % change) which may be one reason modern cars have more accurate speedos than older ones.

[bodhi]

Tiglon said:

Isn't it the case that analogue speedos read higher than actual speed, but digital speedos are generally accurate?

That's been my experience over the years.

Assumed it was due to perceived/predicted potential lack of accuracy in analogue speedos, hence the margin for error. No such worries with digital.

Not so sure, as the Civic we've just scrapped with a digital speedo always used to overread compared to the speed on the sat nav, usually by about 10%.

My 1 Series with an analog speedo is pretty much bang on, maybe overreads by 1 - 2 mph.