Speedometers in cars lie?

[Cinnabunbun]

Please help me settle this. I am a terrible backseat driver but it drives me mad when I can see the speedometer dial showing 23-25 miles per hour when we are in a 20 mph zone.

My partner behind the wheel says that the speedometer in every car overestimates so when it says 25 we are really only going 20. Is this true?

I openly admit that my backseat driving is a royal pain in the backside but can you help me settle the facts of the situation please?

The speedometer tells the truth (YANBU)
The speedometer in all cars always underestimates (YABU)

The speedo in our Skoda overreads by 10% so shows 22 when doing 20, 55 when doing 50 etc. However the one in the Freelander is spot on and I occasionally remind DH of this when he’s driving. It’s not as simple as all car speedos show slower than you are doing. You can calibrate your own speedo to a sat nav speed or the flashy speed limit signs though.

Yabu like others say he's right that the speedometer over estimates, you can't see it accurately and you are distracting him either drive yourself or chill out you already know you're being a pain why would you carry on?

I can set the speed limit in my car, which I always do so I never go over.
My satnav will always show 2-3km lower (we're kms in Ireland)

Guidance is leeway of 10% +2 mph due to differences in speedometers so fine for travelling over 35mph

I must be the only person in the world with an accurate speedo !
Ford focus with built-in satnav, which always shows exactly the same as the speedo.
After reading similar threads on here, I checked at a few places where they show your speed or flash if you are going faster.

Absolutely bang-on, every time.

You need to control your back seat driving. It's so annoying when people do that. If you don't like how others drive, drive yourself.

It’s normally reads about 2/3 mph over what you are doing.

It was a bit if an urban myth that the police gave you a 10% margin for error, their equipment is calibrated and accurate. However car manufacturers will always calibrate their speeds to read over, can you imagine the law suits waiting to happen if it was the other way around, so they will always err on the side of caution.

My car is about 10% over too, but DP's car, which is a year old is absolutely bang on.

Yes all speedos do say a slightly higher speed than you are actually doing but only about 10% or so, very unlikely to be 20 when showing 25 but 23 is quite possible. A speedo can also be out if you change your tyres to a different size than recommended for your vehicle.

I would say in the old days yes speedos had inaccuracies - today less so. However if you are caught speeding saying your speedo showed 20 is no defense. He is the driver his responsibility.

legally, you speedo is not allowed to be more than 10% out.

That means that it can read 22 when you are doing 20, but no more.

Satnavs are more accurate, and when mine is on I use that rather than speedo, but you have to be careful, as if your car speedo is that wrong, then your car needs fixing.

Most show a speed of 5% higher than your actual speed. It’s calculated by measuring how fast the wheels are spinning using a sensor and are calibrated never to show a reading that is lower than your actual speed.

I believe GPS shows speed as a measurement of distance over time. This is why you’ll get a variance between speedo and sat nav/smug smiley face signs on the roadside.

The OPs DP is trying to justify speeding. Hope no-one is ever hit and suffers a worse injury than would otherwise have happened as a result.

Inaccurate yes, but not necessarily overestimating.

Agree with others, speedometers do over estimate, you can't see the real speed from any seat other than the drivers (unless it's a number speedometer rather than a dial), and you are being annoying by commenting.

I spoke to someone who run speed awareness courses recently, he mentioned that a study has found angry people drive faster and more dangerously than relaxed drivers. It's worth thinking that you might be making your DP a worse /more dangerous driver by complaining about their driving.

Put on some relaxing music, it's proven to be the best thing for slowing people down. don't comment.

If it bothers you that much, perhaps you should insist on driving?

I was told (whilst on a speeding awareness course) that speedos are out by 3-5mph over what you are actually doing, so when your Speedo says your doing 35 you are probably doing closer to 30.

Lovemusic - I was told a bit more nuanced, that it depends on your tires how much it's out by, but usually around 2-3MPH. I guess if you have newer tires on it can be more like 5MPH out.

My sat nav say 27 when the speedo says 30, satnac says 65 when speedo is at 70. Speedos lie and are normally up to 10% overreading.

The speedo in my BMW overestimates by about 5%, which is fairly normal.

YABU for being a backseat driver. Anyone who tried that in my car would be given two options : shut up or get out.

They always overestimate. My sat nav says I'm doing 70, the speedo shows around 76.

Back seat driving is incredibly irritating, even more so when the person concerned hasn't got a licence.

OP leave your partner alone. If you don't like how he drives then drive yourself.

If you actually drive then you would realise that road design has a lot of influence on how people drive. Some roads naturally prevent people going over 15mph let alone 20mph due to the fact they are narrow due to parked cars etc.

Like with speed cameras forcing people to drive at a certain speed when the road hasn't been designed properly causes more incidents as drivers are too busy looking at their speedo rather than the hazards around them.

I often go past one of those flashing speed signs in a 30 mph limit near us. I have my speedo needle exactly on 30. I am careful of that. Last time it flashed up saying that I was doing 34.

There is a small degree of inaccuracy, but it isn't supposed to be wider than 10%, or that is my understanding anyway.

It's inaccurate in most cases in my experience but more importantly than that, you need to leave the driver alone.

Back seat drivers are a distraction which is at best bloody irritating and at worst could cause an accident.

If there is a reason that you are not able to drive, download an app or watch videos on your phone (and watch with headphones) for when you are a passenger so that you don't feel the need to interfere and complain.

Speedos overestimate by around 10%, for the reasons others have given.

Please stop backseat driving. It's awful.

They're often inaccurate in my experience.

But please don't backseat drive! DH does it and I have been known to pull over and tell him he either shuts up or walks home! It's so distracting and it drives me mad.

Either drive yourself or let the other person get on with things!

I have my speedo needle exactly on 30. I am careful of that. Last time it flashed up saying that I was doing 34.

I think it's very unusual for your speedo to show you going slower than your actual speed.