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AIBU?

Share your dilemmas and get honest opinions from other Mumsnetters.

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Original poster

Speedometers in cars lie?

70 replies

Cinnabunbun · 27/10/2018 10:42

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)

OP posts:

DiamondsInTheMud · 27/10/2018 10:43

Mine tells me im going faster than i am. My sat nav always tells me im doing 4/5 mph slower than my car tells me. Ive always gone by my sat nav speed and never got a ticket.

Witchend · 27/10/2018 10:45

I thought it was fairly standard knowledge that speedometers underwear by a small amount. 25 in a 20 I'd have thought was pushing it.
But I've gone past one of those signs which tells you your speed with the speedometers dead in 40, and it's reading 36.

Ilovetolurk · 27/10/2018 10:45

I have a vw which shows my speed as a number

When i drive through a speed camera that tell you your speed eg in a 20 zone mine says 22 when the camera says 20, or mine 32, camera 30

My DM also has a VW hers does this too

I think there’s a good chance my speedo is 2 mph too high

worridmum · 27/10/2018 10:47

Speedometers are not purely accrute and so there is a 10% acceptable variance for them. But it should show you a higher figure then 10% lower.

SoupDragon · 27/10/2018 10:49

My satnav always shows around 10% higher than my car speedo.

SoupDragon · 27/10/2018 10:50

Lower FFS. My sat nav shows a LOWER speed than the car speedo.

Sethis · 27/10/2018 10:52

In general your speedo will tell you a number 5mph or so higher than your actual speed. A speedo of 25 means you're probably doing 20-21.

But tbh YABU to get this worked up over 25 in a 20 zone anyway. If he was doing 30+ in a 20 zone, or 85+ on a 70 road then I'd say you have cause to be annoyed.

RatUnholyRolyPoly · 27/10/2018 10:52

Uh-huh, speedos overestimate. Not by a much as showing 25 when you're actually doing 20 though; but 22 wouldn't surprise me.

redsummershoes · 27/10/2018 10:54

car speedometers are not acurate.
and it's not predictable just how unacurate they are.
it's better to stick to the numbers on the dashboard.
speed limits are there for a reason, not just to piss drivers off.

BatsAreCool · 27/10/2018 10:55

Speedometers tends to over estimate speed.

However, I feel sorry for your DP if your backseat driving all the time.

redsummershoes · 27/10/2018 10:56

tbh I don't think it's unreasonable to remind the driver of the speed limit if you think they are driving too fast.

MrsStrowman · 27/10/2018 10:59

If you don't like his driving why not drive yourself. The worst kind of back seat drivers are non drivers

BatsAreCool · 27/10/2018 10:59

tbh I don't think it's unreasonable to remind the driver of the speed limit if you think they are driving too fast.

OP says it drives her mad to see 23mph on the Speedo when in a 20. I would say that she should drive herself and then there would be no arguments.

Socksey · 27/10/2018 11:01

Also if you are in the passenger seat you are seeing it from an angle which means that often what you will see is a higher speed being recorded than what the driver sees (assuming a dial rather than numbers)...

minivampsmakebloodwork · 27/10/2018 11:02

I'd say they're not wholly accurate. My sat nav estimated speed and would always be different to my speedometer. Same with those signs which flash up either your speed or thank you/frowny face depending on whether you are on or above the limit. I can cruise control through them at about 3-4 mph above the limit according to my speedo and still get a thank you.

Or we could use the police car in front of me today as an example. I was on cruise control at 50mph in a 50 limit and it was pulling away from me the whole time. Mind, they also appeared not to have working indicators so...

Either way, I feel it's safer to assume the Speedo is correct rather than risk fines or accidents by assuming it's always going to read a few miles slower than the actual speed.

Tinty · 27/10/2018 11:03

My SatNav also says 22 when speedo says 20 (It is a numbered speedo not a dial).

Racecardriver · 27/10/2018 11:03

I had a car that did this. 2mph under every time.

Sisgal · 27/10/2018 11:05

You need to chill out. Nothing worse than a backseat driver. If you don't like seeing 23 on a speedo in a 20 zone, then either don't look or close your eyes! Or you could walk? Get the bus? A taxi? Would you comment on their driving?. You sound like a terrible passenger

flowery · 27/10/2018 11:06

I think your partner might be over optimistic when he says a speedo reading 25 means the car is actually going 20. But 22/23 is probably about right. If I were going 23 in a 20 zone and DH started moaning I would be unimpressed. But then, he wouldn’t be looking that closely at my speed anyway, just as I don’t check what speed he’s doing, other than when it feels way faster than it should be then I check in case he’s missed a temporary reduced limit sign or something.

gamerchick · 27/10/2018 11:06

Mines 2mph over as well. Leave him alone OP before he dumps you on a pavement somewhere

NearlySchoolTimeAgain · 27/10/2018 11:07

Also the angle you look at the Speedo from makes a difference I think.

The variation between my sat nav and speedo varies by speed. 74 on my car is 70 on my sat nav.

BigChocFrenzy · 27/10/2018 11:13

YABU because backseat drivers can be a dangerous, irritating distraction

When you drive, do it your way

When you are a passenger, stop pestering the driver - unless they are doing something really dangerous and illegal

Firesuit · 27/10/2018 11:15

Assuming the speedometer is within legal limits, and that I've done the maths correctly, it will show between 20 and 22mph when you doing exactly 20mph. At the other end of the scale, it will show between 70 and 77mph when you are doing 70mph.

It will be out by a consistent percentage, so if it shows 21 at 20 it will show 73.5 at 70.

My current car is 5% out, so I travel at 1mph over in 20 and 30 zones, 2 over in 40 and 50 zones, and 3 over in 60 and 70 zones.

My previous car was 10% out, so I travelled at 22mph over in 20, 33 in 30, 44 in 40, 55 in 50, etc.

maggiecate · 27/10/2018 11:15

I think Speedometers calculate the speed of the vehicle from the RPM of the wheels so they overstate the speed because tyre wear affects the RPM. But they are unlikely to be more that 10% out, so 25 in a 20 zone is well outside what the police would accept. GPS systems are more accurate because they measure against a fixed spot (the satellite).

Sirzy · 27/10/2018 11:16

Backseat drivers in my car get told to shut up or walk!

And yes the speedo isn’t accurate as others have said. According to my partner who drives for a living for a normal car the best way to get an accurate idea is a sat nav that uses GPS

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