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)

Don't backseat drive. Why don't you drive?

And yes speedometers aren't accurate.

Topseyt - you need to get that checked, if your speedo is showing slower than you are actually going that's pretty worrying. Over 10% under is quite dangerous.

Always been clocked to about 5mph lower in any car I've owned

Only realised it when I had a satnav that showed your speed and it was always 5mph short of what speedo said

Owned pugs, vdubs and toyotas

Your DP is right.

My DH used to say the same, then one time I was caught by a speed camera. I was doing 40mph in a 30 zone, or so my speedometer told me.
When the letter came through I was actually doing 36mph.

I agree with the PP, I feel sorry for your partner. Quibbling every time he goes a couple of mph over the exact speed limit.

I'm not even kidding you, I would breakup with someone if they did that.

Yep, many of them are calibrated to make it look like you are going 5-7 mph slower than you are. Good idea if you ask me.

Speed is measured by counting the revolutions of the driveshaft leading to the wheels, so tyres of an incorrect size, over or under inflated, and the the effect of having just the driver, or a car full of people and luggage will all vary the 'speed' indicated.
The indication should always be greater than your actual speed but I don't know what the 'tolerance' should be although it might be 10%.

The UK law is that a speedo must never show less than the actual speed, and must never show more than 110% of actual speed + 6.25mph.

So if your true speed is 40mph, your speedo could legally be reading up to 50.25mph but never less than 40mph. Or to put it another way, if your speedo is reading 50mph, you won’t be doing more than 50mph but it’s possible you might actually only be travelling at 40mph.

To ensure that they comply with the law and make sure that their speedometers are never showing less than true speed under any foreseeable circumstances, car manufacturers will normally deliberately calibrate their speedos to read ‘high’ by a certain amount.

As your satnav is not the designated device by which a car’s speed is measured, it does not need to incorporate any fudge factoring. Satnavs will show your actual speed.

I get upset with DH when he takes a certain cor er near our house at 30. It's a 30 zone. The road is worn and multiple pot holes plus it's a fucking corner. I ask him politely to do 20 and he ignores me every single time.

Equally, when I drive, he tells me to speed up. When I'm doing 30 in a 30 zone.

I think marriage vows should be explicit re backseat driving and speed limits. Would be so much easier.

Mines is about 4 mph out

And absolutely if a passenger was telling me how to drive they'd be a pedestrian pronto

Sorry @cinnabunbun I meant...... Yep, many speedometers are calibrated to make it look like you are going 5-7 mph FASTER than you are. Good idea if you ask me. Makes it less likely for you to break the speed limit.

Thank you for your replies. Some very good points made especially the angle I'm looking at the dial and the margins for error. I also accept that my nagging is probably more dangerous than being 5mph over the limit too.

Ugh.. I'm just going to have to try harder to keep my mouth shut

Ugh.. I'm just going to have to try harder to keep my mouth shut

As Yoda would say, "do or do not do. There is no try".

Mine’s definitely overcalibrated. Unless you have something preventing you from driving, I suggest you learn so you can drive as you prefer.

A speedometer can be 10% out but only above not below so it can say 33 when you are doing 30 but not 27

you really do have to be quiet you would have a point if someone was doing 30+ instead of 20 or 40 instead of 30 but it is quite hard keeping exactly 30 most people trying to do 30 would waver over a distance between 28-33 easily

Also if you constantly talk about it you a) distract driver or b.) you do it so much that even if it were a genuine reminder as doing well above you would be ignored like a stuck gramophone record as with any time of nagging about anything it rarely achieves anything except annoyance

it's fine to say if you suddenly notice an overhead altering things from 70- to 40 on the motorway or to say look there is a police car ahead but I know a backseat driver who also says constantly you could be in 5th gear now, normally just as you place your hand on gear lever to change, I reckon if you want to travel by car you keep quiet as 3rd class seating is better than 1st class walking

Speedometers are out by 2/3 mph, I thought that was common knowledge.

I was told at a speed awareness course that they do very marginally overestimate one or two miles an hour, to allow for age and that they are never calibrated and to avoid being sued. He said the best advice was to believe them then you can never go wrong. It will be no defence if you get caught speeding

I have a new Fiesta and mt speedo is pretty much bang on, in my DD's old (old) KA her speedo was out by 10% (which was damned annoying).

Speedometer accuracy can never be completely exact because all that can be measured is the rotation speed of the wheel axes and the actual ground speed displayed will therefore be based on a pre-programmed value for the wheel radius which will be reasonably accurate but can vary according to tyre wear, tyre inflation pressure and the load of the cargo in the car.

When speedometer parts are tested and certified there will always be a slight spread of possible speedometer readouts for any given "real" speed. Calibration must legally be set so that it pretty much never under-reads but can legally over-read by up to 10%

SatNav speedometers aren't very accurate. Satellites can only tell your position to the neatest 10 meters, and their calculation of speed doesn't take land gradient into account. If you are going down steep hill the actual amount of land you travel over in each second could be 5% longer than the distance the satellite can see, making the satellite under-read.

But as for the OP - if you don't trust the driver's judgement then do not get in the car. Make other travel arrangements. If you get in the car then keep your opinions to yourself.