Do speedometers show a faster speed than the car is traveling at?

Neil

Speedometers are not allowed by law to under-read, but are allowed to over-read by no more than 10%. So at a true 30mph, they can legally read anything between 30mph and 33mph.

Because they read the speed via a sensor in the transmission, not the true speed of the vehicle across the ground, they have to be calibrated such that they under-read slightly when the vehicle is lightly laden and has brand new tyres fully inflated. As the tyres wear, and if they are slightly under-inflated, or compressed by a greater load in the vehicle, the speedometer will over-read by more.

A sat-nav is not bound by the legal requirement to not under-read, and reads the speed relative to satellites. This means it does not suffer the differences caused by tyre wear, etc. nor does it have to be calibrated to read slightly under to meet legislation. However, it reads a horizontal speed only - if you are on a flat surface it will read your true speed, but if you are on a hill the sat-nav will only display the horizontal element of your velocity, not the true surface speed (which includes a vertical [uphill or downhill] component).

Bardic

Not to be relied upon! Speedos are permitted to over-read by up to 10%, NEVER under-read, but there's no guarantee yours isn't precisely accurate. Having said that, it probably DOES over-read. In any case, most police forces allow a margin of 10% plus 2mph before they consider "doing" you. Not all, though, N Wales being one . . .

Flup

Speedometers do generally over-read -- it's a lot safer than under-reading! Use a GPS receiver if you want to know your true speed.

There is always some error with "speedometers".
Tyre wear for example.(new tyres to bearly-legal tyres)
I believe there a margin of small % with all "speed camera's".

Jackson P

Speedometers are allowed by law to overread by upto 10%.
They are NOT allowed to under read.

So potentially going at an indicated 30mph you could in fact be doing around 28.
or showing 33 you could in fact be doing 30.

This is why ACPO guidelines give a possible leeway of 10%+2mph for speeding.

The sat nav would be more accurate on a longer drive - but not in stop start town driving.

nan's bread

Sometimes,i was in a friends car and he had a very expensive sat nav which also gave the mph we were travelling at .there was a 10% difference in speeds and he said the sat nav was the accurate one,this seemed to be true when we passed a police car at 85 on the speedo and 73 on the sat nav.

The Tank

Car speedos are allowed a tolerance of +10% / -0%. This means that it is legally allowed to read up to 10% fast but is not allowed to read slow,

Speed cameras usually allow for an excess speed of 10%+2mph. This means that it may not go off until you are doing 46mph in a 40mph zone. But, if it is calibrated to go off at 42 mph you won't have any legal defence.

ken613uk

Most of the answers were only partially correct when quoting the permissible error of 10%. The law states that an error of 10% is permitted at 30 mph. It gives no such discretion at any other speed.

bondlincoln17

i have noticed this...when i m doing 30 (according to my speedometer)...my sav nav speed calculator shows that i m doing 27 or 28...which is dead weird...i dunno which one to follow...i m confused....

i would go with your speedometer man...and tell your friend to slow the hell down...what s her beef

Eddie

When the car is new the speedo will show the true speed. However changes to tyres or wheels can make the speedo show a different speed to the car's actual speed, and you cannot tell what changes have been made by previous owners.

The speed cameras will usually allow a margin of 10% or so, so a few mph over will not usually trigger the camera.

J G

Sort of: A speedometer may be out by up to 10%, and this can be greatly increased by having different tyre sizes fitted to the car.
But how does she know it's not reading correctly & by how much?...
Speed cameras don't trigger at exactly the speed limit, as every car may well have a speedo reading 10% low!

Yawn

There is an acceptable tolerance. It can be a little higher than your actual speed, but it must never register lower.

MickeyT

Unless the law has changed car speedometers have to be within 10% of the correct speed. They can't be exact because as your tyres wear they get smaller so effectively they change the cars gear ratio. Speed cameras generally allow for this 10% but because you don't know the accuracy of the speedo then it's not a good idea to try to use this. Your speedo could be reading low, so if you assume it's reading 10% over but it's showing 10% under, you'd drive so that it said 33mph in a 30 limit but you'd be doing about 36mph, not 30.

UCANTCME

Nope.............I have checked this for hundreds of my customers that have receive a ticket and when I hook their vehicle up the the scanner the speed is right on the mark.

Keith Lemon

No. Speedometers show a true speed, if your friends doesn't then she should take her car to the garage.

I don't understand why I have 4 thumbs down. A speedometer should show the correct speed, if it doesn't then you should get it sorted out.

The myth that speed cameras allow for a 10% difference is complete rubbish. I've been done going at 53mph when the speed limit is 50mph, it even says on my speeding fine.

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