Speedo vs Actual speed

[Haytch]

Having read a few other threads it is widely believed that speedos over read by quite a lot and by more, the faster you are actually traveling.
Ive noticed that my speedo shows exactly (give or take 1mph) what the sat nav says at any speed up to about 90, not tried anything above this.
So is my speedo broken or is there just a big misconception and speedos don't actually under read by as much as people believe.

Edit: had my 'under' and 'over' read mixed up

Edited by Haytch on Sunday 13th March 23:12

[deveng]

Varies from car to car, my A3 is almost cock on but the missus old fiesta was nearly 10% out.

[jtwaring]

Mines 5% out the whole way up

[Cock Womble 7]

Mine over-reads by about a notch and a half.

[Efbe]

every vehicle is different. and not always the same all the way through the speed range.

[LMC]

My 330 was bang on, my Forester is way off, going by these roadside speed light things. They thank me for doing 30mph when the speedo shows 34.

I guess when it reads 130 I'm only doing about 50, by my calculations

[McSam]

Nobody's ever said (correctly, anyway ) that speedos under-read - they never do, and if they do then they're faulty. They're designed with a small amount of over-reading, so that they are "safe" and you're always doing the same or a lesser speed than you think you are. Obviously you can't very well get pulled for doing 80mph if your speedo said you were doing 70, nor can you expect to stop quite so sharply if you thought you were doing 30 but were in fact doing 35.

It seems to vary wildly from car to car, but German stuff is generally the most accurate - presumably because Autobahn speeds would generate a massive error! My Escort said it was going about 13% faster than it actually was, while the Audi is only about 4% optimistic.

[hardcorehobbit]

70mph on my speedo is more like 64-65, maybe a bit more.

Isn't accurate at more than 30mph tbh....

[deveng]

Also, there is a tolerance to how accurate the system will be, and that means that the target value is over reading so that at the limits of tolerance you never under read.

[Glosphil]

My Civic's speedo reads 5% fast from 30-80mph. My previous Accord seemed to be absolutely accurate and only started to show a 1mph readable error at 80mph. In the past I have owned cars with speedos that read up to 10% fast.

[Codd]

Compared my 5s again the Sat Nav on a 75 mile trip last weekend and it's almost spot on. My Dad's fiesta compared to Sat nav was about 6/7MPH out all the way round.

[georgejoshington]

My E46 is bang on.

[WhoseGeneration]

LMC said:

My 330 was bang on, my Forester is way off, going by these roadside speed light things. They thank me for doing 30mph when the speedo shows 34.

I get varying messages from them and suspect those who fit and calibrate them have agendas.
Some appear to be spot on, from gps, others not.

[PJ S]

Margin of error included for tyre rolling circumference, but if wheel size changed from standard, then your accuracy will be affected as a result.

[lipadier]

On my current old banger the speedo is accurate over the whole range:

[5678]

Mine are both about 2mph over the GPS speed all the way up to 3 figures.

[Efbe]

lipadier said:

On my current old banger the speedo is accurate over the whole range:

show off :/

[parapaul]

The Mundaneo's speedo is suprisingly accurate: +1 at 30mph, +2 at 70mph.

[LuS1fer]

GPS works on a 2D basis though doesn't it ie disregarding mountains and abysses?

I think modern cars are more accurate being electronic rather than the good old cable operated guesswork.

[Little Dave]

I was a little surprised about the inaccuracy of mine. Digital speedo was showing 204, the needle was pretty much bang on the 200 mark and the GPS recorded a max of 192. (Between 4% and 6% out depending on which way you measure it)