Car speedo

Reggie.

Had a conversation today with a mate and he said all car speedos are set to overread as in if the speedo says 50 miles your actually doing 45 miles. never heard of this before as he claims car manufacturers do it as they can't be held responsible for speeding offences. Anyone confirm this?

colm_mcm

That's true. There's no set percentage, but they pretty much always overread,

Reggie.

colm_mcm wrote: »

That's true. There's no set percentage, but they pretty much always overread,

Damn it, I didn't believe him
He reckons it can be upto 10%, seems very high

CiniO

Car speedometer by law can show speed according to this:

x - real vehicle speed
y - speed indicated by speedometer.

x <= y <= x*1.1 + 4km/h

I hope I didn't get that wrong.

So when car is doing 100km/h, speedometer can show anything between 100km/h and 114km/h.

Frynge

is it also true that they become more inaccurate over time but only in one direction.
eg read 50kph actual 45
read 50kph actual 44
read 50kph actual 43 etc.

something similar to traffic lights can only break on red?

Reggie.

CiniO wrote: »

Car speedometer by law can show speed according to this:

x - real vehicle speed
y - speed indicated by speedometer.

x <= y <= x*1.1 + 4km/h

I hope I didn't get that wrong.

So when car is doing 100km/h, speedometer can show anything between 100km/h and 114km/h.

That's even higher than 10%

CiniO

Frynge wrote: »

is it also true that they become more inaccurate over time but only in one direction.
eg read 50kph actual 45
read 50kph actual 44
read 50kph actual 43 etc.

Can't think of any reason why would it work that way?

Maybe except from tyre tread wearing, but once you put back new tyres, it should come back to initial 45

Reggie.

CiniO wrote: »

Can't think of any reason why would it work that way?

Wear and tear on the vehicle over time?

CiniO

Reggie. wrote: »

That's even higher than 10%

Yes.
In short words it can overread by 10% + 4km/h.

CiniO

Reggie. wrote: »

Wear and tear on the vehicle over time?

Can't see any relation.
Really.

colm_mcm

Frynge wrote: »

is it also true that they become more inaccurate over time but only in one direction.
eg read 50kph actual 45
read 50kph actual 44
read 50kph actual 43 etc.

something similar to traffic lights can only break on red?

That makes no sense tbh

Frynge

CiniO wrote: »

Can't think of any reason why would it work that way?

not sure, just something i was told years ago.

on a related note i was caught speeding recently when i had cruise control on and the reading on the speed gun was 13kph less than what the speedo, out by ~7%.

debabyjesus

When my speedo says 60mph the sat nav usually says 56/57mph.

CiniO

Frynge wrote: »

not sure, just something i was told years ago.

on a related note i was caught speeding recently when i had cruise control on and the reading on the speed gun was 13kph less than what the speedo, out by ~7%.

Quick calculations tell me you were doing 185 km/h
And your speedo showed 198 km/h :cool:

Reggie.

CiniO wrote: »

Quick calculations tell me you were doing 185 km/h
And your speedo showed 198 km/h :cool:

Good man :eek:

Dartz

"Car speedo"

Thank **** it's about measuring the scalar component of velocity and not another stupid car-fashion item like car-bras or eyelashes or ****. I never understood the point of those things, not when the tits who buy them are on the inside...

Also. If you put on larger diameter wheels/tyres you can make them overread. And that's not even accounting for parallax with conventional gauges. Mine's pretty accurate around 50, getting worse and worse the faster I go.

Henry Ford III

My experience suggests it's true. I've checked speedometers against GPS on several cars new and old.

SleeperService

Dartz wrote: »

And that's not even accounting for parallax with conventional gauges. Mine's pretty accurate around 50, getting worse and worse the faster I go.

Drive a MINI for super parallax speed guessing fun

No Pants

Anecdotal evidence to follow:

When I see one of those wee signs that give the speed reading as you approach, I consider it an insult if the numbers don't come up in red. If it flashes 65 in red at me, my speedo is normally reading higher.

DaveJac

older cable driver speedo's are less accurate espically the faster you go, remember trying to convince a young lad before in my boyracer days that just beceause the clock says he was doing 130 flat out probably only doing 100 in a 1.0 liter corsa and it couldnt go that fast

most modern cars are pretty spot on ive tried a few compared with a app i downloaded works through GPS

debabyjesus

Dartz wrote: »

Also. If you put on larger diameter wheels/tyres you can make them overread.

Think you mean under-read, i.e the car will be going faster than the speedo says assuming its accurate with the standard size wheels.

biko

Henry Ford III wrote: »

My experience suggests it's true. I've checked speedometers against GPS on several cars new and old.

Indeed, there has been several threads where people compare to GPS.

Murt10

One of the reasons, I would imagine that the car companies allow this to continue, is that you are left with the impression that you are getting far better MPG than you really are.

I rely more on the speed shown on the satnav than I do on the speedo.

Frynge

CiniO wrote: »

Quick calculations tell me you were doing 185 km/h
And your speedo showed 198 km/h :cool:

7.222222222222222% might be a more accurate number.

CiniO

Murt10 wrote: »

One of the reasons, I would imagine that the car companies allow this to continue, is that you are left with the impression that you are getting far better MPG than you really are.

I rely more on the speed shown on the satnav than I do on the speedo.

How could overreading speedo give you impression of better MPG?

Murt10

CiniO wrote: »

How could overreading speedo give you impression of better MPG?

Say you filled up the car and went for a drive at 100 kph on your speedo for an hour, You stopped and filled the car up again.

The car would think you have done 100 kms. When you do the calculations you work out that you are getting 20 kms per litre or whatever, but in fact you have only in truth travelled 90 kms, so your actual consumption was 18 kms per litres

CiniO

Murt10 wrote: »

Say you filled up the car and went for a drive at 100 kph on your speedo for an hour, You stopped and filled the car up again.

The car would think you have done 100 kms. When you do the calculations you work out that you are getting 20 kms per litre or whatever, but in fact you have only in truth travelled 90 kms, so your actual consumption was 18 kms per litres

Ahh all right
However I usually would look at my odometer to see what distance I traveled, instead of traveling at exact equal speed for exactly an hour