Car's speedometer accuracy?

shepherd

When I drive through Raheny village, there is a roadside digital speed sign showing my speed - but the speed it shows is always a bit less than what my car's speedometer says. Which should I trust?

_Kaiser_

Several posters here say that your speedometer is about 10% over the actual speed .. dunno if that varies by car though - I'd imagine so?

Alun

That's pretty much the case ... car's speedo over reads by approx 10%. Other factors like incorrect tyre pressures, excessive tyre wear or incorrectly sized wheels or tyres can affect it too, but if everythings bog standard and your tyres are OK and at the correct pressure, then 10% about covers it.

You can verify it using a GPS unit if you like.

miju

pass by the one yer talking about shepard very regularly i usually find that when i do (and it's on cos it doesnt always seem to be) it's usually 5-8mph slower than what my speedometer says

subway

what speeds are you passing it at?

the limit there is 30mph [i think] so 8 mph would be a *huge* percentage.
ive passed it at *speed* and found it fairly acurate.

miju

the speed limit for that road is 60km (and after the camera changes to 40km) so on that strecth of road i always stick no higher than 60 just to get a read off the camera and compare the two

colm_mcm

if your car has a trip computer, and you're on a straight strecth of road, reset the average speed meter, and drive at a constant speed for a minute or two, the correct speed will show up on the trip computer i.e about 5-8 mph lower than your clock states.

bp_me

The inaccuracy varies from car to car. Some can be 10% out across the range (very bad) while some will be calibrated to be 3/4 km/h out across the range (much better)

Hal1

My speedometer broke, anyone know a quickfix? tis a polo mk4. Dont mean to hi-jack this thread but since we are talking about speedometers

bp_me

Hal1 wrote:

My speedometer broke, anyone know a quickfix? tis a polo mk4. Dont mean to hi-jack this thread but since we are talking about speedometers

Depends what actually broke.

GPS is a possible solution. Or new speedo/sender/whatever broke.

Hal1

bp_me,
Hey, what I meant to say was its the rev counter thats broke. Cant say what broke in it exactly, it goes up to about 0.10 of rev but no more than that. It can be annoying at times. Just hoping for a quickfix really got my hanes manual here too heh.

milltown

AFAIK the legal requirement is that a speedo must be accurate at 30mph and within 10% elsewhere. Otherwise the most likely cause of inaccuracy, by far, is aftermarket wheels and tyres.

colm_mcm

If you had bigger aftermarket wheels or tyres, chances are your speedo would be understating your true speed, I think manufacturers overstate speed on speedos to allow for this.

shepherd

When I drive past at 60kph( by my car's speedo), the digital one says around 55kph.

Nobody yet has suggested the digital one might be inaccurate. Thats good.

Once I dont get done for speeding because my spedo underestimated my true speed, Im happy.

parsi

Alun wrote:

You can verify it using a GPS unit if you like.

Go to one of the Usenet GPS groups and suggest that and you'll end up with post after post arguing about whether GPS is an accurate measurer of speed in terms of distance travelled..

bp_me

parsi wrote:

Go to one of the Usenet GPS groups and suggest that and you'll end up with post after post arguing about whether GPS is an accurate measurer of speed in terms of distance travelled..

Having done this, the gps (garmin of some description) reads the exact same as the obd port in the car, but the speedo is 10% under at every speed.

punter281

I have a GPS Tracker and my speedometer always is 7% Higher than the actual speed. I queried it with the tracker suppliers and they claim that speedometers are on average 7.5% above the actual speed.

DublinWriter

shepherd wrote:

When I drive through Raheny village, there is a roadside digital speed sign showing my speed - but the speed it shows is always a bit less than what my car's speedometer says. Which should I trust?

Is that the one outside the Garda retirement home?

My old Primara would always register 10% faster than what the sign displayed, but my new BWM is spot on.

jimmycrackcorm

Of course the digital sign could be calibrated to show a higher speed than actual.

Kimbot

Your speedo is actually wrong!! If you have a performance car and get an Apexi RSM in it you will see by how much!! The RSM gives an accurate digital readout and I go by there where possible!!

Rovi

shepherd wrote:

Once I don't get done for speeding because my speedo underestimated my true speed, I'm happy.

...and that's exactly why the manufacturers produce cars with speedometers that read faster than actual speed.
Can you imagine the litigation they'd be involved in if they tried to produce instruments that read exactly right?

Big Nelly

Kaiser2000 wrote:

Several posters here say that your speedometer is about 10% over the actual speed .. dunno if that varies by car though - I'd imagine so?

They put a speedometer in your car? can you not tell how fast you are going by how much you wind up the elastic band???

Alun

parsi wrote:

Go to one of the Usenet GPS groups and suggest that and you'll end up with post after post arguing about whether GPS is an accurate measurer of speed in terms of distance travelled..

It's more accurate as a measure of speed than it is as a measure of position, because the main factors that influence position inaccuracies (atmospheric conditions, satelite constellation geometry) tend not to vary much, if at all, over short distances. So the two positions measured to determine speed may actually be both quite inaccurate, but they'll be equally inaccurate thus cancelling each other out, if you get my drift.

highdef

My analogue speedo also gives a lower than reality at this speed check in Raheny. Analogue says 41MPH when I am doing the 37.5mph limit. Would qoute in Km/hr but very hard to give an accurate speed in Km/hr. Digital speedo is bang on and when I tested the digital speedo at varies speeds up to 120 motorway speed using GPS, it was spot on. So when Driving, I have the real speed limits memorised in my head so I can do the REAL limit, once conditions show it's safe to do such. So an indicated speed of 81mph in my car = 120Km/hr or 74.5 mph.

Bruthal

milltown wrote: »

AFAIK the legal requirement is that a speedo must be accurate at 30mph and within 10% elsewhere. Otherwise the most likely cause of inaccuracy, by far, is aftermarket wheels and tyres.

The reason the speedo is reading higher than actual speed is a legal one, the car speedo can not read below your actual speed, so they are biased to read above your actual speed. This is so if you go through a speed trap or cam etc, you will have to be going a fair bit above it according to your speedo, EG if limit is 60kph and you are going 65kph actual speed, your clock will read 70 or more and so you have plenty of warning you are above the limit, if you keep to the 60 on your clock you will be well within the limit. The speed cam is just an example, its all based around giving the driver a speed indication that influences being within safe limits.

Gary ITR

The thread is 3 years old. Don't bump ancient threads