Like lots of people I have noticed that modern cars over read current speed (but not Odometer reading).  Does anyone know if this is adjustable in the Octavia?   My dealer said they could easily change the wheels at the dealer if I wanted a different size, this makes me think that they can also adjust the Speedometer fudge factor?   

 

See this link for discussion of why speedometers are inaccurate on modern cars.

 

http://www.thecarexpert.co.uk/how-accurate-is-a-car-speedometer/

As the link says your car has to be within 10% above and cannot read slow at all.  If it is outside those parameters then you should go to your dealer and have it sorted under warranty because it is illegal.  If it is within those parameters then they will not change it.  Different tyre sizes will obviously change it but then that is a whole different can of worms.

It used to be 10% +3mph, when caught speeding, it was generally accepted the speedos were 10% optimistic, therefore if you were caught doing up to 47 in a 40, a succsesful speeding conviction would have been difficult for the police to achieve

Edited 24 June, 201510 yr by cortina63

Talking about speeding, what's the latest on the subject of fixed roadside speed cameras not being legally accurate below 30 mph?

We have loads of local 20 mph limits now and it is almost impossible to keep to them, though mid to upper 20's is manageable.

I wondered if they got around yet to using cameras that are legal below 30 mph?

 

Apparently there have been some prosecutions by police cars actually following the car in question, so seemingly that is legal.

I heard from reliable source sometime ago about the speed camera accuracy thing. Don't know if it's improved though.

While a tyre is new it obviously has a bigger circumference and so travels further on one rotation than when worn, so speedo's will never be totally accurate whilst taking readings via a wheel size. But the question is, how accurate is a GPS speed reading on a sat nav ?

Let me ask my question more clearly, nothing about the standards which I understand.

 

Since dealers are able to fit different size wheels after delivery, does this mean that they can also adjust the speedometer to correct for this?

I wouldn't think it would be required. When you change the wheel size, the tyre size (and aspect ratio) also changes to maintain a similar rolling radius, within working tolerances.

That's what I'd assume anyway.

I wouldn't think it would be required. When you change the wheel size, the tyre size (and aspect ratio) also changes to maintain a similar rolling radius, within working tolerances.

That's what I'd assume anyway.

Just put the numbers into an online calculator and you are dead right, obvious now 

 

 

225/35 R19 => Circumference = 79.1"

 

225/40 R18 => Circumference = 78.8"

Should the speedo take its reading from the wheels or gearbox shaft anyway?

These days I'd have thought they could use some kind of sensor aimed down at the road surface and registering how fast the car is passing over the surface!

Should the speedo take its reading from the wheels or gearbox shaft anyway?

These days I'd have thought they could use some kind of sensor aimed down at the road surface and registering how fast the car is passing over the surface!

 

If you want to add a few grand to the price of the car for a doppler radar speedo please be my guest.

 

I seem to remember that you can get some of the police garages to do speedo calibration, so it's spot on.

The current method seems fit for purpose, so yeah, no need for additional expense to fix something that isn't broken in the first place.

Talking about speeding......

We have loads of local 20 mph limits now and it is almost impossible to keep to them, though mid to upper 20's is manageable.

Keep it in third or even second gear and/or use cruise control if safe. It is no more "impossible" than sticking to 30mph.

Get a free app on a phone to get an accurate speedo measurement, and then get someone else to check whilst you drive it at various speeds, eg. 20, 30, 40, 50 etc and keep the correction in mind. It ain't rocket science.

My car is pretty consistently 2mph overreading throughout the speed range, on a trip in my brothers car, his was 3 mph over from 30-70 mph. It changes as the tyres wear down, by a very small percentage ( new tyres are closer to being accurate)

So if it says 22/23 mph on the speedo on my car, it is actually doing 20/21

Edited 24 June, 201510 yr by Bristolf2b

Talking about speeding, what's the latest on the subject of fixed roadside speed cameras not being legally accurate below 30 mph?

We have loads of local 20 mph limits now and it is almost impossible to keep to them, though mid to upper 20's is manageable.

I wondered if they got around yet to using cameras that are legal below 30 mph?

 

Apparently there have been some prosecutions by police cars actually following the car in question, so seemingly that is legal.

Use your CC, it'll keep you at the required speed, you'll annoy the F*ck outta other motorists though. 

Use your CC, it'll keep you at the required speed, you'll annoy the F*ck outta other motorists though.

I don't give a damn about what others think about me keeeping to speed limits to be honest, I am quite happy to let people pass.

Edited 24 June, 201510 yr by Bristolf2b

Just put the numbers into an online calculator and you are dead right, obvious now 

225/35 R19 => Circumference = 79.1"

225/40 R18 => Circumference = 78.8"

 

Dealers are only allowed to fit "approved" tyre sizes to vehicles they sell these days so they will all give almost the same rolling circumference.

Its the same also for online tyre websites which is why you have to define your make & model before you can buy them otherwise they can be fined for selling non-approved sizes.

When I bought my tyres in Switzerland I have to give the VIN number & build code of the car before I could purchase them.

 

 

The speedo calculations & display on the dashboard are homologated by the relevant authorities for each market of the car.

Therefore, I doubt you would be able to "fudge" or "calibrate" the speedo of a modern car.

It used to be 10% +3mph, when caught speeding, it was generally accepted the speedos were 10% optimistic, therefore if you were caught doing up to 47 in a 40, a succsesful speeding conviction would have been difficult for the police to achieve

I wouldn't rely on that.

 

Modern speed measuring devices are accurate. Speeding is an absolute offence so if the police device (assuming it has been correctly set up and maintained) says you were doing 47 it is irrelevant what your speedo says inside your car. Indeed, there is a specific offence if your speedo doesn't work correctly (within the allowable tolerances). If the police are relying on the speedo in their own car then it will also be one that is calibrated on a very regular basis and is spot on in terms of accuracy.

 

Try going to court and arguing that you are innocent because your speedo was wrong.....you won't get very far!

This has been discussed elsewhere quite recently.  It WAS 10% plus 2 MPH (not 3mph).  If your speedo has to by law be accurate to within 10% you cannot really be prosecuted for exceeding the limit within that 10% because your speedo is not accurate enough to keep your speed to the limit to 1mph.  Bearing in mind that all speedometers should read fast slightly and cannot by law read slow if you were doing 36mph in a 30 limit your speedo would actually be reading more than that.  From experience I can say that generally I have found them to be accurate to within 2-3mph so if you were caught at 36mph that would be over the 10% plus 2mph but it would most likely be reading in the region in the car at 38-39mph and therefore well ver the limit.

 

It is along winded read but the ACPO guidlines are here and should dispel all of the myths and urban mutterings about speed and prosecution.

 

http://www.cambs-police.co.uk/roadsafety/docs/201305-uoba-joining-forces-safer-roads.pdf

This has been discussed elsewhere quite recently.  It WAS 10% plus 2 MPH (not 3mph).  If your speedo has to by law be accurate to within 10% you cannot really be prosecuted for exceeding the limit within that 10% because your speedo is not accurate enough to keep your speed to the limit to 1mph.  Bearing in mind that all speedometers should read fast slightly and cannot by law read slow if you were doing 36mph in a 30 limit your speedo would actually be reading more than that.  From experience I can say that generally I have found them to be accurate to within 2-3mph so if you were caught at 36mph that would be over the 10% plus 2mph but it would most likely be reading in the region in the car at 38-39mph and therefore well ver the limit.

 

It is along winded read but the ACPO guidlines are here and should dispel all of the myths and urban mutterings about speed and prosecution.

 

http://www.cambs-police.co.uk/roadsafety/docs/201305-uoba-joining-forces-safer-roads.pdf

I fully understand the guidance with regards to when police can or should prosecute or use other methods.

 

My point is that you can't go to court and defend yourself by saying your speedo wasn't working so you're not guilty. It is an absolute offence if the police can prove the speed and as with any part of a car it is the motorists responsibility if something isn't working on it.

 

The police guidelines allow for the built in errors in the majority of speedos so common sense generally prevails.

Use your CC, it'll keep you at the required speed, you'll annoy the F*ck outta other motorists though. 

The only CC I've got is my right foot !!!

Sorry didn't read your sig. you could always leave it in 1st and bounce it on the rev limiter !

Actually the 20 limit is pretty daft as on most of the roads round here you can't even reach 20 most of the time!

 

Unless you are on a bike - they get up a lot of speed 'cos they don't stop at red lights or crossings........

 

 

p.s. I'd love to know how much the council spent on the scores of painted white "20's" on the roads and even more steel poles and signs - and another lot telling you when it's gone back to 30. 

Edited 24 June, 201510 yr by ednmra

Interestingly I did some 50 odd miles on cruise control set to a specific speed (on empty motorway last night) just after filling up with fuel at the services - the Columbus doesn't tell you your speed (like a Tom Tom would) but the average speed on the 'since last fill' after these 50 miles was exactly 7% lower than the speedo - I literally had cruise set for these miles and didn't change speed so in theory they should have matched!

Interestingly I did some 50 odd miles on cruise control set to a specific speed (on empty motorway last night) just after filling up with fuel at the services - the Columbus doesn't tell you your speed (like a Tom Tom would) but the average speed on the 'since last fill' after these 50 miles was exactly 7% lower than the speedo - I literally had cruise set for these miles and didn't change speed so in theory they should have matched!

The short bit from the gas station and while accelerating will affect the avg speed.

Try zeroing it while at cruisespeed and how big the difference is.

I quite like the fact that speedos tend to over exaggerate.

My Yeti does about 76 when the speedo says 80, so I feel quite happy driving at 80 on the motorway, feeling pretty secure that Its highly unlikely that I am going to get done..

GPS cross referencing agrees with the lower figure.