Given all the fancy tech these days with accelerometers and GPS I'm surprised it still is solely based on wheel rotations

On another note. There is now the Smart Motorway system on the M62 between J30 and J25. Initially when the system was first up and running but the screens were blank (so no visible Smart indications in operation) the cameras were flashing and apparently tickets being issued for anybody driving over 70 mph, something which was eventually switched off.

 

However, just before Christmas I observed 2 cars heading down from J27 towards J26 at what was clearly more than 100 mph and both of them triggerred the cameras.

 

Does anybody know if they are set to catch drivers who are clearly doing over the ton?

Different countries around the world have differing regulations regarding "real" speed versus displayed speed so a difference between the speedo & GPS systems is normal.

 

In the UK and most countries in Europe, the displayed speed is configured by the manufacturer to be 5-8% higher than the real speed.

In the US for example, the displayed speed is the real speed.

However, when displaying "real" speed manufacturers will obviously make sure that a car never shows a speed lower than the real speed so there will always be a small positive tolerance to account for tyre wear etc.

I have never been in a car either here in Australia or the UK with an accurate speedo.

Variations have been from 1% to 10% optimistic but once you know what it is then it is no big deal and you allow for it.

My current Octavia3 is 4kph out at 60kph and 8kph out at 110kph.

My old Octavia2 was slightly worse.

Fortunately the Odometers were/are far more accurate being less than 1% out.

I had one Hyundai with an odometer that was nearly 8% out (10% on speedo at all speeds) and that was a real pain for working out real fuel consumption.

Hmm, so with larger wheels (18" vs 16") each revolution will carry you about 6" further per revolution. One rotation of the 18" takes you 141cm, the 16" 126cm so over a mile that's 1135 rotations vs 1273 rotations?

 

So will the mileage in the larger wheeled car appear less?

 

This is starting to feel like a GCSE maths test 

Hmm, so with larger wheels (18" vs 16") each revolution will carry you about 6" further per revolution. One rotation of the 18" takes you 141cm, the 16" 126cm so over a mile that's 1135 rotations vs 1273 rotations?

So will the mileage in the larger wheeled car appear less?

This is starting to feel like a GCSE maths test

All of that looks pretty correct, if you're driving without tyres.....

Other way round, TMWNA - worn tyres rotate faster for a given speed so the speedo reads higher than when they are new.

 

Sorry I obviously didn't make my point clearly enough.  A fully worn tyre is at its smallest, possibly 12.8mm diameter smaller or 40mm less rolling circumference and therefore rotates more times per mile than a new tyre hence, the speedo then reads higher speeds closer to their GPS actual speed and as nearly everyone is getting higher speedo readings than GPS actual speed, my point is valid.  Speedos read high to compensate for fully worn tyres when they will read accurately.

Doh. . . fail. I suppose tyres are generally useful.

Edited 12 January, 201511 yr by lpt100

Hmm, so with larger wheels (18" vs 16") each revolution will carry you about 6" further per revolution. One rotation of the 18" takes you 141cm, the 16" 126cm so over a mile that's 1135 rotations vs 1273 rotations?

 

So will the mileage in the larger wheeled car appear less?

 

This is starting to feel like a GCSE maths test 

Forget the maths, try this link http://www.tyresave.co.uk/tyresize.html

 

Tyre side walls profiles are the bit you forgot whilst busy doing the math.

This is starting to feel like a GCSE maths test 

 

This is just the 11 Plus.  Major fail

 

 

Sorry I obviously didn't make my point clearly enough.  A fully worn tyre is at its smallest, possibly 12.8mm diameter smaller or 40mm less rolling circumference and therefore rotates more times per mile than a new tyre hence, the speedo then reads higher speeds closer to their GPS actual speed and as nearly everyone is getting higher speedo readings than GPS actual speed, my point is valid.  Speedos read high to compensate for fully worn tyres when they will read accurately.

 

Correct. The only point I'd add is that the speedo is at its most accurate when the tryes are new so as they wear, as you say, the speedo will read higher and higher for the same speed as measured by the GPS so become less accurate.

 

I quite like this wheel/tyre calculator as it also works out the spacing etc if you want to change wheels.  

Edited 12 January, 201511 yr by Smeghead

10% + 2 is a recommendation by (I think) The Association of Chief Police Officers. It is not mandatory - some police authorities operate a zero tolerance policy. So +1 mph over the limit = a ticket in some areas.

Just checked and my memory is correct its a non mandatory ACPO recommendation only. Funnily enough there is an article in the Sunday Times today about this. Average speed cameras are being introduced by stealth on some motorways. There is a new hidden Hadec3 average speed trap at Clackets on the M25. Kent police refuse to say what grace (if any) they will apply to this trap. Locals believe it might be zero. So watch out!

I think these Hadec3 cameras are in a few places now, apparently they are painted grey, there are no road markings for them for reference, and they may not be gantry mounted. http://www.driving.co.uk/news/new-cameras-to-kerb-motorway-speeders/

 

Going by this report it seems the cameras also record speeds over 70mph even when they are apparently set to national speed limit http://www.bbc.co.uk/news/uk-england-bristol-28329966

 

And some time ago 14,000 motorists were caught in the average speed cameras at Bristol in the road works section with a 50mph average limit http://www.bristolpost.co.uk/14-000-motorway-speeders-caught-months/story-17039453-detail/story.html

 

However, when displaying "real" speed manufacturers will obviously make sure that a car never shows a speed lower than the real speed so there will always be a small positive tolerance to account for tyre wear etc.

I was in the US just a few weeks ago and hired a 2014 Ford Explorer. When the digital cruise was set to exactly 70mph my Garmin also showed exactly 70mph - the analogue speedo was showing 68mph, I found when the analogue speedo was set to 70mph the digital reading on both the car and Garmin was indicating 72mph, so it was showing low.

I quite like this wheel/tyre calculator as it also works out the spacing etc if you want to change wheels.  

But Skoda's aren't an option vehicle

But Skoda's aren't an option vehicle

 

Indeed, it's not restricted to any make in particular - it's just a good generic calculator.  

 

Put in the tyre and wheel size in the left pane and it tells you the revs per mile, tyre height and other measurements.  Use the right hand pane to put an alternative wheel/tyre size and it gives you the differences so you get a good idea whether they will fit or not.