30 =27
 

Driving up Pole Lane in Darwen the other day I was confronted with an illuminated sign on the left side of the road that showed “27mph” and words to the effect of ‘thank you for keeping to the speed limit’. I can’t remember the exact words because I was concentrating on not hitting a parked car and avoiding doing the same to cars come towards me.

I was doing exactly 30 mph at the time as was shown on my speedo. So if proof was ever needed that traffic speed equipment is set to give a small allowance (+10%) on the actual speed then this has to be it.

Must have a trip down the road one day to see it targets traffic coming down the road as well.

I wonder what would happen if I drove faster than about 33 mph? I wonder what the wording would be? Maybe a copper would leap out from behind a car brandishing a radar gun?

Re: 30 =27
 

Could the sign have been right and your speedo wrong? Tempted to get Busman to drive me over there to see what it says to us. :D

Re: 30 =27
 

Speedos are notorious for being inaccurate always overstating your speed. The most accurate method of checking is by using a GPS system as these have <1% error

They are also not linear in their inaccuracy, e.g. in my volvo teh following happens


Actual speed(GPS) vs Speedo
30 32
40 44
50 53
60 62
70 77
80 87
Cough Cough plus some :D

Re: 30 =27
 

Of course it could but not very likely on a car just one year old with less than 2,000 miles on the clock.

However if my speedo is wrong then I now know by how much – er more or less.

Re: 30 =27
 

Re: 30 =27
 

As I mentioned, they are inaccurate and teh manufacturers make sure ther are optimistic so they don't get sued by irate motorists caught speeding when their speedo said they weren't

Re: 30 =27
 

yeah i've flown past a few speed cameras at certain speeds adn got nothing thru

mind u at times i've been flashed at and got nothing thru

Re: 30 =27
 

A traffic policeman friend of mine once told me that speedos are wrong on all cars and are usually 2-3mph above the speed you are actually doing. As Entwisi said, this is probably a car manufacturer's way of covering themselves.

Re: 30 =27
 

richard lives in darwen, we where driving through Bolton the other day and they have loads of these new speed thingies

Re: 30 =27
 

The coppers over here will usually give you 10 to 15k/h over the limit ... even more if they are munching on a triple chocolate donut. We have the "speed thingies" too. Good for checking how accurate your speedometer is and that's about all.

Re: 30 =27
 

Your speedo will definitely be out. They all are.
The accuracy of you speedo also depends on the amount of wear on your tyres. The more worn they are the faster your car will think it is going.

Re: 30 =27
 

neil thats crap

Re: 30 =27
 

its not to be fair, not massively but it does have an effect

<geek mode on>

take a car with 15" wheels and 205/65 tyres, teh overall diameter of a new tyre is

15*25.4 = 381mm

add teh 'height of the tyre (205/100*65) =133.25

=514.25mm

Circumferance of a 'new tyre therfore is Pi(d) =1615.564021805mm

now a tyre has 8mm of tread when new, 1.6mm when worn so a diff of 6.4mm this is doubled as it affects both 'sides' so it become (514.25-12.8)Pi = 1575.351635847

so a difference of 30mm for each and every revolution. or in percentage format

1575.351635847/1615.564021805 *100 = 97.51 % so a difference of 2.49%


<geek mode off> :D

Re: 30 =27
 

I would explain why but Ian did it for me, thanks mate. You can of course explain why my statement was wrong if you like.
Tyre pressure can also effect you speedo as can vehicle loading. I did not say it would effect it a lot but 2.5% is quite a bit.
It would be worth checking you car has the correct tyres on it. I bought a car a few years ago that had the wrong profile tyre on that made quite a big difference to the speed displayed.

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the police want to put the time to something else in stead of booking motorist

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right so if we have identicale cars but i le t some air presure out of my tyers and both cars say 30 miles hr you would beat me

Re: 30 =27
 

There is some truth in that - you only need to pay any attention to speeding cases that go to court etc. Often, independent reports carried out on Police cases (particularly when assessing accuracy of equipment) will point to the margins of error - like those affected by pressure and the amount of tread. This is not just for a possible speeding vehicle, but also for the equipment say in a police land rover. You imagine if it hasn't been calibrated correctly (and land rover's have pretty big wheels) - at 100mph for instance you can be traveling X meters per second (cant remember what it is off the top of my head now) and have a certain number of revolutions per meter traveled, but this can vary with pressure or tread and the police have to account for it in their margin of error. A friend of mine is a former traffic Sergeant, and he now does expert evidence for court cases so I've had the geek lesson a few times now...;)

Re: 30 =27
 

well they say you learn something new every day and i have

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I once got fined 10 shillings for exceding the 30 mph speed limit between Sparth Rd. and the Hyndburn bridge on my Lambretta LI 150 , (a lot of money for a 4 pound a week apprentice) still have the little red driving license, (with endorsment) stashed away with the family documents

:D :D :D

Re: 30 =27
 

I could not say because you don't know how accurate the speedo is in each car.


If anyone drives a Ford Focus, Mondeo and probably others with the digital milage reading under the speedo you could try this - With the ignition key off press and hold the trip meter reset button, turn the ignition to position 2 while still pressing the trip reset, after a couple of seconds the milage reading will say gauge and all the gauges (temp fuel speed revs) will move to the max value and back. Let go of the reset. This puts the dash in test mode. Keep pressing the reset button and it will change the test. You will find one that shows speed in mph. I thing is shows spd and a number, another press shows it in kmh. Go for a drive and you will see the speedo will read different to the digital reading. My Mondeo does and so does the works Focus. Both were a couple of mph out at 60mph. When you turn of the ignition the dash resets to normal. Another useful thing it shows is battery voltage, you can see how well your battery is charging, it should be over 14 volts with the engine running and all your lights, heater fan etc on. ( Don't be surprised if it drops lower when you switch on the heated quick clear windscreen, that pulls a lot of current ).

Re: 30 =27
 

If your worried about soft tyres or punctures then you need Michelin Tweels

Taken from here http://www.carbibles.com/tyre_bible.html

http://www.carbibles.com/images/michelintweel2.jpg

Also useful if you plan a bank job and need to avoid police stingers :D