Speedo Accuracy
Discussion
I'm learning (at the expence of Points and Pounds) that there are margins in excess speed. In the event of a speeding allegation, the calibration records of the device can be used, for or against...
But how sure are we of our own speedometers?
I sometimes look and try to work out the 'safe' margin but with a speedo that goes up in increments of 20, it's almost impossible to work out the 'actual' speed I'm doing before having to look up to avoid a rear-ender!
But how sure are we of our own speedometers?
I sometimes look and try to work out the 'safe' margin but with a speedo that goes up in increments of 20, it's almost impossible to work out the 'actual' speed I'm doing before having to look up to avoid a rear-ender!
Buy a GPS device such as a Road Angel or Drivesmart, then hold a steady speed, and compare GPS speed to your speedo speed.
At 30mph an unmodified production car speedometer is supposed to be accurate within +/- 1mph.
70mph on your speedo is probably only about 67mph on a GPS.
GPS is very accurate at a steady speed when I have tested.
At 30mph an unmodified production car speedometer is supposed to be accurate within +/- 1mph.
70mph on your speedo is probably only about 67mph on a GPS.
GPS is very accurate at a steady speed when I have tested.
ive tested all my vehicles using tomtom.
all 3 vehicles are at least 10% out as in showing 33 when doing 30
its further out at higher speeds too, the ducati had 175 on the clock but was 155 on the GPS, landrover had 115 and was 92, not found a closed road long enough for the pork at top end yet, will feedback when I have.
all 3 vehicles are at least 10% out as in showing 33 when doing 30
its further out at higher speeds too, the ducati had 175 on the clock but was 155 on the GPS, landrover had 115 and was 92, not found a closed road long enough for the pork at top end yet, will feedback when I have.
shuvitupya said:
At 30mph an unmodified production car speedometer is supposed to be accurate within +/- 1mph.
At any speed a legal speedo MUST not under-read, i.e. if it says XX mph, then you must not physically be going faster than XX mph. It may under-read a bit though, up to "about" 10%...I know in my car that an indicated 70mph is just over 64mph. Odometer (number o fmiles travelled) should be bob-on though. All this can be screwed up by putting new wheels on without modifying other bits, apparently...
esselte said:
tigger1 said:
All this can be screwed up by putting new wheels on without modifying other bits, apparently...
Not if you careful and select the correct tyre profile to maintain the rolling radius.
Fal-zackerly. That's why I said it "can be screwed up by", not "is screwed up by" - but you're right, if it's done properly it's not a problem. That said, I still think there's scope for fiddling a cars mileage like this...could easily reduce your "apparent" miles by about 10% methinks, if you up the tyre size by an inch
Flat in Fifth said:
Glassman said:
So, is there enough of a doubt for me to defend a speeding allegation?
what are you (or your mate?
) actually accused of in numbers? NIP # 1. 39 in a 30 (sent it back with licence so they can hang me)
" 2. 61 in a 50 (issued on 22/06/06 and I'm still deciding what to do about it even though I don't know if it was me or the missus driving)
" 3. 36 in a 30. No laughing matter that I've had THREE in the space of 10-days, but NIP #3 is lost... dunno where it's gone... maybe I dreamt they sent it... maybe they didn't send it... I just have to tell them I never received it (I have two complaint ref nos for Royal Mail).
Tick, tock.... I can afford +3pts... any more and I'll be forced to turn to crime.
OK then no chance to bring in speedo accuracy and ability to accurately read scale.
as far as I am conerned any alanlogue intrument cannot be accurately read to closer than +/-(0.5*minor scale division interval)
Therefore even though the numbers may be in 20 mph intervals there will probably be a minor scale marker every 5mph. Thus closest accuracy to which speedo can be read is +/-2.5mph.
Apply this rule to your own case. Couple that with a check test against a measured mile location as advised from your County Surveyor.
However I guess, and it is a guess that considering the numbers you quote 1 and 2 are definitely non starters and 3 almost certainly the same.
This does open up a question (yet another question!) which has always troubled me about electronic enforcement.
The justice system to a large extent is based upon the premise that the sequence goes as follows.
a) Offence detected
b) Justice procedure resulting in case proven or hands up.
c) Sentence = warning don't do it again. (the sentence could be nothing more than a warning, or fine up to imprisonment dependant upon severity of offence)
d) If behaviour continues and is detected the sentence racks up, reiterate until offending stops or ultimate sanction.
However with automatic enforcement there is every possibility that someone can get to the ultimate sanction (or too close for comfort on both sides of the equation) before ever getting to the b) or c) stage.
Unlike multiple speeding offences on a single trip on a single road in a single force area there seems to be no mechanism to ask for things to be considered as a single or multiple offences to be taken into consideration together.
To me this cannot be considered justice, more a case of being processed.
Comments from legal beagles?
as far as I am conerned any alanlogue intrument cannot be accurately read to closer than +/-(0.5*minor scale division interval)
Therefore even though the numbers may be in 20 mph intervals there will probably be a minor scale marker every 5mph. Thus closest accuracy to which speedo can be read is +/-2.5mph.
Apply this rule to your own case. Couple that with a check test against a measured mile location as advised from your County Surveyor.
However I guess, and it is a guess that considering the numbers you quote 1 and 2 are definitely non starters and 3 almost certainly the same.
This does open up a question (yet another question!) which has always troubled me about electronic enforcement.
The justice system to a large extent is based upon the premise that the sequence goes as follows.
a) Offence detected
b) Justice procedure resulting in case proven or hands up.
c) Sentence = warning don't do it again. (the sentence could be nothing more than a warning, or fine up to imprisonment dependant upon severity of offence)
d) If behaviour continues and is detected the sentence racks up, reiterate until offending stops or ultimate sanction.
However with automatic enforcement there is every possibility that someone can get to the ultimate sanction (or too close for comfort on both sides of the equation) before ever getting to the b) or c) stage.
Unlike multiple speeding offences on a single trip on a single road in a single force area there seems to be no mechanism to ask for things to be considered as a single or multiple offences to be taken into consideration together.
To me this cannot be considered justice, more a case of being processed.
Comments from legal beagles?
code monkey said:
shuvitupya said:
GPS is very accurate at a steady speed when I have tested.
how do you test a GPS system?
do you get another gps system, or compare with the car system? both of which may have a flaw?
not being silly just picky.
How do you calibarate a calibrated speedo?
esselte said:
code monkey said:
shuvitupya said:
GPS is very accurate at a steady speed when I have tested.
how do you test a GPS system?
do you get another gps system, or compare with the car system? both of which may have a flaw?
not being silly just picky.
How do you calibarate a calibrated speedo?
By traceability back to a known standard maintained by the National Body responsible.
Ignorance (of the offence...not your IQ!!) is no defence. Just because you didn't know that you were going faster than the speed limit, doesn't mean you weren't.
That sounds really preaching and poncy, sorry. I usually know when I'm breaking the speed limit. It's normally every time that I'm sitting in the driver's seat!
That sounds really preaching and poncy, sorry. I usually know when I'm breaking the speed limit. It's normally every time that I'm sitting in the driver's seat!
Forums | Speed, Plod & the Law | Top of Page | What's New | My Stuff