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10% + 2 on top of speedometer or maps/sat nav speed?

Posted 14th Sep 2021
I use Google maps and the speed it shows is higher than the car speedometer, the leeway that is permitted, which device does it apply to?
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The only device that matters is the device used by the police. You can try this defence



A man was driving his car at speed when he saw blue lights flashing in the mirror. He floored it to 100 mph, then 110, then 120. Suddenly he thought, "What am I doing? I'm too old for this!" and pulled over to await the policeman's arrival.

Pulling him over, the policeman got out of his patrol car and walked up to the car. He looked at his watch, then said, "Sir, my shift ends in 30 minutes. Today is Friday. If you can give me a new reason for speeding -- a reason I've never heard before -- I'll let you go."

The man paused then said: "Three years ago, my wife ran off with a policeman. I thought you were bringing her back."

"Have a good day, sir," replied the policeman.
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Technically neither, it's the speed you're measured at by the enforcement device. Normally a car speedometer will show a speed slightly higher since the speedometer is allowed to overread but it's not allowed to underread. Normally the GPS calculated speed is more accurate and will be slightly lower than the car speedometer, I've not seen a car speedo that is lower than the GPS speed but I don't know if that's an issue with your car or mobile device.
12 Comments
  1. Avatar
    The only thing you should be using to monitor the speed in your car is it's speedometer. Good luck arguing in court, that Google maps said you were fine
  2. Avatar
    You don't automatically get 10% leeway anyway it's only recommended.

    Below that 10% can be a speed awareness course and fine anyway. Depends on local force.
    Edited by: "MonkeysUncle" 15th Sep
  3. Avatar
    The only device that matters is the device used by the police. You can try this defence



    A man was driving his car at speed when he saw blue lights flashing in the mirror. He floored it to 100 mph, then 110, then 120. Suddenly he thought, "What am I doing? I'm too old for this!" and pulled over to await the policeman's arrival.

    Pulling him over, the policeman got out of his patrol car and walked up to the car. He looked at his watch, then said, "Sir, my shift ends in 30 minutes. Today is Friday. If you can give me a new reason for speeding -- a reason I've never heard before -- I'll let you go."

    The man paused then said: "Three years ago, my wife ran off with a policeman. I thought you were bringing her back."

    "Have a good day, sir," replied the policeman.
  4. Avatar
    Technically neither, it's the speed you're measured at by the enforcement device. Normally a car speedometer will show a speed slightly higher since the speedometer is allowed to overread but it's not allowed to underread. Normally the GPS calculated speed is more accurate and will be slightly lower than the car speedometer, I've not seen a car speedo that is lower than the GPS speed but I don't know if that's an issue with your car or mobile device.
  5. Avatar
    ccnp14/09/2021 20:38

    The only device that matters is the device used by the police. You can …The only device that matters is the device used by the police. You can try this defenceA man was driving his car at speed when he saw blue lights flashing in the mirror. He floored it to 100 mph, then 110, then 120. Suddenly he thought, "What am I doing? I'm too old for this!" and pulled over to await the policeman's arrival.Pulling him over, the policeman got out of his patrol car and walked up to the car. He looked at his watch, then said, "Sir, my shift ends in 30 minutes. Today is Friday. If you can give me a new reason for speeding -- a reason I've never heard before -- I'll let you go."The man paused then said: "Three years ago, my wife ran off with a policeman. I thought you were bringing her back.""Have a good day, sir," replied the policeman.


    Lol good one
  6. Avatar
    Most speedometers give a higher speed than the actual speed & this gets bigger with the relative speed. So 31mph is more likely 30 and 74 is more likely 70mph.
    As above I would not rely on Google speed, there is usually a bit of a lag to recognise the actual speed
  7. Avatar
    Assume any leeway is based on the speedo, but I'd you know there's a camera then assume there is no leeway.
  8. Avatar
    GPS. Learned that on a speed awareness course, so those guys should know. 10% +2 is not to be relied on because there may be no leeway. Is up to the local constabulary
  9. Avatar
    10% + 2 isn’t a thing and never has been
  10. Avatar
    howarth315/09/2021 00:56

    10% + 2 isn’t a thing and never has been


    It's the ACPO guideline that the majority of forces still follow to prevent administrative overload. Officer discretion still exists.
  11. Avatar
    Johnmcl714/09/2021 20:41

    Technically neither, it's the speed you're measured at by the enforcement …Technically neither, it's the speed you're measured at by the enforcement device. Normally a car speedometer will show a speed slightly higher since the speedometer is allowed to overread but it's not allowed to underread. Normally the GPS calculated speed is more accurate and will be slightly lower than the car speedometer, I've not seen a car speedo that is lower than the GPS speed but I don't know if that's an issue with your car or mobile device.


    I've always found my speedo to be around 1 but usually 2mph lower than my GPS, that's using both Google maps and I'm pretty sure Waze too. I had to change the speeding notification to more than 5mph otherwise it would beep at me when I was doing the limit according to my car. I mean I've never been done for speeding, so I've always assumed my speedo is correct or that if it's incorrect it's not enough to trigger.
  12. Avatar
    There are a few villages near me that flag your speed when you enter - our car tends to show 1-2 mph faster on the speedo against a 20 or 30mph marker. Google maps never seems to show my speed, despite being turned on.

    I have a friend who is a police inspector, and he used to say that his force worked on the basis of 5% + 2mph for speed limits.
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