Incorrect speedo
Discussion
Hi folks
Recently bought a car and have been driving it since Monday. I have just compared my speedo to the sat nav and its reading approx 6-8 mph too slow. I thought something wasn't right, I have been through approx a dozen fixed cameras in the meantime however have not been flashed.
I now have that sinking feeling that I'm due a letter/s in the post, I have been driving 20 yrs point free. I'm a truck driver for a living and I'm worried about my job.
I'm hoping that I subconsciously held back at the cameras knowing I was going too fast.
Would this stand up in court if my speedo is incorrect?
Regards
Recently bought a car and have been driving it since Monday. I have just compared my speedo to the sat nav and its reading approx 6-8 mph too slow. I thought something wasn't right, I have been through approx a dozen fixed cameras in the meantime however have not been flashed.
I now have that sinking feeling that I'm due a letter/s in the post, I have been driving 20 yrs point free. I'm a truck driver for a living and I'm worried about my job.
I'm hoping that I subconsciously held back at the cameras knowing I was going too fast.
Would this stand up in court if my speedo is incorrect?
Regards
What I would do:
Get the car tested at a reputable place, and have the speedometer measured and the results recorded.
If you bought it from the dealer, return it to them and ask for the speedometer to be fixed. It's a serious fault. Get them to confirm the repair.
If it was a private sale... you're probably stuck with repairing it yourself.
If you get tickets, chat to someone like AGTLAW who can advise on the likelihood of successful appeal;
Get the car tested at a reputable place, and have the speedometer measured and the results recorded.
If you bought it from the dealer, return it to them and ask for the speedometer to be fixed. It's a serious fault. Get them to confirm the repair.
If it was a private sale... you're probably stuck with repairing it yourself.
If you get tickets, chat to someone like AGTLAW who can advise on the likelihood of successful appeal;
There's a margin of error in all speedo's - all the cars I've owned in recent years have shown at least a few MPH difference between the speedo and a Sat-Nav, hell, my old V70 had a 10mph difference (although I think that was down to me having taken the dash apart and perhaps not put the needle back on properly).
It's perfectly normal for there to be a difference between the two, and as mentioned different sized wheels to what it had from the factory can also have an effect.
It's perfectly normal for there to be a difference between the two, and as mentioned different sized wheels to what it had from the factory can also have an effect.
Bigger wheels fitted that cancelled out the built in over-reading from the factory? wrong number of teeth in the ABS ring? could be a few causes, best to have it checked, or go on a run with a few GPS units/phones set up to show speed (make sure the road is flat for GPS to be more accurate) and see if there is much of a descepancy.
geof said:
Hi folks
Recently bought a car and have been driving it since Monday. I have just compared my speedo to the sat nav and its reading approx 6-8 mph too slow. I thought something wasn't right, I have been through approx a dozen fixed cameras in the meantime however have not been flashed.
I now have that sinking feeling that I'm due a letter/s in the post, I have been driving 20 yrs point free. I'm a truck driver for a living and I'm worried about my job.
I'm hoping that I subconsciously held back at the cameras knowing I was going too fast.
Would this stand up in court if my speedo is incorrect?
Regards
Before you panic and go seeking redress try this:Recently bought a car and have been driving it since Monday. I have just compared my speedo to the sat nav and its reading approx 6-8 mph too slow. I thought something wasn't right, I have been through approx a dozen fixed cameras in the meantime however have not been flashed.
I now have that sinking feeling that I'm due a letter/s in the post, I have been driving 20 yrs point free. I'm a truck driver for a living and I'm worried about my job.
I'm hoping that I subconsciously held back at the cameras knowing I was going too fast.
Would this stand up in court if my speedo is incorrect?
Regards
1. Set your car to do a steady speed, say 40mph, preferably with cruise engaged
2. Note the GPS speed
Repeat 1 and 2 at 70 and as many speeds as you wish.
Note the difference between the speedo readings and the GPS readings.
You could use a scatter plot and a linear trend line if you have Excel or similar. The formula of the straight line wil reveal the error factor for your speedo.
Whatever you do don't note the differences in speed unless your vehicle is at a steady/constant speed and has been for 30 seconds or so.
If th e GPS speed is higher than the speedo readings you need to get that fixed.
Is it a defence to a speeding fine? That will depend upon your credibility, the circumstances of the error coming into your possession and whether the magistrates accept that.
Brigand said:
There's a margin of error in all speedo's
(Snip)
It's perfectly normal for there to be a difference between the two, and as mentioned different sized wheels to what it had from the factory can also have an effect.
A speedo has a permitted tolerance for manyfacture. It may over read by up to 10% but not under read. In the case of the OP it is under reading. This is a fault and needs fixing. The cause could be in the sender, the speedo unit or with the wrong wheel / tyre combination. (Snip)
It's perfectly normal for there to be a difference between the two, and as mentioned different sized wheels to what it had from the factory can also have an effect.
I understand that your car speedo is calibrated to be exactly correct at 30 MPH. That is if you have not been messing about with gearbox etc...
once you start increasing speed error sets in, hence most Police forces allow an "error" rate or allowance for camera's etc....
Your sat nav signal has to go out of your sat nav up to space, then back again, it is a simple device unlike military gps systems which cost £1000000's and are designed to pin point to cm's where you are on the planet so they can drop a bomb through your bedroom window (ask the Taliban this for verification)
The sat nav also assumes you on on a flat bit of ground, so I would never believe the sat nav over a car speedo
Secret is stay on or below your car speedo, you can't rely on the sat nav to be accurate, I doubt anyone has been "done" for speeding off a car one unless it's known to be faulty/ had some DIY work done on it
You sweat for 2 weeks from when you think you committed the speeding offence for the envelope to drop onto the door mat otherwise the NIP (notice of intended Prosecution) will not count
good luck
once you start increasing speed error sets in, hence most Police forces allow an "error" rate or allowance for camera's etc....
Your sat nav signal has to go out of your sat nav up to space, then back again, it is a simple device unlike military gps systems which cost £1000000's and are designed to pin point to cm's where you are on the planet so they can drop a bomb through your bedroom window (ask the Taliban this for verification)
The sat nav also assumes you on on a flat bit of ground, so I would never believe the sat nav over a car speedo
Secret is stay on or below your car speedo, you can't rely on the sat nav to be accurate, I doubt anyone has been "done" for speeding off a car one unless it's known to be faulty/ had some DIY work done on it
You sweat for 2 weeks from when you think you committed the speeding offence for the envelope to drop onto the door mat otherwise the NIP (notice of intended Prosecution) will not count
good luck
Thanks for all your comments folks.
Just been out with a gps and recorded the following
@30mph it is +4mph out
@40mph it is +3.5 mph out
@50mph it is only +3mph out, weird that it becomes more accurate at greater speed.
So hopefully panic over as I always keep it just under 30 ( indicated ) mph for a camera so would guess might of gone through @32mph. Like i say no flashes so fingers crossed.
Great advice given. The car was private sale
Thanks again for your comments.
Just been out with a gps and recorded the following
@30mph it is +4mph out
@40mph it is +3.5 mph out
@50mph it is only +3mph out, weird that it becomes more accurate at greater speed.
So hopefully panic over as I always keep it just under 30 ( indicated ) mph for a camera so would guess might of gone through @32mph. Like i say no flashes so fingers crossed.
Great advice given. The car was private sale
Thanks again for your comments.
watchnut said:
I understand that your car speedo is calibrated to be exactly correct at 30 MPH. That is if you have not been messing about with gearbox etc...
once you start increasing speed error sets in, hence most Police forces allow an "error" rate or allowance for camera's etc....
Your sat nav signal has to go out of your sat nav up to space, then back again, it is a simple device unlike military gps systems which cost £1000000's and are designed to pin point to cm's where you are on the planet so they can drop a bomb through your bedroom window (ask the Taliban this for verification)
The sat nav also assumes you on on a flat bit of ground, so I would never believe the sat nav over a car speedo
Secret is stay on or below your car speedo, you can't rely on the sat nav to be accurate, I doubt anyone has been "done" for speeding off a car one unless it's known to be faulty/ had some DIY work done on it
You sweat for 2 weeks from when you think you committed the speeding offence for the envelope to drop onto the door mat otherwise the NIP (notice of intended Prosecution) will not count
good luck
Well if any of your advice was heeded luck would indeed be required. Your post is guff in its entirety. once you start increasing speed error sets in, hence most Police forces allow an "error" rate or allowance for camera's etc....
Your sat nav signal has to go out of your sat nav up to space, then back again, it is a simple device unlike military gps systems which cost £1000000's and are designed to pin point to cm's where you are on the planet so they can drop a bomb through your bedroom window (ask the Taliban this for verification)
The sat nav also assumes you on on a flat bit of ground, so I would never believe the sat nav over a car speedo
Secret is stay on or below your car speedo, you can't rely on the sat nav to be accurate, I doubt anyone has been "done" for speeding off a car one unless it's known to be faulty/ had some DIY work done on it
You sweat for 2 weeks from when you think you committed the speeding offence for the envelope to drop onto the door mat otherwise the NIP (notice of intended Prosecution) will not count
good luck
Utter guff
ETA something useful for watchnut... https://spaceplace.nasa.gov/gps/en/
ETA something useful for watchnut... https://spaceplace.nasa.gov/gps/en/
Matruk said:
watchnut said:
I understand that your car speedo is calibrated to be exactly correct at 30 MPH. That is if you have not been messing about with gearbox etc...
once you start increasing speed error sets in, hence most Police forces allow an "error" rate or allowance for camera's etc....
Your sat nav signal has to go out of your sat nav up to space, then back again, it is a simple device unlike military gps systems which cost £1000000's and are designed to pin point to cm's where you are on the planet so they can drop a bomb through your bedroom window (ask the Taliban this for verification)
The sat nav also assumes you on on a flat bit of ground, so I would never believe the sat nav over a car speedo
Secret is stay on or below your car speedo, you can't rely on the sat nav to be accurate, I doubt anyone has been "done" for speeding off a car one unless it's known to be faulty/ had some DIY work done on it
You sweat for 2 weeks from when you think you committed the speeding offence for the envelope to drop onto the door mat otherwise the NIP (notice of intended Prosecution) will not count
good luck
Well if any of your advice was heeded luck would indeed be required. Your post is guff in its entirety. once you start increasing speed error sets in, hence most Police forces allow an "error" rate or allowance for camera's etc....
Your sat nav signal has to go out of your sat nav up to space, then back again, it is a simple device unlike military gps systems which cost £1000000's and are designed to pin point to cm's where you are on the planet so they can drop a bomb through your bedroom window (ask the Taliban this for verification)
The sat nav also assumes you on on a flat bit of ground, so I would never believe the sat nav over a car speedo
Secret is stay on or below your car speedo, you can't rely on the sat nav to be accurate, I doubt anyone has been "done" for speeding off a car one unless it's known to be faulty/ had some DIY work done on it
You sweat for 2 weeks from when you think you committed the speeding offence for the envelope to drop onto the door mat otherwise the NIP (notice of intended Prosecution) will not count
good luck
Edited by BertBert on Thursday 4th May 22:30
geof said:
Thanks for all your comments folks.
Just been out with a gps and recorded the following
@30mph it is +4mph out
@40mph it is +3.5 mph out
@50mph it is only +3mph out, weird that it becomes more accurate at greater speed.
So hopefully panic over as I always keep it just under 30 ( indicated ) mph for a camera so would guess might of gone through @32mph. Like i say no flashes so fingers crossed.
Great advice given. The car was private sale
Thanks again for your comments.
If your speedo is reading more than your sat-nav then it's pretty certain you have an issue somewhere with your car (Ignore the post from watchnut, it's basically complete bJust been out with a gps and recorded the following
@30mph it is +4mph out
@40mph it is +3.5 mph out
@50mph it is only +3mph out, weird that it becomes more accurate at greater speed.
So hopefully panic over as I always keep it just under 30 ( indicated ) mph for a camera so would guess might of gone through @32mph. Like i say no flashes so fingers crossed.
Great advice given. The car was private sale
Thanks again for your comments.
ks!)What car is it?, what are the wheel sizes?, and what are the tyre sizes?
The wrong size/profile tyres for the car can easily throw out the speedometer by the amount you're talking about, and is the most likely cause of you issue.
(You'll be surprised how many people buy tyres that are the same diameter and width as their previous tyres, but go up a couple of profiles for a more comfortable ride, not realising that they are altering the rolling radius of the tyre and therefore the speedometer reading)
er...
Speedo should read more than the sat nav. Speedos are allowed to over-read ie read more than the actual speed. They are not allowed to read under the speed!
Speedo should read more than the sat nav. Speedos are allowed to over-read ie read more than the actual speed. They are not allowed to read under the speed!
4rephill said:
If your speedo is reading more than your sat-nav then it's pretty certain you have an issue somewhere with your car (Ignore the post from watchnut, it's basically complete bks!)
ks!)Do you have a trusted friend that drives? If so, follow him through the town at a fixed distance with him driving at 30 and see what your speedo reads, then repeat for the NSL. He could give a single flash of the hazards to let you know when he's settled at the speed you want to test.
If anything, gives you a second opinion, but as others have said already, I'd be looking at wheel/tyre combo first.
If anything, gives you a second opinion, but as others have said already, I'd be looking at wheel/tyre combo first.
V8LM said:
If you have a tachometer then you can work out the speed from that (assuming you can find the gearbox and diff' ratios) and compare.
The ratios are based on the correct wheels/tyres been fitted and as that may be the problem here it could give the same error as the speedo.The ratios are fixed. Depending on how the speedometer is driven (either from the back of the gearbox or from the diff) then it's a way to show whether the speedometer, its sensor, or the wheels/tyres are the issue.
If the OP uses the measurements of the tyres fitted to calculate the speed from the tacho It will be correct. Even better to measure the actual radius from the ground to the centre of the driven wheel.
If the OP uses the measurements of the tyres fitted to calculate the speed from the tacho It will be correct. Even better to measure the actual radius from the ground to the centre of the driven wheel.
Edited by V8LM on Sunday 7th May 09:03
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