View Full Version : The 10% +2mph rule...
SVeeedy Gonzales
03-05-05, 03:53 PM
So come on then, lots of people have mentioned this and there must be a sizeable portion of the population who talk in hushed whispers about this "rule"...
Who believes in it? Have you tested it? I've pushed it a few times and so far have not suffered, e.g. indicated 56 through a gatso in a 50...
Does anyone believe that it's purely down to the individual vehicle, or are we all maybe driving slower than we think? I've read several tests in bike mags (ok, not the most reliable source...) and the "10% or so" error seems very common in datalogged speeds vs indicated speeds.
I always drive past police cars with this rule (assuming they're going slow enough!) and they never mind... so... place your vote and let me know your thoughts on this phenomenon!
I've pushed it a few times and so far have not suffered, e.g. indicated 56 through a gatso in a 50...
Isnt that a bit risky?! :shock: :? I always err on the side of caution and go on or even under....!
I thought the rule, 10% + 1mph was a Police guideline in determining if you were aware that you were speeding - not a calibration for your speedometer.
The speedo, by law, must never under-indicate your speed. 10% is a large error, but an accepted one, when viewing and operating analogue, clock like gauges - like the average speedometer.
Ceri JC
03-05-05, 04:20 PM
I thought the rule, 10% + 1mph was a Police guideline in determining if you were aware that you were speeding - not a calibration for your speedometer.
The speedo, by law, must never under-indicate your speed. 10% is a large error, but an accepted one, when viewing and operating analogue, clock like gauges - like the average speedometer.
Same here :?
10%+2mph is the guideline set down by (believe it or not) Brunstrom and the ACPO (Association of Chief Police Officers). Basically police officers are not supposed to stop you unless your breach this rule of thumb unless there is a reason to do so (driving carelessly etc). However it is only a guideline so don't take it as licence to exceed the limit. Naturally it doesn't apply to speed cameras.
All speedos read slightly proud, in order to remove the manufacturer from blame as they mustn't, by law, read under. 10% is quite a large amount but certainly not unheard of. I would suggest 5% is probably more common.
.
Most manufacturers use common componants in the speedo systems across various bike models, given the variety of tyre choices etc without specific calibration the speedo will not show the correct speed. It will amost certainly be out by a good few miles perhour. On the GS with knobbly tyres the speed is out by 5 mph at 30 mph so a displayed 30 is in fact 25 mph.
Mike1234
03-05-05, 06:04 PM
That's assuming your speedo is actually accurate in the first place!
Naturally it doesn't apply to speed cameras
That's my understanding, too - "10% + 2mph" is the coppers "book 'em" threshold. Between the limit and "10% + 2mph" you still risk being stopped by a uniformed officer, but you'll likely get the "wagging finger" and a lecture. Unless you're very bolshie and you were wheelying at the time of the offence.............. :roll:
Go through a Gatso at "10% + 2mph" and unless your certain of it's "snap 'em and book 'em" setting you're taking a big risk. I have it on good authority that all of the Gatsos in 30mph limits in the South Wales Police area are set at 39mph. Do I risk it? Not on your nellie :lol:
Following on from Patch's post, an indicated 70mph on my bike is a true 65mph - I got bollocked by a copper on a Bikesafe course for going too slowly on a motorway.......... :oops:
Wiltshire7
03-05-05, 06:10 PM
i read a while ago in bike magazine that bikes often show 10% higher speeds than actual speed. thats why they use GPS when speed testing as the spedo is never spot on.
my in corsa i can go through 30mph speedos in barnet at about 40! no joke. ive been flashed a fair few times (and been lucky), and the point for my car seems to be about 14mph over the limit at 30.
no idea how far it goes for faster speeds but ive never wanted to try it.
i dont think my speedo is anywhere near 14mph too fast at 30, i think its mostly the speed cameras, plus a bit of my car being over.
Last time I was nicked the cop told me that if there had been a camera on that 40 stretch, it would be set to trigger at 46 and that he sets his own limit higher.
Unfortunately it wasn't higher than 56 :oops:
I think the speedo error is dependant on the speed you're doing. I've tried it out a few times on those "SLOW DOWN! YOUR SPEED IS..." signs and find that below 25mph my speedo is showing about 5-10% less than what the sign is showing, yet above 30mph my speedo is showing about 5-10% more than what the signs show. This is assuming that those signs are calibrated correctly.
chazzyb
03-05-05, 06:40 PM
Following on from Patch's post, an indicated 70mph on my bike is a true 65mph - I got bollocked by a copper on a Bikesafe course for going too slowly on a motorway.......... :oops:
I did too, though it wasn't a bollocking as such - he pointed out that with me doing 30 by my clock, it was couple of mph under by his, so I could (or was it should?) go faster!
This is assuming that those signs are calibrated correctly.
I'm pretty sure they're not. Bit like the speed cameras in that they seem to have individual speeds programmed in.
.
northwind
03-05-05, 10:58 PM
The carbed SV's speedo underreads anyhow... By, funnily enough, approximately 10% (on mine at least). Checked it against the Veypor which still isn't 100% accurate, but is a whole lot closer.
timwilky
04-05-05, 06:22 AM
When I took my test the examiner asked "why were you going slow?". "what when " Oh poo Ive failed thinks I.
He then explained in the 30s I was doing 28 in the 40 36, 55 in the 60s and 64 in the 70s. Because it was consistant, he was prepared to believe it was a speedo error and passed me with 1 minor.
On the way back from the test centre we calibrated the test bike against my instructors. When I hit an indicated 30 I held my hand up, same at 40, 50, 60 and 70. In every case I was overreading.
This inaccurate speedo could have cost me my license, had not the examiner been wise. top man
SVeeedy Gonzales
04-05-05, 07:14 AM
I've pushed it a few times and so far have not suffered, e.g. indicated 56 through a gatso in a 50...
Isnt that a bit risky?! :shock: :? I always err on the side of caution and go on or even under....!
Yes... 99% of the time I go through at the speed or under... but sometimes it's easy to get caught up in something else and leave it too late, when overtaking, etc. :oops:
SVeeedy Gonzales
04-05-05, 07:19 AM
i read a while ago in bike magazine that bikes often show 10% higher speeds than actual speed. thats why they use GPS when speed testing as the spedo is never spot on.
my in corsa i can go through 30mph speedos in barnet at about 40! no joke. ive been flashed a fair few times (and been lucky), and the point for my car seems to be about 14mph over the limit at 30.
no idea how far it goes for faster speeds but ive never wanted to try it.
i dont think my speedo is anywhere near 14mph too fast at 30, i think its mostly the speed cameras, plus a bit of my car being over.
I've heard all speed cameras can be set up for whatever speed, so some areas may have it lower or higher than others... can you imagine the complaints if they were all set to 31mph... half the population would be getting flashed.
Although I almost always go through right on the limit, I'm always aware, especially on the bike, of cars behind and to the side going faster... every time, whether I'm in the car on on the bike... so the cameras must be set higher than the limit, to be sure there's no mistake... but as someone else said, unless you know the exact trigger point for that particular camera, it's not worth risking it...
Wiltshire7
04-05-05, 09:36 AM
what is fun is when u set off a camera thats meant for traffic on the other side of the road, then just watch the drivers faces as they get flashed when they're under the limit! great to see.
Ceri JC
04-05-05, 09:53 AM
Go through a Gatso at "10% + 2mph" and unless your certain of it's "snap 'em and book 'em" setting you're taking a big risk. I have it on good authority that all of the Gatsos in 30mph limits in the South Wales Police area are set at 39mph. Do I risk it? Not on your nellie :lol:
I've definately (on more than one occassion) seen the ones downhill on cowbridge road east getting triggered at 36/37. I've been matching the speed of cars in front, they've gone through at that speed, been flashed and I've then chucked on the anchors :wink:
Oh and before somone gives me a "35mph+ plus in a 30 is tantamount to murder", yes, I know that and I usually do 20-25 in 'real' 30mph roads. This particular road is really (and used to be) a 40mph road. 'They' just dropped the speed limit to 30 a couple of years back (despite it being a dual carriageway :roll: )
My bike reads 'about' right, my car reads 130kmph when I'm doing 120 (tested with GPS) and when driving at 130 by the clock the bike is about 120.
If that makes sense...
The Spit on the other hand...
Clock = Real
25 = 30
40 = 50
55+=Anything over 60
I drive it on the rev counter - it's mor reliable...
Stu
SVeeedy Gonzales
04-05-05, 10:54 AM
what is fun is when u set off a camera thats meant for traffic on the other side of the road, then just watch the drivers faces as they get flashed when they're under the limit! great to see.
I've done that! :oops:
On a more serious note, is it fair that people are driving around at 32 mph, feeling like they're verging on the edge of criminality, being worried that they're risking getting busted, not paying as much attention to the road cos they're keeping one eye out for cameras and mr plod... when really at an indicated 32 they're actually running at 28 or something?
If they're going to impose these limits then they need to make sure that our speedos are reading spot on, or let us know what our safe "indicated" speed should be... If I knew my speedo was spot on 30 then I'd happily go through cams and stuff at 29. Right now, I feel cheated, as I can feel like a crim when I'm within the law, and if I stick with the (essentially faulty, under-reading) manufacturer speedo/speed limit combo that's out there then I'm being forced to drive and ride slower than I'm entitled...
Does that make sense? Or is that just a mindless rant over a few more mph? I work in Engineering and I know they can make speedos far more accurate than they are doing. :evil:
I know my speedo on my car (Punto) consistantly reads 5mph over up to 50mph (on the clocks) by using one of those score boards.... I mean display that makes you aware of your speed,
When the new mondeo came out a couple of years back Autocar (I think) tested 100mph (clocks) as 90mph (actual) and thats on a fairly modern car.
Speedo's should ALWAYS over read but probably by varying amounts so I wouldn't rely on anything. As has been said its a rule of thumb for cops and not for speedo over read.
Wasn't the CBR1000RR recently recalled because the speedo UNDER READ by accident. That would have been interesting......
Benji
Nick762
04-05-05, 12:01 PM
The carbed SV's speedo underreads anyhow... By, funnily enough, approximately 10% (on mine at least). Checked it against the Veypor which still isn't 100% accurate, but is a whole lot closer.
I think mine tends to overread by a small percentage which becomes more apparent the faster I go (at least according to my GPS). Thought I was doing 90 once but the GPS showed only about 88 as maximum speed.
Wiltshire7
04-05-05, 12:13 PM
i know how to solve it...
on top gear ages ago they did a test to see how fast u have to go through one to get away with it. if u go at 176mph then ur out of the area by the 2nd flash :lol:
simple. 176mph. good luck.
i know how to solve it...
on top gear ages ago they did a test to see how fast u have to go through one to get away with it. if u go at 176mph then ur out of the area by the 2nd flash :lol:
simple. 176mph. good luck.
No this was what was frightening, at 176mph the camera didn't even know something had passed, it didn't flash AT ALL.
Benji
SVeeedy Gonzales
04-05-05, 12:51 PM
i know how to solve it...
on top gear ages ago they did a test to see how fast u have to go through one to get away with it. if u go at 176mph then ur out of the area by the 2nd flash :lol:
simple. 176mph. good luck.
Now is that 176mph *indicated* or is it a true 176mph?? That's nearing 200mph... I'll just get the 'busa and check... :twisted:
vBulletin® , Copyright ©2000-2025, Jelsoft Enterprises Ltd.