The 10% +2mph rule...
 

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!

Re: The 10% +2mph rule...
 

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.

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.


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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.

That's assuming your speedo is actually accurate in the first place!

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:

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: