Dodgy speedo

[MrGrumpy]

Now I always berated my old Burgman as having an 'optimistic' speedo, but I've come to the conclusion that the speedo on the mk4 Tmax is even worse! In Scotland, there are a lot of those signs that display your speed - and I was surprised that on every occasion, the speed they were displaying was way lower than the speedo speed eg 35 on speedo, 29 on the sign!
Now obviously, it is preferable that speedos are slightly optimistic, but this is silly. Are there any rules on how optimistic a speedo can be? I seem to recall there is a 10% rule, but presumably thats for pessismistic speedos - I guess the law doesn't care if you think you are going faster than you are?

[michaelphillips]

Now I always berated my old Burgman as having an 'optimistic' speedo, but I've come to the conclusion that the speedo on the mk4 Tmax is even worse! In Scotland, there are a lot of those signs that display your speed - and I was surprised that on every occasion, the speed they were displaying was way lower than the speedo speed eg 35 on speedo, 29 on the sign!
Now obviously, it is preferable that speedos are slightly optimistic, but this is silly. Are there any rules on how optimistic a speedo can be? I seem to recall there is a 10% rule, but presumably thats for pessismistic speedos - I guess the law doesn't care if you think you are going faster than you are?

i always thought around 8%, on my nexus i go through various speed indicator boards and gathered that my onboard speedo is about 4mph over the actual

[Hexatony 2]

I`m not sure how accurate these "speed signs" are with scooters (or motorcycles), whenever I pass one they seem to flash up a random series of numbers before going blank! However, when I temporarily fixed my Garmin gps to the Hexy., the speedo was anywhere between 5 to10 mph optimistic. (it was showing 58 when the Garmin said 50). Oh those Italians!!

[gn2]

The speed signs up here never show the correct speed.
I think that as well as the well documented speedo errors there could be a delay between the speed being detected and the display being activated, so that by the time you see the speed on the sign, your speed has changed.
Some of them near me often show totally nonsense numbers which could even be kph rather than mph.
As for regulations on speedo accuracy, this should clear it up: http://www.unece.org/fileadmin/DAM/tran ... 039r1e.pdf" onclick="window.open(this.href);return false;