Prius T4 Speedo

[JamieDow]

10% is very common which is why the police have to allow for the 10% when they give you a ticket. You can get them professionally calibrated if they are manual but I have no idea about the digital kit.

It would make sense to goto the upper 10% as you are less likely to exceed the speed limit and make you safer. (Well that seems the most politically correct statement at the time!)

Got my Rover 75 checked by a local speed enforcement officer and it was 9% lower than my actual speed.

[timberwolf]

10% is very common which is why the police have to allow for the 10% when they give you a ticket.<snip>

I would thought the police would only need to give a 10% allowance if they were using their panda car's speedometer to measure your speed when following you? If they were using another device e.g. a radar gun pointed at you from the side of the road then I'd have guessed the allowance would depend on the accuracy of that device?

[Opifex]

IIRC speedometers have a tolerance of -0% to +10%. In other words the speedo must always read higher than the true speed. As a consequence, if the true speed is over the speed limit the indicated speed must be even more over the limit. As regards how much tolerance the police will allow before prosecution, I understand that this is at the discretion of the local Chief Constable and varies across the country. The former Chief Constable of North Wales was famously anti-motorist and enforced the limit with no allowance at all.

[timberwolf]

Ta. I always thought that chief constable was a great advert not to visit North Wales :)

[inspiredron]

The speedo indication is a deliberate overstatement. A friend with a Gen 2 also has a Scangauge attached to the diagnostics socket. Surprise - surprise - the Scanguage speed agrees closely with the GPS. So the engine diagnostics shows true speed but the big numbers on the dash that it feeds are fiction. I queried this with Customer Service and was told that the speedo was set to be optimistic to allow for tyre wear. Think about it - Tyre wear will make the error worse as each revolution of the tyre takes you LESS distance! EU regs say that the permitted error is -0 + 10%+6km/hr (I think). With analogue gauges we never noticed thi stoo badly. Now with GPS and digital it sticks out like a sore thumb.

[JamieDow]

Granted GPS is generally a more accurate indication but I would also edge on the side of caution. Depending upon satellite availability and a number of other aggravating factors they can read a little under. Having conducted some trials with GPS on vehicles to accurately account for speed and location my last employer found them to have some occasional inaccuracies.

[kithmo]

Granted GPS is generally a more accurate indication but I would also edge on the side of caution. Depending upon satellite availability and a number of other aggravating factors they can read a little under. Having conducted some trials with GPS on vehicles to accurately account for speed and location my last employer found them to have some occasional inaccuracies.

GPS speed is only 100% accurate on a flat terrain, it varies on hills.