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speedo or satnav?
- Thread starter p90ade
- Start date
SatNav should be 99.9% accurate as its based on GPS
SMURF!!!
My speedo on the fiesta (shock/horror - its ok im getting a clio soon :rasp: ) is about 5mph faster!
M135i
Its illegal for speedos to tell you hat your going slower than you are. By law they have to overread.
Oopps yep wrong way round your right they show hihger speeds than your realy doing.
Oopps yep wrong way round your right they show hihger speeds than your realy doing.
They have different sorts of inaccuracies.
Being analog the speedo is a fundamentally inaccurate device that reads high by varying amounts depending on the speedo and the speed you're doing. But its inaccuracy is consistent. The same speedo's error at the same speed is always the same. You can rely on how wrong it is.
Being digital GPS is fundamentally accurate, its reading can be relied on to within one unit, ie, one mph or km/h. But its not displaying the speed you are doing right now when you are reading it. What it does is calculate an average speed over a second based on the distance covered, then display that for the next second. So if you're accelerating it will display a low reading, and if you are slowing it will display a high reading. The only time it actually displays the speed you're doing now is if your speed has been constant for 1-2 seconds. It can't be relied on to tell you your current speed except in that circumstance.
The combination of speed and GPS can be very accurate. You use the GPS to tell you how far out your speedo readings consistently are. Then when you want to know what speed you are doing now you read the speedo and mentally adjust for its error.
So for example on my 172 I know from my GPS that my speedo reads 66 km/h at a real 60 km/h, and the enforcement margin here on police speed cameras and guns is 8 km/h, so as long as I don't exceed an indicated 73 km/h (according to the speedo) I won't get booked in a 60 zone. And I never have been.
In an ideal world Renault would build its sporty cars the way sporty car buyers would want: with a speedo that was accurate so they could go as quickly as possible without getting booked. Not deliberately engineer it so the speedo reads significantly high to appease the safety nazis who if they can't coerce us into driving slowly want to trick us into. Or at least it should be possible to get it re-calibrated so it was accurate.
Being analog the speedo is a fundamentally inaccurate device that reads high by varying amounts depending on the speedo and the speed you're doing. But its inaccuracy is consistent. The same speedo's error at the same speed is always the same. You can rely on how wrong it is.
Being digital GPS is fundamentally accurate, its reading can be relied on to within one unit, ie, one mph or km/h. But its not displaying the speed you are doing right now when you are reading it. What it does is calculate an average speed over a second based on the distance covered, then display that for the next second. So if you're accelerating it will display a low reading, and if you are slowing it will display a high reading. The only time it actually displays the speed you're doing now is if your speed has been constant for 1-2 seconds. It can't be relied on to tell you your current speed except in that circumstance.
The combination of speed and GPS can be very accurate. You use the GPS to tell you how far out your speedo readings consistently are. Then when you want to know what speed you are doing now you read the speedo and mentally adjust for its error.
So for example on my 172 I know from my GPS that my speedo reads 66 km/h at a real 60 km/h, and the enforcement margin here on police speed cameras and guns is 8 km/h, so as long as I don't exceed an indicated 73 km/h (according to the speedo) I won't get booked in a 60 zone. And I never have been.
In an ideal world Renault would build its sporty cars the way sporty car buyers would want: with a speedo that was accurate so they could go as quickly as possible without getting booked. Not deliberately engineer it so the speedo reads significantly high to appease the safety nazis who if they can't coerce us into driving slowly want to trick us into. Or at least it should be possible to get it re-calibrated so it was accurate.
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