Speed accuracy

[teamgers]

Hi, can anyone tell me which is the more accurate, the speed indicated on your sat nav? or, the speed indicated on your speedo in the car?




Many thanks

[PaulB2005]

Moved to more appropriate forum

The Sat Nav will be more accurate as speedos tend to under read the speed.

[Reap]

[mikealder]

The GPS speed is only updated once per second so there will always be a slight lag, as for going up and down hills degrading the accuracy yes it will do, but try working out by how much and you will find it is a very small degradation, the GPS is always more accurate than the crude device the vehicle manufacturer supplies (exept in tunnels 0 - Mike

[Anita]

[PaulB2005]

[Robin2]

Satnav speed is very accurate if, and only if, you are travelling at a steady speed, on the level and in a straight line. Under those circumstances you can rely upon it to much better than 1 mph to calibrate your speedo. Cars sold in Europe must have speedos that are zero to 10% fast. To allow for changes due to tyre wear, under inflation etc most manufacturers aim for a few percent fast, to ensure that if using the speedo to keep to the limit there is no possibility of unknowingly exceeding it. My Ford shows 72 mph at a satnav speed of 70, my wife's Citroen shows 73. My previous car, a VW, was 74! I use the satnav to set up my cruise control to a GPS speed of 70 on the motorway.
The Construction and Use regulations don't cover speeds as low as 20 mph, so car speedos may be much less accurate at these speeds. With the increasing number of 20 mph zones a GPS check may be even more important to avoid accidentally exceeding the limit.
Robin
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[if1977]

I would also like to say that if you change your wheel size i.e. fitting alloys that are bigger or smaller will also give you different readings on your speedo as the rotation of the speedo pick-up sensor in the gearbox changes it's revolutions due to the different sized wheels . I know that the speedo heads are not accurate anyway in most cars but if you buy a car that is fitted with non-manufactured alloys this will make it worse. So if you buy the car like this don't assume that it is all down to the speedo.
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[Andy_P]

[quote="Reap"]

[xda]

[philpugh]

[quote="Andy_P2002"]

[robertn]

One point to consider is the algorythms used by the GPS manufactures. The GPS receiever reports a speed to the Sat Nav, which is probably pretty accrutate, BUT will depend on the mode of the device - usually Automotive, which means it is averaged over the last 3 or 5 seconds. Do this test - driving along at a constant speed, stop suddenly (When it's safe)
As soon as ou are stopped, watch the speed on the GPS..... I bet you are still doing 30mph.

Then the Sat Nav software converts it to Miles/Hour, and either rounds of truncates it. Heres the thing - we don't know which, so 60 could be between 60 and 61, or 59.5 and 60.5.

Slightly less pedantic than the previous posts , but at least it does not change with tyre wear.

[Snudge]

[Andy_P]

[mikealder]

And I thought you had the correct answer the first time due to the curved base of the triangle used - Mike

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