sat nav v speedometer accuracy?

[Tom55]

Hi all

probably been covered before and I am too lazy to track it down, I have a garmin streetpilot 2610, which indicates my speed, now this is different from my speedometer by around 10%, so which is more accurate?

[audidoody]

The GPS is more accurate. Your speedometer has a built-in over-reading error of between 5% and 10%

[rogert]

hi the sat nav should be more accurate as no two car speedos record exactly the same speed as difference in tyres and road surface changes the overall reading but for safety your probably better off using the car speedo as they are usually slightly lower than they say.
hope that helps mate

[waremark]

Typical accuracy for GPS speed up to 100 mph is generally said to be within 1%. More sophisticated devices can be much more precise.

[Adam_BGT]

whats the accuracy over 100 like? my speedo only goes to 120, but with the new engine, i frequently have to rely on mr garmin.

[AMG Merc]

Adam_BGT said:

whats the accuracy over 100 like? my speedo only goes to 120, but with the new engine, i frequently have to rely on mr garmin.

I'd say it depends on the SatNav's processor speed - the faster you travel the more work it has to do talking to the available birds to calculate your speed. IMHO, SatNavs are generally very precise across the speed range and no comparison to your inaccurate speedo reading.

[markmullen]

Speedos have to read over to allow for changes in the diameter of the wheels as the tyres wear down. Speedos can legally read over up to 10% to allow for this.

GPS is very accurate when you are at a steady speed, due to the sampling rate of most GPS devices (typically once a second) they due lag during acceleration or deceleration.

There are some datalogging devices on the market for roadtesters and the like which operate on much higher sampling rates but they are very expensive compared to navigation systems.

[matt540]

speedos read higher than actual speed

E.G 30MPH = 28MPH

[S600VXR]

Glad I read this after wondering if my 190mph ish last week in Germany was real...

[markmullen]

As an example my Porsche was reading 173mph on the speedo at a GPS 161

[GTIR]

On vehicles fitted with tachographs like trucks and coaches the speed is very close to the sat nav speed, usually only about 1 or 2 mph out.

I suppose these need to be more accurate and thats why they are calibrated every 2 years.

Edited by GTIR on Saturday 1st September 09:27

[kevin ritson]

Bear in mind that if you're on an incline the GPS speed will be slightly flawed - it's using your position to calculate speed and not elevation - eg: if you're walking up a steep staircase you may have travelled 20 metres but your position has only changed by a couple of metres

[AMG Merc]

kevin ritson said:

Bear in mind that if you're on an incline the GPS speed will be slightly flawed - it's using your position to calculate speed and not elevation - eg: if you're walking up a steep staircase you may have travelled 20 metres but your position has only changed by a couple of metres

I thought GPS needed a view of 3 sats for position and 4 for elevation, no?

[munky]

the TVR's speedo is 2mph overstated at indicated 80mph so 2.5%, not bad.

[Cider Andy]

kevin ritson said:

Bear in mind that if you're on an incline the GPS speed will be slightly flawed - it's using your position to calculate speed and not elevation...

The OP's StreetPilot 2610 navigates in 3D: it does use his elevation in the calculation.

[ajg31]

S600VXR said:

Glad I read this after wondering if my 190mph ish last week in Germany was real...

190 in a VXR, thats bloody good going