Speedo or GPS?

mrk · 6 Feb 2011 at 13:03

Colbaker said:
Surely it depends on what sort of signal the Sat Nav is getting to how accurate the speed is?

I would just go by my speedo as this is fixed/

SatNavs lock on to multiple satellites but that's mainly for showing your location on the map and that location being accurate to the meter. Speed is calculated regardless and it's more accurate than your speedo in most cases!

At 75 my phone GPS reads about ~77MPH and the speedo is consistently around 3-5MPH slower above 70MPH, more matched below that speed though.

[TW]Fox · 6 Feb 2011 at 13:03

mjt said:
That's irrelevant. If the Fiesta was unsafe to drive at those speeds then it would have been limited from the factory and that's a fact.

Are you saying it's a fact that the VMAX in every car in production is a safe speed at which to drive that car?

Kenai · 6 Feb 2011 at 13:06

Khaaan! said:
At 75 my phone GPS reads about ~77MPH and the speedo is consistently around 3-5MPH slower above 70MPH, more matched below that speed though.

Have you fitted the wrong sized tyres or something at some point? Surely a speedo should never be under reading unless something along the line is functioning incorrectly, or isn't to correct specifications?

mrk · 6 Feb 2011 at 13:10

Kenai said:
Have you fitted the wrong sized tyres or something at some point? Surely a speedo should never be under reading unless something along the line is functioning incorrectly, or isn't to correct specifications?

Bah, I deleted out a word or two that didn't make sense while proof reading the post (proof reading fail?).

What I meant to post was

At 75 on speedo, GPS reads about 77
At below 70 the GPS is more inline with the speedo, maybe ~1MPH over reading?
With cruise on btw, stock 17" wheels.

If my car was driven at 120+ then it it would also be about 3-5MPH over reading on the speedo. That's if it had been driven at those speeds of course!

Overlag · 6 Feb 2011 at 13:10

[TW]Fox;18402514 said:
130mph in a Ford Fiesta

No wonder they cant trust people enough to increase the motorway speed limit.

why not? top speed 146 and ive done it. (just).

mjt said:
That's irrelevant. If the Fiesta was unsafe to drive at those speeds then it would have been limited from the factory and that's a fact.
The brakes and tyres on the ST are not the same as the ones on a 1.2 are they?

nope different wheels, different brakes, lower down stiffer springs etc etc... i felt much safer at 130mph in my FiestaST that i ever did at 70mph in my Fiesta 1.25. And People do 70mph in small Fiesta's, Corsa's etc every day.

[TW]Fox · 6 Feb 2011 at 13:13

Top speed of the ST is 129mph not 146mph.

Overlag · 6 Feb 2011 at 13:22

[TW]Fox;18403738 said:
Top speed of the ST is 129mph not 146mph.

hmm maybe i'm confused with the 185 kits. Never mind.

Disco Boy · 6 Feb 2011 at 13:31

GPS, provided that the road is flat.

whitecrook · 6 Feb 2011 at 13:42

Khaaan! said:
SatNavs lock on to multiple satellites but that's mainly for showing your location on the map and that location being accurate to the meter. Speed is calculated regardless

Rubbish

How is speed calculated ? You need to know the distance travelled therefore the starting location and the next location at the next data sample. Speed and location are intrinsically linked

mrk · 6 Feb 2011 at 13:43

On mine the speed starts to show as soon as you see the GPS logo appear in the status bar, this is before it's locked on to all available satellites and when it does the speed remains the same.

[TW]Fox · 6 Feb 2011 at 13:44

Khaaan! said:
On mine the speed starts to show as soon as you see the GPS logo appear in the status bar, this is before it's locked on to all available satellites and when it does the speed remains the same.

So how do you think it tells your speed before its locked onto satellites?

mrk · 6 Feb 2011 at 13:46

[TW]Fox;18403950 said:
So how do you think it tells your speed before its locked onto satellites?

Well that's why I underlined all, it locks on to a few which wouldn't mean your location is accurate (might be 20 metres out for example) but the speed reading always is. Once it's locked on to more and the location is accurate to a metre or two the speed reading hasn't changed.

Kenai · 6 Feb 2011 at 13:50

The speed reading will only be as accurate as the positioning is though, as it needs to use the position to calculate the speed.

mrk · 6 Feb 2011 at 13:52

Maybe accuracy is different on different GPS devices then? Not all devices use the same hardware after all

Cosimo · 6 Feb 2011 at 13:54

Khaaan! said:
Maybe accuracy is different on different GPS devices then? Not all devices use the same hardware after all

Well there’s a surprise.

mrk · 6 Feb 2011 at 13:58

Sorry, obvious realisatiom is obvious.

CaptainRAVE · 6 Feb 2011 at 14:11

R420LA6X2/4MNA said:
That said, crash @ 130MPH and it'll probably matter little what you were driving....

True enough, always reminds me of this -

http://www.wreckedexotics.com/articles/017.shtml

UKDTweak · 6 Feb 2011 at 14:11

I have tested three different GPS devices in my current car, one was a dedicated GPS, one was a a Nokia N97 and the other was my current Galaxy S... in All three, I have seen a difference from the speedo of 4-5 mph @ a constant 70mpg.

So when my speedo has show 70mph, All three devices have shown my speed as 65-66mph.

PMKeates · 6 Feb 2011 at 18:10

Almost certainly GPS - most speedometers are designed to over-read (though in a modern vehicle the car's systems know almost exactly how fast it is going).

mjt · 6 Feb 2011 at 18:49

[TW]Fox;18403685 said:
Are you saying it's a fact that the VMAX in every car in production is a safe speed at which to drive that car?

If all components are in good condition, yes. And of course the road is clear, etc.