What is correct, Speedo or sat nav?

Flyboy152

Stephen_Leak wrote: »

On my old Mondeo, the speedo' was 10% under (ie. 30mph on the dial, 27mph on the road. 70mph on the dial, 63mph on the road). On my new Fiesta, the speedo' is 3mph under (ie. 30mph on the dial, 27mph on the road. 70mph on the dial, 67mph on the road).

Unless you have had your speedometer calibrated properly, how on Earth can you really know this. Satellite navigation is not going to give you an accurate comparison.

cyclonebri1

birkee wrote: »

I'm afraid you lost me!..... What point missed?

You are required by law to have a working speed indicator that works within the prescribed tolerances, and my comment was, ... "let them catch you with a non working speedeometer then" with reference to someone saying you didn't "need a speedometer to indicate speed, as a sat nav will do the job".

Feds? I'm not in the USA!
"you / we end of "..... what does that mean?

The point I was making is that a satnav is an acceptable method of speed indication so a working speedo then wouldn't be an issue, they don't even check them at the annual MOT even though they use the mileage figure derived from it. (seems very strange that, obviously not seen as a safety issue as is everything else checked during the test).

Also I did have a one of car pass it's SVA, single vehicle approval as it was a few year ago, guess what it had a satnav speedo


The point I felt you'd missed is that speeding is speeding, end of story, the feds/plod/old bill or whatever you want to call them don't give a monkeys how you measure speed, as long as you/we do.

Flyboy152

Gloomendoom wrote: »

I have noticed that the sat nav's do have a bit of a delay in responding to changes in speed although, overall, they are very accurate.

But those two statements are contradictory. Unless you know the exact length of the delay, you cannot calculate your real-time speed.

Flyboy152

Lum wrote: »

Now think of the people who think they are enforcing the law by doing exactly "70" in the outside lane.

Strider590 wrote: »

Or 60 whilst mumbling "it's a limit, not a target" under their flat cap.

Again, without knowing exactly how much your speedometer is overreading, it would be impossible to extrapolate what your speedometer should be indicating.

mikey72

Flyboy152 wrote: »

Because "average" is not an accurate representation of real-time speed.

If you are doing a constant speed, the satnav is accurate.
It may not be responsive enough to cope will rapidly changing speeds, but that's not accuracy. That's the refresh rate.

Lum

You know, now that I've seen it's 10%+6.25, I was thinking about the story I posted earlier on the thread, where my old Escort speedo was reading 27 while my mate's Micra was reading 35.

It's quite possible that my speedo was actually pretty accurate and his was at the far end of allowable overread. What prompted the test was him complaining about being tailgated all the time while doing the limit and someone else pointing out that he always drove around town at 5 under.

I also notice it seems in this thread that BMW/Merc/Audio seem to have the most accurate speedos. Could that be a contributory factor to the whole image of them being driven by complete [male hen]s as perpetuated by Claskson etc. To them everyone on the motorway is doing 10 under the limit on a road where the limit is a lot lower than the maximum safe speed anyway!

Flyboy152

Lum wrote: »

You know, now that I've seen it's 10%+6.25, I was thinking about the story I posted earlier on the thread, where my old Escort speedo was reading 27 while my mate's Micra was reading 35.

It's quite possible that my speedo was actually pretty accurate and his was at the far end of allowable overread. What prompted the test was him complaining about being tailgated all the time while doing the limit and someone else pointing out that he always drove around town at 5 under.

I also notice it seems in this thread that BMW/Merc/Audio seem to have the most accurate speedos. Could that be a contributory factor to the whole image of them being driven by complete [male hen]s as perpetuated by Claskson etc. To them everyone on the motorway is doing 10 under the limit on a road where the limit is a lot lower than the maximum safe speed anyway!

Nah....I think it's becasue they are driven by c***ks.

jase1

Flyboy152 wrote: »

But those two statements are contradictory. Unless you know the exact length of the delay, you cannot calculate your real-time speed.

Are speedometers, especially digital ones all that much different? Just because the sample rate is higher doesn't make the measurement any more of less accurate.

As has been said, if you are driving along at a constant, indicated 70mph on a flat, straight road there is no reason to believe that the (unchanging) speed as read by the satnav is anything other than accurate.

I'd also say that on my car, the average speed measure consistently shows a speed slightly lower than the main speedo, and the same as the satnav, given the above.

adouglasmhor

birkee wrote: »

Surely the sat nav is a calculator of speed? Not a vehicle linked indicator.

What has that got to do with anything?

adouglasmhor

birkee wrote: »

Thank you Lum.
Exactly my point. The speed indicator needs to be vehicle linked otherwise it's a calculator.

Can you please point out where in the regs it says that?
Also how do they define linked, my radar one was attached to the bike and powered by it but like many modern speedometers was not mechanically driven by it. Some read a pulse form a cable others read when the magnet passes the cup sensor on the cable.