What speed am I doing

James_N

The other thread on here re Speed Reminder Displays made me wonder about the wider picture.

My speedo says (say) 30 dead.
The roadside reminder 29.
My car sat nav 28
My iphone sat nav 27
(Made up example but typical - and differences widen as speed increases)

- Which to believe?

lemontart

assume you are doing the highest figure that way you can be sure to watch you are not breaking the speed limit

spadoosh

Surely the only one your supposed to?!

Your speedo is calibrated to the car (though not 100% accurate) i would imagine adhering to the speed by this method will be most likely to quash a conviction.

Sat navs track over wider distances and can constantly be a few metres out therefore totally unreliable.

Roadside reminders are theyre to give and indication of your speed if your not bright enough to use your speedometer.

Would always go off speedo and should it be out, this will be the only form of suitable evidence i think.

bryanb

Speedos can over read by up to 10%, and most do!

bryanb

spadoosh wrote: »

Your speedo is calibrated to the car (though not 100% accurate) i would imagine adhering to the speed by this method will be most likely to quash a conviction.


Would always go off speedo and should it be out, this will be the only form of suitable evidence i think.

Good luck with that!!

fivetide

bryanb wrote: »

Speedos can over read by up to 10%, and most do!

This. sounds like if you are doing 30mph you actual speed is around the 28 mark (28 + 2.8mph being 31ish and all). The road side things tend not to be too accurate, they are there to make a point more than anything so the sat nav/ phone assuming they are checking with all the satellites, should in theory be the accurate measure.

Go with the speedo though, they might over read but they can't under read and it will stop you getting a ticket or attracting the attention of plod!

5t.

shaun_from_Africa

bryanb wrote: »

Speedos can over read by up to 10%, and most do!

The law actually gives a far wider figure than this.

Provided that the indicated speed is within 10% + 6.25 MPH of the actual speed then the car speedo is within its permitted accuracy. (but as already mentioned, this is only for over reading).

So if your speedo is showing 30mph, you could in fact be going as slow as 20.75mph, although in reality I doubt if many instruments would be this far off.

bap98189

Your car speedo will almost certainly be reading too high.

I wouldn't trust the mobile phone. The GPS systems tend not to be reliable if you are travelling at speed - mine is fine if I'm walking but can't keep up in the car.

The car SatNav is probably the most accurate, especially if you keep the speed constant over a long distance.

Hintza

bap98189 wrote: »

The car SatNav is probably the most accurate, especially if you keep the speed constant over a long distance.

This is the one I use. It however will not be as accurate if accelerating or braking.

Hurri

spadoosh wrote: »

Sat navs track over wider distances and can constantly be a few metres out therefore totally unreliable.

GPS receivers typically use carrier phase rate (doppler effect) to calculate speed, and is not just a simple distance/time calculation.

Not sure how mobile phone gps systems do the calculation however.

CKhalvashi

bryanb wrote: »

Speedos can over read by up to 10%, and most do!

Especially when the tires are just over the legal limit (I think mine are just on the limit atm, and it's going in tomorrow morning).

There are a range of factors, and no one calibration will ever be completely correct for a number of reasons.

CK