SPEEDO INNACURACY

[Yostumpy]

Whist driving towards one of these 'tell tale speed reading, slow you down' jobbies I was doing 35mph, and the thing flashed 'slow down 32mph!' therefore approx 10% innacurate OR 3 mph at 35mph. Now is this a progressive innacuracy, not knowing how these thngs are calibrated, I mean will I be about 7mph out at 60 or still only 3 mph out! I could drive towars it at 60, but not advisable in a 30 zone!

[jad]

speedo will be a certain percentage out all the time.

i.e. mine reads 10% under all the time

speedo says 30 im doing 33

speedo says 70 im doing 77

etc

mine was checked against sat nav as well

Edited February 10, 2010 by jad

[mark1200]

when i got my satnav i checked my speedo against it. Satnav reads 30mph speedo says 32mph,a bit faster satnav says 60mph speedo says 64mph took it up to 90mph and the speedo said 97mph

[Retroanaconda]

I think speedos are allowed to over-read by up to 10%, but not under-read.

It's fairly standard for them to be out a bit.

[LandyManLuke]

In answer to your question, it will be proportional, i.e. a percentage difference, rather than a fixed mph difference.

[Jimmy Two-Jacks]

In answer to your question, it will be proportional, i.e. a percentage difference, rather than a fixed mph difference.

The sat nav method is ok but you have to bear in mind that the reading is not indicating your actual speed at that moment but the reading taken from the information tracked from a number of co ordinates sent to the satellite a fraction of a second earlier once the information has been processed it registers on your gps. Thats why if you use something like GPS SPEEDO with a PDA you get fluctuating speeds that don't seem to correspond with your speedo or indeed road speed.

[LandyManLuke]

The sat nav method is ok but you have to bear in mind that the reading is not indicating your actual speed at that moment but the reading taken from the information tracked from a number of co ordinates sent to the satellite a fraction of a second earlier once the information has been processed it registers on your gps. Thats why if you use something like GPS SPEEDO with a PDA you get fluctuating speeds that don't seem to correspond with your speedo or indeed road speed.

Nothing's sent to the satellite, and the update rate is typically only 1Hz. That said, GPS speed readings at constant velocity (constant speed and direction - horizontal and vertical) are pretty accurate.

[crwoody]

When I check mine against my TomTom, I find it's pretty much spot-on all the way from about 20 up to 70, I have got slightly oversize tyres on though so that helps.

IMHO Sat nav is the way to go if you want to make a fairly reliable check, you just need to be travelling at a steady speed when you cross check it.

Certainly more reliable than trying to read your speed off a roadside sign.

[Ex Member]

....but the reading taken from the information tracked from a number of co ordinates.....

Actually, that is wrong. GPS speed is determined directly via the doppler effect. It does not calculate speed based on position change.