Yostumpy Posted February 10, 2010 Share Posted February 10, 2010 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! Quote Link to comment Share on other sites More sharing options...
jad Posted February 10, 2010 Share Posted February 10, 2010 (edited) 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 Quote Link to comment Share on other sites More sharing options...
mark1200 Posted February 10, 2010 Share Posted February 10, 2010 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 Quote Link to comment Share on other sites More sharing options...
Retroanaconda Posted February 10, 2010 Share Posted February 10, 2010 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. Quote Link to comment Share on other sites More sharing options...
LandyManLuke Posted February 10, 2010 Share Posted February 10, 2010 In answer to your question, it will be proportional, i.e. a percentage difference, rather than a fixed mph difference. Quote Link to comment Share on other sites More sharing options...
Jimmy Two-Jacks Posted February 10, 2010 Share Posted February 10, 2010 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. Quote Link to comment Share on other sites More sharing options...
LandyManLuke Posted February 10, 2010 Share Posted February 10, 2010 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. Quote Link to comment Share on other sites More sharing options...
crwoody Posted February 10, 2010 Share Posted February 10, 2010 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. Quote Link to comment Share on other sites More sharing options...
Ex Member Posted February 10, 2010 Share Posted February 10, 2010 ....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. Quote Link to comment Share on other sites More sharing options...
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