I have the Garmin GPS Map76CS. When driving my speedometer shows 5 mph faster than the speed readings on my GPS. Should I trust the GPS speed readings?
Not necessarily, though your GPS (in my experience) is more likely to be closer to actual. When I test four or five GPS devices in my car at one time with them all displaying MPH they don't always agree with each other, let alone the speedometer.
Joined: 12 Jul 2007 Posts: 544 Location: Bloomington, Il
Thu Dec 13, 2007 2:36 pm
Tim wrote:
When I test four or five GPS devices in my car at one time with them all displaying MPH they don't always agree with each other, let alone the speedometer.
Joined: 12 Jul 2007 Posts: 544 Location: Bloomington, Il
Thu Dec 13, 2007 10:30 pm
Tim wrote:
Wouldn't it be hard to test auto GPS devices otherwise?
I suppose you could be a passenger while your chauffer drives. I'm just not sure I want to be on the same road with someone who is testing and comparing 5 GPS units while he drives.
Joined: 26 Oct 2007 Posts: 183 Location: Philadelphia
Re: Trust Speed Reading! Thu Dec 13, 2007 11:13 pm
Lew wrote:
I have the Garmin GPS Map76CS. When driving my speedometer shows 5 mph faster than the speed readings on my GPS. Should I trust the GPS speed readings?
I do. I have known that speedometer is a few MPH above actual speed. My TT720 shows a slower (and I think more accurate) speed than my speedometer.
Further support of this is with the "your are driving this fast" radar-speed displays that the Police set up on roads, the posted speeds match my gps, and not my speedometer.
Our police Dept puts out the little white speed trailer regularly around the county and I am going to call and find out where it will be the next few days. I can setup the GPS and check with the trailers speed readings. It could be the faster you go in your car the speedometer will be off more. At slower speeds maybe it's right on. Anyway, I am going to make several passes at the speed trailer. The trailer is usually in a 35 or 45 speed zone. We will see what happens and let your know.
By law, vehicle speedometers can indicate a higher than actual speed but not lower. This however is only the case when the original tyres are on the car. Under-inflation causes a smaller rolling radius and thus a higher indicated speed which would still meet the law regarding the speedometer. Putting on larger wheels/tyres would have the opposite effect.
Joined: 15 Dec 2007 Posts: 939 Location: Central PA (FN10) - USA
Sat Feb 23, 2008 3:12 am
I'm not sure your statement is completely accurate, for the USA anyway, as related to the statement regarding vehicle speedometers can indicate a higher than actual speed but not lower. In the US there is a US federal standard that accuracy must be within 5% - it can be higher or lower. Foreign standards can be more stringent.
Some info I gleaned from sources I believe is accurate and enlightning to the original topic/question:
Speedometers are not totally accurate, and most speedometers have tolerances of some 10% plus or minus due to wear on tires as it occurs. Modern speedometers are said to be accurate within 5% but as this is legislated accuracy (ISO 1997), this may not be entirely correct. This can make it difficult to accurately stay on the speed limits imposed; most countries allow for this known variance when using RADAR to measure speed. Although levels of some 3 km/h, or 3% are also used, where tough enforcement is used. This causes many arguments due to motorists complaining that they were not doing the speed as reported. Revenue[1] is being increasingly blamed for these stricter measures. There are strict United Nations standards in place but it seems not being enforced leaving this matter in limbo for many countries. Excessive speedometer error after manufacture can come from several causes but most commonly is due to nonstandard tire diameter, in which case the
GPS devices may indicate the true speed of travel on the user interface. Unlike instrumental speedometers which provide a continious reading, the GPS speed readouts have a one second update interval.
The reading is based on reception of data from the satellites in orbit, and is therefore independent of the car's transmission components. Discrepancies between the two readings may be caused by instrument error (on the vehicle), or by changing directly influencial factors, such as tire sizes.
Hi Spyder,I am pretty sure that in Europe the tolerance on speedometers is minus nothing. Any tyre wear or under inflation that reduces the rolling radius will then indicate a higher than actual speed which is the intent of the exercise. In France the Police allow a small margin of error to compensate for inaccuracies in the radar like temperature, off angle etc.
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Or I've got someone with me to operate them and take notes while I drive.
