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Old 11-10-2012, 02:15   #1
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Which would you trust more in your car GPS or Speedo

Just curious, I have two vehicles that for trips I plug in a Garmin GPS. For both vehicles at relatively low speed rates of say 45 mph the speedometer of the vehicles matches the speedometer of the car pretty closely. However, at interstate speeds of around 70 mph the gps gives a speed of about 10 mph below the speedometer. The Garmin manual says not to trust the speed but I cannot imagine why not.

So far no tickets but what do you experts have to say? Trust the GPS or the car speedometer?

Rich
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Old 11-10-2012, 20:18   #2
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Re: Which would you trust more in your car GPS or Speedo

No expert but I would guess the GPS speed is more accurate. Several factors can affect the automotive speedometer.

GPS speed will not be a defense in case of ticketing.
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Old 11-10-2012, 20:39   #3
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Re: Which would you trust more in your car GPS or Speedo

If you want to check find out which is more accurate, find a spot with a measured mile and time it.
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Old 11-10-2012, 20:41   #4
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Re: Which would you trust more in your car GPS or Speedo

gps hands down. 5 or 500 mph it's just a calculation. With a mechanical speedo you can have a 3% error that just gets worse and worse. Then you have changing tire diameters etc as well.

And yeah, there are speedo check sections on the freeway for exactly this purpose
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Old 11-10-2012, 20:51   #5
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Re: Which would you trust more in your car GPS or Speedo

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GPS speed will not be a defense in case of ticketing.
This is a very good point.

I have a garmin that I use in my car and pickup and whatever the speed, they match pretty good. Trust the car first. If you get a ticket when the speedometer showed you were not speeding, then it's a problem with the car that needs fixed.
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Old 11-10-2012, 20:58   #6
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Re: Which would you trust more in your car GPS or Speedo

A broken speedo is not a very good defense either. If you show a judge a $150 repair receipt he *MIGHT* let you off for a small ticket... maybe.
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Old 11-10-2012, 21:13   #7
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Re: Which would you trust more in your car GPS or Speedo

The Garmin car GPS has a "maximum speed" reaudout, which might get you out of a ticket, but mine always reads something like 82 mph--don't think I'll be showing it to any cops.
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Old 11-10-2012, 22:11   #8
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Re: Which would you trust more in your car GPS or Speedo

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The Garmin car GPS has a "maximum speed" reaudout, which might get you out of a ticket, but mine always reads something like 82 mph--don't think I'll be showing it to any cops.
LOL, My motorcycle gps has speed limits in it, and it's super hard to read because the speed is always red.
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Old 11-10-2012, 23:41   #9
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Re: Which would you trust more in your car GPS or Speedo

Speedos in cars always err on the side of caution some reading ten percent more than you are doing, the gps however is accurate in the flat but they dont take into consideration inclines, so if going up a ten percent hill they will be out by ten percent.
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Old 11-10-2012, 23:54   #10
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Re: Which would you trust more in your car GPS or Speedo

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Speedos in cars always err on the side of caution some reading ten percent more than you are doing, the gps however is accurate in the flat but they don't take into consideration inclines, so if going up a ten percent hill they will be out by ten percent.
Same issue with curves in the road, changing lanes, or not driving straight. And, not just an incline...any change in altitude will cause an error in actual speed...hills, overpasses, etc.

Also, I believe all GPSs have a speed smoothing algorithm whereby the GPS takes an average of the actual speeds over a certain time period. Remember GPS measures speed in all directions (right, left, forward, reverse) and displays only a positive number. Try this by going 5mph in reverse...you will see 5mph. This is the reason that your GPS on your boat will show speed while at anchor. It is the swinging from port to starboard that is "smoothed" and displayed. We are at anchor, but out GPS currently displays 0.2kts.

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Old 11-10-2012, 23:57   #11
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I have slightly over size tires wheels that leaves the Speedo about 25% off. It is the GPS all the way.


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Old 15-10-2012, 02:02   #12
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Re: Which would you trust more in your car GPS or Speedo

I just bought a 'truck specific' GPS for my Swedish Peterbilt (Volvo truck). I think the GPS is a bit more accurate, as it always coincides with the radar warning trailers that many states have hanging out in construction zones. My speedo has always been 5mph off.
BTW- It's a Rand McNally, for trucks it's been rated the best for routing and ease of use. Good thing, too. Some of the new drivers need 'easy' more than others.... ;-)
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Old 15-10-2012, 02:25   #13
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Many iPhone users lost the Google map app when they upgraded to ios6. A lackluster TomTom product was the replacement.

For those in that predicament like I was, someone in another thread suggested an app by mapquest. I tried it and like it best of any GPS cellphone app I have tried. Voice directions if you want, switch off if you just want maps. This thread is about mph and I do not believe most GPS map apps list speed. Some do. Some list speed, g's in turns and altitude
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Old 15-10-2012, 02:36   #14
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Re: Which would you trust more in your car GPS or Speedo

Did you change the units from knots to mph when you put it in your car?

70 mph is close to 60 knots. Slower becomes closer.

John
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