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GPS and speedometer

Old 06-12-08, 07:39 PM
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GPS and speedometer

- note to mods: not sure what thread would apply, feel free to move if necessary -

Picked up a Garmin Nuvi 200 GPS today ($149 at Office Depot).
Anyone have this particular model?

It seems to have the basic features I'm looking for, without spending $300+, but I'd love to hear opinions from users.

My question has to do with the way it tracks vehicle speed. One of cars has a speedometer that is sketchy, and a replacement unit was priced out at a much higher price than the cost of the GPS. Much higher. Plus, it's an older car.

Can the Garmin (or any GPS) accurately track vehicle speed, much like a speedometer? I know there's probably some satellite lag time occasionally...
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Old 06-12-08, 07:50 PM
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I have the 330, but the speed gauge on it seems to be pretty darn close to the speedometer on my car. The one thing that's screwy is the "max speed" thing. Right now my max speed since last reset is shown as 266 MPH. I speed, but not that much. I have taken it on planes, but it's been turned off.
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Old 06-12-08, 07:52 PM
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Originally Posted by 12thmonkey
- note to mods: not sure what thread would apply, feel free to move if necessary -

Picked up a Garmin Nuvi 200 GPS today ($149 at Office Depot).
Anyone have this particular model?

It seems to have the basic features I'm looking for, without spending $300+, but I'd love to hear opinions from users.

My question has to do with the way it tracks vehicle speed. One of cars has a speedometer that is sketchy, and a replacement unit was priced out at a much higher price than the cost of the GPS. Much higher. Plus, it's an older car.

Can the Garmin (or any GPS) accurately track vehicle speed, much like a speedometer? I know there's probably some satellite lag time occasionally...
Yes, they can. I don't know if the Nuvi line can as it is very "map" focused. But the feature has always been available in any of the "sport" oriented (boating, hiking, etc) handhelds. I use mine to check my speedo for accuracy. It indicates speed and bearing, accumulates trip distance like an odometer, etc.

If the Nuvi does estimated arrival time, speed is in there somewhere. The question is whether it is available, and I am not that familiar with the Nuvi line.
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Old 06-12-08, 08:18 PM
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A GPS receiver can give an accurate velocity reading. As you said, there is some lag involved. If you accelerate quickly, the GPS will lag until your speed stabilizes.
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Old 06-12-08, 08:51 PM
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more precise question, which is more accurate gps speedometer or the one on the car.
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Old 06-12-08, 08:58 PM
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I have a Garmin Legend CX. It's a handheld model I mostly use for kayaking, hiking, etc. I noticed that is shows I'm going slower than my truck speedometer says I'm going. Not necessarily lagging, like mentioned before, but at highway speeds, cruise control on.
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Old 06-12-08, 09:16 PM
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GPS is going to be more accurate. Speedometers have mechanical errors which increase with speed as well as time.

See: http://en.wikipedia.org/wiki/Speedometer#Error (Note the GPS section that follows )
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Old 06-12-08, 09:37 PM
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My old car was off by 3-4 mph at highway speeds. new car is within 1 mph, but I'd venture that with cruise control set on a flat highway, GPS will be more accurate.
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Old 06-12-08, 10:17 PM
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Technology is amazing.

Took my Garmin Streepilot on a 1500 mile trip and the difference from my odometer to the GPS for the whole trip was like .4 miles or something. Not bad at all.

As far as the speedo goes I've always found it to be very accurate. Difference is usually about 1mph or so.
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Old 06-12-08, 10:36 PM
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the Garmin will be very accurate. the speedometer sometimes will read a little lower (or the same, but never higher) than actual speed for legal reasons.
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Old 06-12-08, 11:30 PM
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I think you got it backwards. The speedometer may read HIGHER than actual speed. If the speedometer says you're doing 20 when you're actually doing 70, that's bad. If it says you're doing 70 and you're actually doing 20, that's not as bad.
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Old 06-13-08, 06:54 AM
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With the Nuvi's you cant see the speedometer while you are getting directions, I think.
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Old 06-13-08, 06:59 AM
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Originally Posted by kaze0
With the Nuvi's you cant see the speedometer while you are getting directions, I think.
I'll have to check that out.
I know for $149 it isn't going to necessarily do everything I want it to the way I want it to, but I hope the speedometer function should be helpful more often than not.

And to be honest, I didn't even know GPS units had a speedometer function until yesterday. Like I said in my OP, a part of this unit's use is for one of our cars - an older model with a malfunctioning speedometer that is not cost effective to fix.

I'll fire it up this weekend...

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Old 06-13-08, 07:51 AM
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Originally Posted by whotony
more precise question, which is more accurate gps speedometer or the one on the car.
"Normally" the GPS will be much more accurate although you may get a few completely flakey readings (usually position will be suddenly way off too). These are caused by reception problems or disturbances in the ionosphere.

All speedometers are dependent on tire wear and tire pressure for accuracy. In addition, some systems can only substitute limited gear ratios in sensor to compensate for nominal tire size and rear axle ratio combinations so there may be a bias at nominal tire pressure and size. By Federal law, a wider tolerance exists on the high side, so MOST speedometers read a little high (there will be exceptions)

Good way to calibrate speedo
1) Check your tire pressure
2) Drive speed limit on Interstate, using cruise control
3) Check the speed with GPS. Watch for obviously crazy readings, but if they "make sense" they are probably at least 10X more accurate than speedo. Do this for at least a mile or more, using the "predominate" GPS reading (or average).
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Old 06-13-08, 08:15 AM
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Originally Posted by D.Pham00
the speedometer sometimes will read a little lower (or the same, but never higher) than actual speed for legal reasons.
That's interesting because in and old issue of C/D they mentioned how BMW speedos were always a bit higher than actual speed. I think the BMW speedos are actually the least accurate of the ones they tested.

When I need to be sure, I work with the electronic speedometer: it actually just takes an average, but resetting it every minute or so gives me enough of an idea so I can hit the cruise control.
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Old 06-13-08, 08:52 AM
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I've got a TomTom One 130, Garmin Legend CX and both are pretty spot on for gauging speed.

I use the Garmin as my defacto speedometer on my motorcycle as bikes are notorious for speedo errors. My bike is even worse since the primary units are metric and the MPH are secondary and get pretty far off the faster I go.
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Old 06-13-08, 09:18 AM
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Is there a monthly service fee with a GPS or is it free?
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Old 06-13-08, 10:19 AM
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Originally Posted by The Bus
That's interesting because in and old issue of C/D they mentioned how BMW speedos were always a bit higher than actual speed. I think the BMW speedos are actually the least accurate of the ones they tested.

When I need to be sure, I work with the electronic speedometer: it actually just takes an average, but resetting it every minute or so gives me enough of an idea so I can hit the cruise control.
must be having a brain fart. i meant to say that the speedometer should read the same or higher than the real speed, but not lower. though apparently that's not the case, according to that report. the reason being is that if it reads substantially lower than the real speed, and you get a ticket (or an accident), then the manuf. may have some liability.

also, other things affect the speed - like the diameter of the tire.
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Old 06-13-08, 11:43 AM
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Originally Posted by Jacoby Ellsbury
Is there a monthly service fee with a GPS or is it free?
If you pay US taxes, you've paid for it already.
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Old 06-13-08, 11:50 AM
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Originally Posted by Jacoby Ellsbury
Is there a monthly service fee with a GPS or is it free?
GPS service has no charge itself, though if you get some options like traffic, then those options may and usually cost.
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