Garmin RV760 GPS

[rracer5]

Originally Posted by 5picker

Have you connected it to the internet and done an update?
Might be time since you've had it a while.

Garmin Updates and Downloads

Welcome to the forum!
I see it is your 1st post.
Set-up and add your R/V information in your signature so everyone knows what you have.

That reminded me to do my overdue update. Now I hope it doesn't cause any issues that I didn't have before.

[doc73]

I am still not sure why you need the GPS to see your speed. That is what your speedometer is for. I will not begin to tell you how many times I've been told the GPS told me I was OK.. When they were speeding.

[doc73]

Originally Posted by rracer5

That reminded me to do my overdue update. Now I hope it doesn't cause any issues that I didn't have before.

Same for me.. The hardest part for me.. Finding the damn synch cable! Our Garmins charge vlcable is different from the synch cable and it's been a while..

[NXR]

Originally Posted by doc73

I am still not sure why you need the GPS to see your speed. That is what your speedometer is for. I will not begin to tell you how many times I've been told the GPS told me I was OK.. When they were speeding.

The Ford F53 motorhome chassis speedometer has white numbers on a black background but the pointer is red. It's difficult to see in many lighting conditions including driving during the day with the headlights on. Several threads exist about adding supplemental lighting; I prefer to use the GPS display because it's a lot more visible and clear.

If your GPS is giving faulty speed indications you should consider trashing it. That's a fundamental function of the GPS system and it's used to navigate everything from people walking to jets crossing the oceans.

Ray

[Theo]

Originally Posted by NXR

{snip}...If your GPS is giving faulty speed indications you should consider trashing it. That's a fundamental function of the GPS system and it's used to navigate everything from people walking to jets crossing the oceans.{snip}

IMHO, this may be an over-simplification of "faulty speed indications". Additional factors might need to be considered before you trash your GPS.

GPS units report ground speed (AKA: "longitudinal" speed or "as the crow flies"); this can be different than actual MPH. Your longitudinal change in position is noted every few seconds and the GPS does the ground speed calculation using the change in longitudinal distance and the elapsed time between readings. The problem is the distance change is measured (longitudinally) from a calculated point in space and NOT from the actual distance the wheels have traveled.

To illustrate this, imagine a car traveling due west at 55 MPH on 30 longitudinal miles of flat desert leading to a mountain range. On the other side of the range is another 30 longitudinal miles of flat desert. The longitudinal "width" of the mountain range (as measured from space) is also 30-miles across. Thus, we have three 30 longitudinal mile sections of the earth's surface travelled by a car at a consistent 55 MPH.

The problem is that the road that traverses the mountain is actually 50-miles long due to the need for a comfortable climb and descent via curves and switchbacks. So, because the calculations are made from a point of view in space and are longitudinal, the GPS thinks you are actually "slowing down" in the 30-mile mountainous section since it takes twice the time to travel that 30 longitudinal mile change in distance across the earth.

Now, to put this into perspective, this difference in groundspeed vs. MPH results in an error less than 1 MPH. This is far less that errors induced by tree canopies and urban terrain.

Aviators have a clear, unobstructed view of the sky and are naturally concerned with ground speed for distance calculations. That is why GPS units are well-suited for aircraft.

In summary, your speed reported via a normally functioning gps traveling on roads may not reflect your true speed due to several factors:

1. If the antenna does not have a clear view of the sky;
2. If you are ascending or descending mountains;
3. If you are in an urban area with tall buildings which may cause signal reflections (timing errors).

Accuracy will be best on level ground with an unrestricted view of the sky but may suffer in mountainous country with a canopy of trees or in the "concrete canyons" of large cities.

[DougW]

Originally Posted by Theo

IMHO, this may be an over-simplification of "faulty speed indications". Additional factors might need to be considered before you trash your GPS.

GPS units report ground speed (AKA: "longitudinal" speed or "as the crow flies"); this can be different than actual MPH. Your longitudinal change in position is noted every few seconds and the GPS does the ground speed calculation using the change in longitudinal distance and the elapsed time between readings. The problem is the distance change is measured (longitudinally) from a calculated point in space and NOT from the actual distance the wheels have traveled.

To illustrate this, imagine a car traveling due west at 55 MPH on 30 longitudinal miles of flat desert leading to a mountain range. On the other side of the range is another 30 longitudinal miles of flat desert. The longitudinal "width" of the mountain range (as measured from space) is also 30-miles across. Thus, we have three 30 longitudinal mile sections of the earth's surface travelled by a car at a consistent 55 MPH.

The problem is that the road that traverses the mountain is actually 50-miles long due to the need for a comfortable climb and descent via curves and switchbacks. So, because the calculations are made from a point of view in space and are longitudinal, the GPS thinks you are actually "slowing down" in the 30-mile mountainous section since it takes twice the time to travel that 30 longitudinal mile change in distance across the earth.

Now, to put this into perspective, this difference in groundspeed vs. MPH results in an error less than 1 MPH. This is far less that errors induced by tree canopies and urban terrain.

Aviators have a clear, unobstructed view of the sky and are naturally concerned with ground speed for distance calculations. That is why GPS units are well-suited for aircraft.

In summary, your speed reported via a normally functioning gps traveling on roads may not reflect your true speed due to several factors:

1. If the antenna does not have a clear view of the sky;
2. If you are ascending or descending mountains;
3. If you are in an urban area with tall buildings which may cause signal reflections (timing errors).

Accuracy will be best on level ground with an unrestricted view of the sky but may suffer in mountainous country with a canopy of trees or in the "concrete canyons" of large cities.

Wow!

Since the GPS calculates your location to within a couple of feet and it does so in fractions of seconds the “Longitudinal speed” is moot. Also the GPS calculates altitude with a reasonable amount of accuracy so the speedometer function is quite accurate indeed. Just for giggle I have used the GPS, my phone with a GPS amp and the speedometer in the car. All 3 read the same speed.

[CurtPutnam]

GPS speed readings can be of value of you have rear tires different in diameter than was calibrated at the factory. Unless, of course, the vehicle is smart enough to do that.

[NXR]

Originally Posted by DougW

Wow!

Since the GPS calculates your location to within a couple of feet and it does so in fractions of seconds the “Longitudinal speed” is moot. Also the GPS calculates altitude with a reasonable amount of accuracy so the speedometer function is quite accurate indeed. Just for giggle I have used the GPS, my phone with a GPS amp and the speedometer in the car. All 3 read the same speed.

Agreed. If the difference between the GPS and the real speed is enough for someone to be said to be speeding then the GPS is broked.

A quarter-century in high-end avionics systems with an FAA Class I, II and III Unlimited certification as well as a Class IV Limited certification taught me a lot about navigation systems, radio-based and otherwise.

Ray

[Theo]

As I said, the ground speed vs. MPH issue affects the reading by <1 MPH...good enough for government work! The major inaccuracies are induced by signal reflection and sky cover. In a situation with level ground, a clear view to the sky and no signal reflection, a GPS may actually be more accurate than a vehicle’s speedometer...especially, as CurtPutnam stated, if tire/wheel modifications have been made.

A stationary civilian GPS will minimally report location within a 4-meter (13.1 foot) radius of your location. That means that without augmentation like WAAS (differential GPS), your location is reported within a 26-foot diameter circle. A stationary, high quality GPS in an unobstructed location with an optimal satellite configuration *may* even achieve a 2.5-foot diameter circle resolution.

A phone/tablet GPS sensor traveling in a vehicle will be less accurate than a high end, purpose-built GPS receiver. I fly (VFR) with the latter which also has WAAS augmentation...haven't missed an approach...yet!

[doc73]

Originally Posted by NXR

The Ford F53 motorhome chassis speedometer has white numbers on a black background but the pointer is red. It's difficult to see in many lighting conditions including driving during the day with the headlights on. Several threads exist about adding supplemental lighting; I prefer to use the GPS display because it's a lot more visible and clear.



If your GPS is giving faulty speed indications you should consider trashing it. That's a fundamental function of the GPS system and it's used to navigate everything from people walking to jets crossing the oceans.



Ray

Interesting about the speedometer. I was not refuring so much to the accuracy of the GPS, I guess. more the relying on the speed limit it provided as the "posted limit" on the road to determine the speed limit... Versus just looking outside for the white and black signs all over.

[rracer5]

Originally Posted by Theo

As I said, the ground speed vs. MPH issue affects the reading by <1 MPH...good enough for government work! The major inaccuracies are induced by signal reflection and sky cover. In a situation with level ground, a clear view to the sky and no signal reflection, a GPS may actually be more accurate than a vehicle’s speedometer...especially, as CurtPutnam stated, if tire/wheel modifications have been made.

A stationary civilian GPS will minimally report location within a 4-meter (13.1 foot) radius of your location. That means that without augmentation like WAAS (differential GPS), your location is reported within a 26-foot diameter circle. A stationary, high quality GPS in an unobstructed location with an optimal satellite configuration *may* even achieve a 2.5-foot diameter circle resolution.

A phone/tablet GPS sensor traveling in a vehicle will be less accurate than a high end, purpose-built GPS receiver. I fly (VFR) with the latter which also has WAAS augmentation...haven't missed an approach...yet!

Well....that's a good thing!

[Nancyc7]

I also use the speed limit on my GPS, I need all the help I can get.

I have it set to Most Detail and it does not need an update. What's weird about ours right now, we are in Florida, near Apalachicola. The GPS screen looks like we are in Canada, it displays very little information.

[NXR]

Originally Posted by doc73

Interesting about the speedometer. I was not refuring so much to the accuracy of the GPS, I guess. more the relying on the speed limit it provided as the "posted limit" on the road to determine the speed limit... Versus just looking outside for the white and black signs all over.

Yup, sorry, I mis-interpreted what you meant. My daughter got pulled over because she relied on the GPS "speed sign" when the limit had recently changed. The cop knew about the issue and let her off with a warning.

Ray

[rracer5]

Originally Posted by Nancyc7

I also use the speed limit on my GPS, I need all the help I can get.

I have it set to Most Detail and it does not need an update. What's weird about ours right now, we are in Florida, near Apalachicola. The GPS screen looks like we are in Canada, it displays very little information.

That's cause of all the Canadian snow-birds coming down!

[JLeising]

Believe the GPS gets the displayed speed limits from the maps in its memory. A previous poster said when the limits are not available, his GPS displays “standard” limits for the type of road. My 760 does not do this - the limit box disappears on mine when there is no limit available in its maps.
If the limits disappeared after an update, I suspect the updated maps don’t include limit information for the roads in question.

[doc73]

Originally Posted by NXR

Yup, sorry, I mis-interpreted what you meant. My daughter got pulled over because she relied on the GPS "speed sign" when the limit had recently changed. The cop knew about the issue and let her off with a warning.



Ray

No worries, I was not very clear myself with what I said.

[jimmycaps]

Originally Posted by doc73

I am still not sure why you need the GPS to see your speed. That is what your speedometer is for. I will not begin to tell you how many times I've been told the GPS told me I was OK.. When they were speeding.

Actually I can’t see the speedometer, I have a tilt steering wheel and it stays pulled down. I also have a digital me but I wear glasses and have Transitions lenses, the lenses are to dark to see the digital numbers. I depend on the Garmin. Besides it’s what your brain gets use to looking at. I have the same problem with my TST 507 TPMS, TST changed the psi on the color monitor to the other side. I have Waze but that eats up a lot of data when left on. I am use to glancing at the Garmin. I’ll keep the Garmin as I’m thinking of going to Quartzsite AR next month but after that if the Garmin has not changed back I’ll look at another brand. I think Camping World pushes a gps. I’ll figure it out, maybe something simple. Thanks to all for the advice

[jimmycaps]

Originally Posted by doc73

I am still not sure why you need the GPS to see your speed. That is what your speedometer is for. I will not begin to tell you how many times I've been told the GPS told me I was OK.. When they were speeding.

Actually I can’t see the speedometer, I have a tilt steering wheel and it stays pulled down. I also have a digital one but I wear glasses and have Transitions lenses, the lenses are to dark to see the digital numbers. I depend on the Garmin. Besides it’s what your brain gets use to looking at. I have the same problem with my TST 507 TPMS, TST changed the psi on the color monitor to the other side. I have Waze but that eats up a lot of data when left on. I am use to glancing at the Garmin. I’ll keep the Garmin as I’m thinking of going to Quartzsite AR next month but after that if the Garmin has not changed back I’ll look at another brand. I think Camping World pushes a gps. I’ll figure it out, maybe something simple. Thanks to all for the advice

[NXR]

My polarized sunglasses almost wipe out digital displays so I have to drive without them.

Ray

[Vince and Charlette]

Silly as it may seem, we always travel with 3 GPS receivers: the built-in CoPilot, the Garmin and the Rand McNally. Except in the simplest of situations, we often find at least one will disagree! One may say to turn right, while the other says turn left. So I recommend never traveling with just one. In a pinch, I've even called up the Maps app on my iPhone. We have a 40' DP with a toad - twice in four years, I've had to unhitch and back to get out of a dead end. Never again if I can help it with extra GPS guidance!