I just did a little 5 hr trip with my 06 super duty. I noticed a few hours in that the speedo would read a few km/h slower than the speed on the gps. I also reset my truck trip meter before the start of the trip and reset the gps trip meter about 60 km into the trip. The gps was gaining distance through the trip.

The only thing I could thing it might be is the 18" factory rims (optional aluminum units) versus the 17" factory rims. Has anybody noticed their speedo / odo out?

 

what kind of gps was it?


one thing i learned in the Navy, never trust a civilian gps for accuracy.

 

If it is WAAS or EGNOS anabled then it is as accurate as the MIL standard. (as long as Mr. Bush doesn't pull the plug...the ALL GPS signals will be encrypted to cevilian - or other contries' MIL for that matter)

If it is not, as for point accuracy per givven time sample you are correct, but speed has a constant trajectorial deflection, meaning that it will always compansate for this inaccuracy.

Eysham

 

Not entirely true. Mine is pretty accurate with mileage, although I did it a few times since I have owned this last one, did it a couple of times with my first one, after that I just never remembered to use it and just used my trip odometer(which I have gotten into the habit of not using unless I'm hauling horses for customers, used to do it all the time). The only real inaccuracy on the civilians is the location of you and the circle around you on the screen. On most of your commercials ones it could mean up to 300 yds difference in where the screen says you are and where you actually are, on military I think it has to be 10 yds or less.

 

It may have less to do with your GPS units and more to do with what Ford considers "accurate" for a speedometer. It very common to find a 2-3% variance from actual, usually with the speedo running on the fast side. This is true of any Ford vehicle on the road.
Ford does set the tires RPM (revolutions per mile) at the factory based on the tire size ordered, so tire size is already taken in account by Ford.

 

Dang Eysham, that's pretty deep! I'm gonna have to work on that one for a while!!

 

Mine is just the opposite. I run 35'' tires, and at 60mph truck speedo - GPS is 64.8 ( I even brought another GPS on board and they were excatly the same. ( GPS in 2007 is extremely accurate )

On a 4 hour drive I resest both trips ( Truck and GPS ) after GPS showed I went exactly 100 miles truck showed I went 90.

 

While the gub'ment pulled SA back in 2001, there are still some questions of accuracy and signal manipulation regarding GPS in the US. You can be sure that our GPS receivers are not as accurate as they could be and that this is a designed behavior for several reasons.

Google around a bit...there is a wealth of information on this stuff.
http://www.huntting.com/sawatch/

 

Also, your speedometer is never totally accurate. Normal speedo error is around 2%, but I have seen factory speedmeters off by as much as 10%, which means when it says 60 you could really be going somewhere between 54 and 66 MPH.

 

How much tread left on the tires?

My 2001's speedo is 1 MPH low at 60MPH. But that's with 285/75R16's which are 32.8 inches tall, while the original 235/85r16's were 31.7. So, the speedo must have been WAY high with the stock tires.

My '96 t-bird, when I regeared it, had a more accurate speedo after the re-gearing than before (changed the speedo gear). It was 10% high stock, and 2% low after the re-gearing.

As to the accuracy of GPS units, from memory:

When comparing civilian vs. military units, the accuracy used to be 10 meters for civilian, and almost dead-on for military. They did something a while back that made the civilian units accurate down to 1 meter. I guess they figured if someone stuck a civilian GPS unit in a missile, 10 meters was close enough anyway

I have a Magellan unit from about 5 years ago. Every year I go back to the same place in Upstate NY, and ever year, my GPS shows the exact same spot, down to the resolution of the display.

Going 60MPH on the highway, I think that's accurate enough to calibrate a speedometer.

If anything, a GPS won't register higher or lower speeds over a long distance. It will show speeds at any single point being slightly higher or lower, but overall average speed will be quite accurate. Or "accurate enough" to check your speedo

 

While on the subject of GPS I have a question on distance traveled. Does the gps caculate "flat map" distance or can it factor in elevation change as well? I use it the gps on the bike when laying out trails for enduro and the question came up. In the trail selection use, the elevation changes can be 2000+ feet with many steep ups and downs.

BTW. my 05 f-250 speedo reads ~1.5 fast at 70mph.

 

It would depend on the GPS unit - whether it does 2D or 3D distance traveled.

Something to ask the manufacturer, that's for sure

 

We make our living selling GPS equipment for agriculture. The systems we sell are what is called sub meter acurate. That means that they will be acurate to within +/- one meter. They are ussually much better than that.


The GPS speed is normally very close. I can put my Garmin next to our better units and the speed is identical. The Garmin may update the speed on the screen slower but at a constant speed is right on.

Krewat is right on the distance traveled. I think most would be 2D or flat map. I'll have to look at my Garmin. It may have 3D.

 

Im using a Garmin e-trex vista, any idea where to check 2d v 3d?

 

Sorry but I have ne clue. Garmin is a toy for me and haven't learned it very well. You might go to the adventure rider forum. They have a lot of guys with knowledge on those things.