Ok, I said I'd give an update on the speedo - vs - GPS - vs - dealer calibration:

I've done a couple of test suggested by this group. I ran several 'measured miles' on a straight road - after 4 mile markers my odometer read 4.3 miles traveled. So, as GIJoeCam guessed, my odmometer read high. I also did the measured mile and clock thing - set cruise control for 60 and clocked between 2 mile markers. It too 66 seconds to cover the measured mile. Ran the speedo up to 66 mph on CC (GPS said exactly 60 mph) and it took exactly 60 seconds to do the measured mile.

But, I now know that the GPS is off and that the mile markers are inaccurate BECAUSE my Ford dealer calibrated my speedometer and said it was dead on accurate (he charged me for this info!)
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My '97 F-150 speedo has, for years, read a little high when compaired to my Garmin Street Pilot GPS. Ex: - truck shows 70 mph with cruise control ON, the GPS showed 69 mph. No big deal - actually pretty ok!

3 weeks ago the delta increased to 6 mph - truck speedo shows 70 mph with cruise control ON, GPS shows 64 mph.

So, I cruise the Interstate at 73 mph (just a little over the legal limit), but my GPS shows I'm only traveling at 67 mph.

So maybe I can cruise at 79 mph on the speedo ( GPS actual speed of 70 mph + 3 mph over the speed limit ) - would I get a ticket?

Really, why the increase in the delta from 1 mph to 6 mph three weeks ago. Had Ford dealer do a computer speed verification (the report says "Diag hook to WDS test drove the VSS.Computer and speedo read all the mph" $41 cost.

Reading the newsgroups, most seem to think vehicle speedometers are set slightly high, so the 1 mph delta I had for years would seem to validate this. But why the major change to a 6 mph delta? I tested the GPS over several trips in town and on the highway and the newer/higher delta remained constant. I would have thought the GPS should have been the most accurate but it appears (per the Ford dealer) that the truck speedo is accurate (+/- what ever they define as accurate).

Any thoughts?

Thanks

 

Only way to tell for sure is time a measured mile. That will tell you exactly which one is accurate.

 

Don't know why it would change, but mine is off as well. The truck speedo is 2 mph faster than my Edge, and my Edge is 2 mph faster than a GPS.

 

Travel exactly 60mph and see if you go exactly one mile in exactly one minute. You can compare this to milage markers on interstate highways. These are usually very accurate. Your GPS uses several satellites to triangulate your location and often times do not use the same satellites, which can cause minor difference in speed or minor location differences because of the angle of the satellites in the sky in relation to your vehicle. I have used mine to track a specific path and used it again on the same path and noticed a slight difference in path GPS markings. Walk around a track for ten laps and the GPS trail markings will not be exactly on top of each other and not likely the same exact distance each lap. When camparing to a speedometer or odometer a few MPH difference can be expected. My GPS is a cheap Magellan but all work on the same principle.

 

Your delta between the speedo and the GPS unit could very well just be a lack of satellites from one day to the next, with different satellites being used can record different measurments

 

I'll take the satellite GPS-verified readings every time. The speedo is open to a whole host of errors. The GPS reading when moving, especially, will be consistently more accurate than the speedo. Around town, they occasionally post these radar/speed trailers (sometimes in a construction zone, sometimes in subdivisions, etc). My GPS always matches dead-on with the portable radar, so I trust it to keep me legal. My speedo, on the other hand, is always 2mph slow across the entire range. The odometer disagreed with the GPS by 0.4 miles over an 800 mile trip a couple years ago... but I had to go through the Alleghaney Tunnel, so I lost satellite reception for a minute.

As for the whole 60 mph thing, you don't necessarily need to travel 60 mph. Find a flat, open stretch of freeway, and reset the trip odometer right next to a mile marker. Drive it for some distance (the longer, flatter, and straighter, the better) and stop next to another mile marker. The elapsed distance on the signs should match the elapsed distance on the odometer. In your case, your odometer will likely read higher than the road signs.

As for what happened, look at a couple of things. First, is your speedo off the same amount at all speeds, or is it off proportionally across the range? i.e. does it always read 6 mph slower than the GPS, or does it read 3 mph off at 35 and 6mph off at 70?

Have you changed tires recently? Do you have a gauge overlay in the IP? Have you recently had the truck serviced anyplace they may have over-inflated your tires?

-Joe

 

Several good suggestion provided - so I will do some more tests with the GPS at different speeds and using the mile markers, etc. I will post the info when I get it. I also e-mailed Garmin - is it possible my GPS could be in decline due to leaving it in the truck in the Florida summers (heat affecting the electronics) although a change in the delta from 1 mph to a 6 mph should have been gradual I would have thought.

Anyway, I'll do an update on what I find after more tests.

Thanks for all the 'Replies'

 

In my experience, either the GPS works and is spot on (within the manufacturer's published tolerances) or it's waaaay off. I'd be more inclined to suspect the needle on th speedometer has moved than to think that the GPS all of a sudden is reading 6mph off. I suppose that it's always possible that its internal timer is off, but the way they work is they use the position data between two points to determine the distance traveled since teh last data point, and the elapsed time from the last data point. That elapsed time *should* come from the satellites, not the internal clock though, and if it's using the satellite's data, EVERY GPS using those satellites would be off, and all of them would have to be off the same amount simultaneously.... I doubt that's the case.

I suppose that, if the unit uses its internal clock to calculate the speed, and the internal clock somehow slowed, it would appear as though you had covered the same distance in less time and were traveling faster than you really are. Again, doubtful, but I suppose it's possible.

I'm eager to hear what Garmin has to say about it. What model GPS are you using?

-Joe

 

I'll also vote for the GPS being accurate over anything else....

Steve

 

another thing to remember... if your wheels and tires arent the correct size it will make your speedo and odom.. read off thats is another factor

 

My GPS is an old b&w Street Pilot - bought it years ago. It does have the ability to upload maps from Street Atlas which Garmin trys to sell me each year. I upgrade every two years for my use in locating places or planning routes - but Google Maps is being used by me more and more - especially with the satellite overlay on the roads.

 

Here is Garmin's 1st answer - I sent them my exact model number (again) and hope for a better answer :
"Thank you for requesting Garmin Email Product Support.

It could be that the satellite acquisition log has gotten overloaded over
time. I would recommend an acquisition reset if I knew which specific Street
Pilot you have. If a stp 3, press page and then power; or other combinations
are: PAGE + POWER, PAGE + FIND + POWER, MAP + POWER. Or it could be that it needs a master reset but I would need to know the exact model to list those."

 

Ok, I said I'd give an update on the speedo - vs - GPS - vs - dealer calibration:

I've done a couple of test suggested by this group. I ran several 'measured miles' on a straight road - after 4 mile markers my odometer read 4.3 miles traveled. So, as GIJoeCam guessed, my odmometer read high. I also did the measured mile and clock thing - set cruise control for 60 and clocked between 2 mile markers. It too 66 seconds to cover the measured mile. Ran the speedo up to 66 mph on CC (GPS said exactly 60 mph) and it took exactly 60 seconds to do the measured mile.

But, I now know that the GPS is off and that the mile markers are inaccurate BECAUSE my Ford dealer calibrated my speedometer and said it was dead on accurate (he charged me for this info!)

 

1. The vehichle identafacation software block (vid) has wrong data per the gear ratio, dispite what the dealer did.
2. The speedo gets it's info from the PCM so it will be inaccurate when this condition exists.
3. The EGE tuner will also be inaccurate because it to gets it's signal from the same source.
4. Be sure the stock tire size is on the truck or the VID block data will drive the wrong speed on the speedo.
Considereing some tolerence for absolute tire size, wear and speedo mechanical needle position, there can still be a small amount of difference even when everything is known to be correct.

 

Been through this with both of my trucks.

Calibrated my EDGE using the rolling wheel method and the odometer is right on. Checked by use of milemarkers and gps. As stated before, the stock speedometer is off by 1-3 mph. It's the same for both of my trucks.

I have 4 gps units, one vehicle mounted and 3 small handheld.

All are very accurate.

All units will read the same when moving down the highway. I have tried 2 at a time to check accuracy. And I have checked using the mile markers. Again the same result. I would suggest using at least 3 miles of markers to get a good average. Some have been known to be off by a few feet, depending on the location.

So you need to calibrate your EDGE using the rolling wheel measurement and figure that the speedo will be off by 1-2 mph. If I show 70, my gps will show 68.