Navigation vs. Speedo accuracy

ltnate3

ltnate3

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Which one do you guys think is more accurate. My speedo will say 73 but my Ecplipse Nav says 68.
 
If your Eclipse has a VSS tap then I'd say the eclipse. If the unit relies soley on GPS time hacks, then I'd say the Speedo. If you've just installed the headunit and it does have a VSS tap, give it several hundred miles to learn.
 
My speedo reads about 2-2.5mph high (ie, when it says 65mph, the car is actually going around 62.5-63mph). I have verified this against those construction zone radar units, and by timing against mile markers on the interstate.

FWIW, most GPS units have extremely accurate speed measurements. My 10+ year old Garmin handheld unit is good to within 0.1kts. I'd trust it over the speedo any day.
 
most cars are about 2 mph off at 60. navigation is extremely accurate.
 
The navigation unit is more accurate. It tracks actual speed by comparing movement against a fixed location (earth). A speedometer is calculated based on gear ratios, tire sizes, etc. As your tires wear, your speedo reads faster for example.
 
GPS based systems are accurate only in a steady state. Otherwise they are taking a running average. So if your speed OR direction changes from moment to moment, the average will lag behind the actual velocity(speed AND direction).

Speedos are all over the map as far as what is accurate. Some are high some are low, they are only close enough that most of us won't get a ticket if we think we are doing the limit.

I watch my GPS in my truck and like I said, it lags the actual direction and speed. Try turning a corner from a stop and accelerating. The GPS will be behind a certain amount which I assume depends on how good it is and how good the satellite lock is.

Just my experience.

Jim Henderson
 
Frogman said:
If your Eclipse has a VSS tap then I'd say the eclipse. If the unit relies soley on GPS time hacks, then I'd say the Speedo. If you've just installed the headunit and it does have a VSS tap, give it several hundred miles to learn.
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Yeah the Eclipse has a VSS tap. I was thinking that had to be the most accurate. Thanks!
 
ltnate3 said:
Yeah the Eclipse has a VSS tap. I was thinking that had to be the most accurate. Thanks!
Click to expand...

And unless you have an older vehicle...that's what your speedo uses...VSS.
 
right and vss can be slightly off due to tire pressure, etc. sat should be very accurate but delayed.
 
beaups said:
right and vss can be slightly off due to tire pressure, etc. sat should be very accurate but delayed.
Click to expand...


Ok I think I'm completely confused now. If the speedo and nav are both hooked up to the VSS wire, why am I getting two different readings?
 
Frogman said:
Because OEM gauges aren't known for their uber l33t accuracy.
Click to expand...

So is there nothing I can do to test which one is correct and then callobrate the wrong one?
 
Sure there is.

1. Those goofy speed test markers on the highway
2. Ask any cop with a radar gun if he would tag you doing 30-50 (whatever the speed limit is), and see what his results are. Most police departments calibrate their radar units on a reglar basis.

3. Don't sweat it too much, 2 miles per hour difference isn't the end of the world.
 
This is from an article I read some time back and I verified it with my wife's crown vic she used to have. Tha article stated that speedometers have to be acurate up to a percentage--I cant remember what that was but I think it was 96%. Most new cars calibrate their speedos to "lie" to you and say that you are going faster than you are. The car uses the "real" value for trip computer and mileage data. Do this set your cruise on a flat road so that your speed will not deviate (I know LSes do not have avg speed but this is as universal as I can get) then reset your avg speed. The vic stayed pegged on 60MPH and the avg speed was 58. I think that your navi is using the "real" value.
 
Or buy/borrow a hand-held GPS unit. Like I said, my Garmin unit was accurate to within 0.1kts (0.11mph), and it didn't have a problem with non-steady state driving. It is just a GPS receiver, not a navigation unit, and I never really noticed a lag when I was using it to check the speedo in my '95 SHO. I can tell you that it didn't have a noticeable lag from going from ~60mph to 75mph (the limit on the turnpikes around here), and it showed a uniform "error" of ~2mph at all speeds checked.
 
looking at your photo gallery you have custom wheels? 20's? if so that would explain the speedo errors
 
That would explain the speedo error, but it doesn't explain the discrepancy--the nav unit calculating based on the GPS signal would, however. If it's using the VSS to calculate speed, it should be just as wrong as the speedo, because its data will be coming from the same source (the VSS).
 
Yes, BUT.

The VSS signal is altered with the new wheels. I had not realized you got 20's on there. However, the navigation unit does not know that, and self calibrates to that signal by comparing said signal with calculated speed from the GPS signal.

In other words, it really doesn't care what size wheels you have. One it calibraes itself, it is accurate.
 
I think nav units use sat to calculate speed, not vss. To my knowledge VSS is only used when a signal is unavailable (ie dead reckoning)
 
No sir, they do use the VSS to calculate speed along with/or in lack of the GPS signal.

In tunnels and the like where I don't have GPS signal, the speedometer function on my D1 still shows speed. I can slow down, speed up, etc, and the D1's speedo screen updates in real time.
 
beaups said:
looking at your photo gallery you have custom wheels? 20's? if so that would explain the speedo errors
Click to expand...

Actually I have 18s and I think they are the exact same diameter as the stock.
 
Frogman said:
No sir, they do use the VSS to calculate speed along with/or in lack of the GPS signal.

In tunnels and the like where I don't have GPS signal, the speedometer function on my D1 still shows speed. I can slow down, speed up, etc, and the D1's speedo screen updates in real time.
Click to expand...

I'm not sure I understand your point. What I was stating is that VSS feed is used when GPS signal is not available (like in a tunnel as you said). Pioneer brands this as "dead reckoning" IIRC.
 
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