GPS Speed readout Accuracy

[+trooperdjb]

Just wondering what everyone's experience or belief is. I've got my GPSMap 60CSx mounted to the dasboard and can watch the speed indication on it as I drive to work. It typically shows 3 MPH slower then the spedometer in my truck does so this morning i timed myself over 4 measured miles and calculated what my speed was. The result was about right on with the GPS indication. This difference seems to hold constant from about 40 to 70 MPH. Not that I'm going to change how I drive based on this, more a point of curiosity.

 

Thanks

[+Sputnik 57]

The GPSr is accurate to within about .1 mph. Most vehicle speedometers overstate your speed by several mph. The glass half full crowd say that manufacturers do that so that you wont get a speeding ticket. The glass half empties say it has the added bonus of overstating your milage (and hence making your warranty run out faster).

 

Trust the GPS.

[CenTexDodger]

It seems to lag a bit with acceleration and deceleration.

[+Always & Forever 5]

Just wondering what everyone's experience or belief is. I've got my GPSMap 60CSx mounted to the dasboard and can watch the speed indication on it as I drive to work. It typically shows 3 MPH slower then the spedometer in my truck does...

I, too, have a 60CSx mounted in my truck, and I see the exact same thing as you do, 2-3 mph faster on the truck speedometer than the GPS display. The GPSr is a lot more accurate at steady speeds, but less so during acceleration and deceleration. Tire size will make a difference, also, if you have larger or smaller than stock size tires installed.

[+trooperdjb]

Thanks for the thoughts and I notice the lagging of GPS speed vs. spedometer more on deceleration than acceleration probably the reading 10 MPH when I first stop at a light.

 

I was fairly confident in the GPS readout after my check this morning but the confirmation is good as I didn't want to fall into the "It's a Digital readout it has to be more accurate" trap. Talk to a good machinest sometime on the difference between digital and dial calipers.

 

Thanks again

[+carpediem007]

Most vehicle speedometers overstate your speed by several mph. The glass half full crowd say that manufacturers do that so that you wont get a speeding ticket. The glass half empties say it has the added bonus of overstating your milage (and hence making your warranty run out faster).

 

Trust the GPS.

 

Don't know about the US, but German law requires manufacturers to calibrate their speedometers in such a way, that they overstate your speed (don't know the percentage though) to avoid speeding accuracy related lawsuits etc...

 

On the other hand, AFAIK there is no relation between your speedometer and milage indicator. The milage indicated is the actual distance travelled.

 

And, yes, do trust the GPS...

 

Cheers,

Michael

[Bedrok]

As a trivial addition to this subject, my old Rino 110 proved to be right when it was disagreeing with all 3 of our speedometers (Ford read 3 to 4 % slow, Toyota and Nissan same amount fast). What I notced was the faster I go, the less accurate it was perpendicular to my course. When traveling N or S, it tends to show 200 to 400 feet E. of actual position. When going E or W, it reads the same error to the N. If I slow down to under 20 mph it comes back to almost right on. I was wondering if the processor can't do the calcs fast enough.

[+trooperdjb]

The innacuracy of position isn't one I've noticed with the 60CSx. When driving down the highway it's right on as to which lane I'm in and which cross roads I pass. I have noticed that when in routing, the distance to turn seems to often be off by between 20 and 40 feet. In other words if waited until the distance to turn read 0 I would be off the road by some small (in the grand scheme of things) but significant (as in the ditch) distance.

[+Prime Suspect]

It seems to lag a bit with acceleration and deceleration.

The speed reading is only update once every second.

[cragfast]

The velocity accuracy of my Garmin GPSMap60csx is quoted as 0.05m/sec (steady state)

0.05m/sec is a little over 0.1mph error but this is only true for a steady state, ie NO acceleration or de-acceleration.

So the speed is only accurate to 0.1mph when travelling at a constant speed.

[StrikesFolks]

My Windstar's speedometer reads about 6 kp/h high while going about 100-115.

 

A few times I have put it on the cruise, wait til things level out on both the GPS and the speedometer, and the GPS is reading 6 kp/h lower.

 

I got identical results on my eXplorist 400, my friends Garmin 60C Map, and an older Magellan 315.

 

The difference between the two is proportion to how fast I'm travelling. When I'm going 60-70 kph the difference is about 4 kp/h.

 

I had no clue my speedometer was off until I got my GPS.

[jonners]

Vehicle speedometers can only have limited accuracy because of the way the speed is determined.

 

As far as I'm aware (and this may now be out of date) the speedo is driven off a gearwheel on the final drive shaft of the gearbox. This is transmitted mechanically to a mini gearbox in the speedo which then shows the speed.

 

There are manufacturing tolerances and wear factors in each of these components that can give rise to variations in the speed displayed, and these variations will increase as components age. Furthermore the tyre/wheel size, tread depth and tyre pressure will all affect the rolling wheel diameter which can in turn give rise to indicated speed differences.

 

Hence the speedo reading is only ever going to be a manufacturer's best effort to get near the true reading (plus the 2-3mph that they build in for reasons already stated by others here). The only way to get an absolutely true reading is to compare time elapsed with the vehicle's actual position change on the surface on which it is travelling. And that is exactly what our GPSr does for us (give or take the inaccuracies caused by the curvature of the earth). So, at constant speed the GPSr (when getting strong signal reception) should be more accurate than a vehicle speedo.

 

HOWEVER before you rush out and start driving at those speed cameras using your GPSr speedo reading, don't forget that this only holds for LEVEL roads. If you are on an incline, your actual ground-covering speed will be proportionately higher than it appears from a satellite's perspective.

[Pacific NW]

This is a great thread... I've noticed this while driving, too. I'll have the GPS on and it typically reads less than the speedometer. I was wondering which was more accurate, now I know.

[+egami]

Just wondering what everyone's experience or belief is. I've got my GPSMap 60CSx mounted to the dasboard and can watch the speed indication on it as I drive to work. It typically shows 3 MPH slower then the spedometer in my truck does so this morning i timed myself over 4 measured miles and calculated what my speed was. The result was about right on with the GPS indication. This difference seems to hold constant from about 40 to 70 MPH. Not that I'm going to change how I drive based on this, more a point of curiosity.

 

Thanks

 

I have friend that works in law enforcement and we clocked my truck after he had his unit recently calibrated. My spedometer was dead on (tested at 60 MPH). My 60CSx is always right on with my spedometer. I was curious about this too because my motorcycle reads 5 MPH slower (at open road speeds) than my GPS. Then someone told me that manufacturers make motorcycle spedometers deliberately read slower (not sure if that's true, but I've met a number of different owners that know theirs is off by about that much).

Edited April 12, 2007 by egami

[+Y2KOTA]

Ture about the tires and changeing the size of them. If want to check the difference in size and accuracy vs. the stock ones, this site would help... DML Tire and Wheel Calculator

 

Side note, Its these great guys at the DML I blame for getting me hooked on Geocaching

[+EraSeek]

It seems to lag a bit with acceleration and deceleration.

The speed reading is only update once every second.

On my older GPS it had a speed filter menu. I believe this is what everyone sees when you come to a stop. It takes about 2-5seconds or so for the gps to come to 0. What it is is a filtering method to keep your gps from jumping around with every signal anomoly and to keep your track consistant when you pass through a tunnel and loose signal momentarily.

[xer0piggy]

It seems to lag a bit with acceleration and deceleration.

The speed reading is only update once every second.

On my older GPS it had a speed filter menu. I believe this is what everyone sees when you come to a stop. It takes about 2-5seconds or so for the gps to come to 0. What it is is a filtering method to keep your gps from jumping around with every signal anomoly and to keep your track consistant when you pass through a tunnel and loose signal momentarily.

 

I know on racing type forums, unless you can photo a GPS unit on your dash, you're considered FOS when bragging about how fast you have gone :-). Dash speedo's don't count past 60 mph.