Odometer On My Car Or My Gps?

[+I.R. Geonut]

I think the car would be more accurate but it's a lease car and the thought of them ripping me off crosses my mind every time I compare the two odometers. My GPS is always less. I drove from Southfield, MI to Flint, MI and the GPS had 2 less miles on it. That's not a lot but it would add up after 3 years

 

What do you guys think?

[+GOT GPS?]

Magellan tends to under report the trip odometer for some reason on my Magellan units.

 

So I don't think there is much of any problem with the car odometers.

 

If only you had an Explorist, you could save the track to the memory card, then that way you could compare the tracklog length to the car's trip odometer, when you go turn in the car.

 

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Geoff

Edited February 10, 2006 by GOT GPS?

[ImpalaBob]

Car odometers are not accurate. Tire wear alone causes variations. Certified odometers .... such as in police vehicles ..... have a +/- variation ....certified to +/- 2 m.p.h. between 40 and 50, and that variation can change with speed. Bottom line is your GPS is WAY more accurate than your odometer. ImpalaBob

[+GOT GPS?]

I'm going to try an experiment with my SporTrak Color, and clear the Trip Odometer, and the Tracklog, then compare the two of them, when I get back from cashing a check here, and see how much shorter the Trip Odometer of the SporTrak is compared to it's own Tracklog.

 

-

Geoff

[+geognerd]

On my eXplorist 400, the tracklog interval is set for whatever the factory default is. When I view my track in Google Earth, I notice that the areas where I was traveling over about 45mph, the track wasn't as detailed. As a result, the track sometimes cut across curves instead of following the road. The track in such situations will be shorter than the distance you traveled on the road.

[+GOT GPS?]

I did a test, but forgot to select Auto-Detailed on the SporTrak Color.

 

It is not too bad, I had gotten pretty close values for Trip Odometer, and Tracklog length.

Trip Odometer=5.84mi

Track Length = 5.81mi

 

Track length would have been longer, but the SporTrak Color was set on AUTO, instead of AUTO Detailed.

 

GPS 60Cx on the return leg of the trip:

Trip Odometer=5.87mi

Tracklog length=5.89mi

 

- - - -

 

So what im saying is that you could try comparing the Tracklog Distance to that of the Odometer of the Car.

[Photobuff]

I don't know the answer, but here are some thoughts. If the route has any ups and downs, the car will travel further than the same route on the level. Think of a road as a undulating ribbon- if you pull it straight, it gets longer. In another thread, it was determined that the GPS only reports the distance on a level- it doesn't take elevation into account. Given the errors in elevation measurements, that's a good thing. Car odometers and speedometers can easily be off 5% or more, mostly because of tire diameter variations, but also just because of plain calibration. You might find a long straight flat section of road (Thruway) and pull the coordinates for a couple of exits using USAPhotomaps. I've found those numbers to be extremely accurate. Then use Geocalc to compute the distance. Do it in legs, if the road is made up of straight segments. Then go drive it. See if the car and GPSr agree with the aerial photo data. Or, use the mile markers on the road, as those are also pretty good.

[+Deneye]

I think the car would be more accurate but it's a lease car and the thought of them ripping me off crosses my mind every time I compare the two odometers. My GPS is always less. I drove from Southfield, MI to Flint, MI and the GPS had 2 less miles on it. That's not a lot but it would add up after 3 years

 

What do you guys think?

 

Put on larger tires. They won't turn as much as a smaller one which will keep your odometer reading lower.

 

(just remember to put the recommended size back on when appropriate )

Edited February 10, 2006 by Deneye

[+StarshipTrooper]

I think the car would be more accurate but it's a lease car and the thought of them ripping me off crosses my mind every time I compare the two odometers....

 

...What do you guys think?

 

I think you should look at your lease agreement. I'll bet it states in there somewhere that the mileage shown on the car's odometer will be assumed to be the correct mileage - or words to that effect.

[+roadrage64]

I'd have to think the GPS is more accurate, but there is no way that the car rental company is going to use anything except the odometer.

[+gpsblake]

The thing about using a GPS for a car odometer. The moment you use it for anything else like geocaching, walking etc, it will make the data invalid.

 

I do wish they would allow you to customize the odometer on GPS units. The reason is I would like to customize the odometer so it shows my total bike riding miles for the year. However like I said, because I use the GPS for more than just one activity, the total odometer for me is actually a trip odometer, which the unit already has.

[+Sputnik 57]

I read in the forums that the DOT (or someone) makes car manufacturers overstate the speed shown on the speedometer (and hence overstate the milage on the odometer) to take tire ware and inflation and other matters into account. Not sure if that is true or not, but it has the added benefit of making your warranty run out faster .

[+HaLiJuSaPa]

I would think that unless you have a higher-end GPS that almost never loses lock at in a short underpass or doesn't correct for that (like some car-oriented ones do) that the GPS odometer would always eventually read lower because of the brief loss of satellite lock under most underpasses (and maybe near tall buildings around cities too). I once tested on 10 miles of a stretch of highway that has "mile markers", the car odometer/trip meter indead read 10.0 miles but the GPS odometer only read 9.8.

[Sarjor]

I also had the same question as any vechicle that I was in showed a different speed than the GPSr. Somewhere in these forums there is a thread that discusses this and the point about the Car guys overstating the speed. Recently my Father inlaw bought a Meridian Color and it is exactly the same as mine dissagreeiong with the car, as is my brothers Garmin unit........HMMMMM wonder if I can get enough people to gang up with me on those Car Manufacturers?????????

 

I always did think the GPS receiver would be more accurate than the cars speedometer.

 

SARJOR

 

[Major Catastrophe]

GPS receivers only increment their internal odometer when the distance traveled reaches whatever increment it is using. If you are driving around curves, you will travel a slightly longer distance around the curve before the GPS increments; i.e., the GPS might update every tenth mile, but you may drive 0.105 miles around the curve. Those small differences should always add up to more on the car than the GPS. Thus, a car with an accurate odometer should be expected to always read slightly more (over time) than GPS. But the bottom line is, the car odometer is admissible in court......

Edited February 11, 2006 by Major Catastrophe

[+GOT GPS?]

Theres the other thing about not being able to hold the car perfectly straight, like wandering side to side in the lane, and changing lanes alot, and I don't think anybody can hold the wheel perfectly straight all the time.

 

-

Geoff

[+Jamie Z]

If the route has any ups and downs, the car will travel further than the same route on the level. Think of a road as a undulating ribbon- if you pull it straight, it gets longer. In another thread, it was determined that the GPS only reports the distance on a level- it doesn't take elevation into account.

If you read that whole thread (I know, it's long with lots of big words) you'll also note that it was pointed out that variations in elevation have a negligible effect on distance--well under 1%. The hills don't amount to beans.

 

Assuming correct size and pressure tires, I would say that the car odometer will be more correct. The odometer (unlike the speedometer) has a direct mechanical connection to the driveline of your car. That is, it measures the rotation of your tires and calculates the distance based on the number of turns your tires make.

 

The GPS on the other hand, measures distance as a series of straight lines, which is just a basic approximation of the true path. The GPS distance will always be less than the true distance. This effect is exacerbated by poor signal.

 

Jamie