Odometer vs Hand Held GPS

The gps unit will be the most accurate IMO Factory speedos are only required to be within 10 per cent, and that assumes the tyres are the right ones and correctly inflated!

So a 1 mile difference (2% if my alcohol befuddled brain is working) over 50 miles is fine.

mrcheerful

Most likely, both the odometer and GPS have slight errors although I'd bet the GPS being more accurate.

Is the GPS WAAS-enabled? Is it a newer unit without the built-in margin or error? My Garmin hand-held GPS displays its accuracy, i.e. 10 meters, 20 meters, etc.

The odometer does not actually measure distance. It is measuring transmission output shaft revolutions and converting the revolutions to distance, assuming a given tire circumference. The circumference can vary due to difference in tire suppliers, tire wear, and tire inflation.

A 1.23 mile discrepancy over 50 miles is a 2.46% difference, well within the margin of error for the odometer.

Yeah, get a better GPS! ;D

Actually, its within the allowed percentage.

Agreed. My experience is that most speedometers, coming out of the factory, with factory wheel, tires etc. read about 5 mph too HIGH at 60 mph (8 kph too high at 100 kph). Of course that is a very unscientific observation. For all I know my Toyota speedometer is far more accurate.

Yes it is WAAS enabled and it was indicating a 23 foot margin of error.

I guess I'm just surprised that the error is in my favor.

Your odometer is within 3% of the correct value. You are doing OK. I'd believe the GPS over the odometer. Neither is going to be perfect, but of the two, the GPS is the more accurate. For all of the cars I have owned over the last 6 years I have compared my odometer to a GPS (Garmin Street Pilot) and to the Interstate mile markers (reasonably accurate over long distances). The GPS and mile markers have generally been in close agreement, while all of my recent vehicles have indicated a slightly shorter distance (usually around 3% less that the GPS or mile markers). At the same time, all of the speedometers seem to indicate a slightly higher speed than the GPS (1 or 2 mph at 65). I have decided that this is deliberate. The odometers are purposely "slow" to forestall arguments that the manufacturer's design them to run "fast" to decrease the warranty liability. The speedometer purposely indicate slightly "fast," to forestall complaints that people were caught speeding because their speedometers were "slow" or that they had an accident because they were going faster than the speedometer claimed.

Ed

RG wrote:

As a Police RADAR instructor I will teach how to check the radar unit using tuning forks, certified speedometers etc. I will also use a GPS unit to check the accuracy of the radar and speedometer, just as a backup. I have not found any variance in the radar speed indicated and GPS speed indicated.

My money will go with the GPS on level ground from point A to point B. If measuring over hills and valleys the speedometer will measure the distance along ground level, but the GPS will be less inclined to take into account rise and fall of the roadway into total miles.

Like a surveyor measuring a property line using a laser or a ground rule. Which is more accurate? Depends what you are trying to determine.

C.

A 23 foot margin of error is pretty good - I'd definitely trust the GPS over the odometer.

It seems like errors are rarely in our favor!

A GPS, around here, may indicate a distance of ten miles but the odometer will indicate the actual fifteen miles it took you to go up and over and around the many mountains we have to transverce to get from one point to another. They work great to tell you were you are in the woods hunting, in relationship to where you parked your truck LOL

mike hunt

"C." wrote:

Do you know this from experience, or are you guessing? I've never had the GPS indicate fewer miles than the odometer. In fact, the relationships between by odometers and my GPS has been remarkably constant, no matter where I drive. Admittedly, I have not driven across any real mountains. I guess in the last 10 years, I haven't driven more than a thousand feet above sea level. My GPS does know the altitude (not with great accuracy), so in theory (and maybe in practice) it can calculate the actual track length and not just the projected path length.

Ed

Thing about it. The GPS measures the difference between two points on the earth. If you went around in circles and stopped at a point a mile away from were you started, what do you think it will read? ;)

mike hunt

"C. E. White" wrote:

most hand-held GPS will measure distance between points in actual travelled distance and not as the crow flies if it is left on in a tracking mode so it will function like an odometer. If it can track 3 or 4 satellites at one time, it will measure vertical movement as well as horizontal movement.

If the GPS is turned on at one point, allowed to get a position fix, turned off, and then turned on at the destination, it will measure the distance between the two points as the crow flies, as Mike Hunt suggests because while it was off, it had no way of knowing where it travelled.

It would show a track distance roughly equal to the distance you traveled (assuming it had enough memory to accumulate the entire distance). Now if you wanted to know how far it was to your starting point, it would give you the short distance. I assume you were asking if it would know the track distance...right? I drive round trips all the time. My GPS accumulates the entire distance and gives me average speeds (including or excluding stops).

A GPS measures your position relative to a perfect sphere with a theoretical radius equal to mean sea level. The old simple ones didn't do three dimensions (where the third dimension is you altitude above the sphere) but all the current ones I've seen operate in 3D as long as you can receive an adequate number of satellites. You are thinking like someone looking at a flat map. A GPS knows the world is round (or at least the programers do). There are plenty of good explanations of how they work on the web. Go educate yourself.

Ed

a good example of this is a few years ago I would mow teh local soccar fields. The field was a total of about 5 acres of ground. I found using my GPS that mowing a swath 10' wide I would cover 17 miles with the tractor.

C.

Your instinct is correct, the odometer is wrong the GPS is right. For all practical purposes, the error is meaningless.

Agreed. Even the wrong tire pressure can effect the odo reading.

What instinct?

Mike, in PA, its "mole hills" ;D

Ah yes but so are the mountains where you are, compared to the Alps. My one son said when he was skiing in Switzerland it was like jumping off a cliff compared to the Big Sur or the Pocono mountains. ;)

mike hunt

MDT Tech wrote: