Odometer vs GPS

[IntruderCruiser]

today was the first chance I had to check this out. Filled up with gas cleared the odometer and the gps. In the first 7 miles, the GPS showed I had gone 1 tenth further than the odometer. In about 15 miles, it showed almost 2 tenths difference. at the end of our short ride, the gps read 62.6 and the odometer read 62.3. It seemed strange to me that the initial 1 tenth in 7 miles didn't keep progressing that way.

[CrashTestDanny]

today was the first chance I had to check this out. Filled up with gas cleared the odometer and the gps. In the first 7 miles, the GPS showed I had gone 1 tenth further than the odometer. In about 15 miles, it showed almost 2 tenths difference. at the end of our short ride, the gps read 62.6 and the odometer read 62.3. It seemed strange to me that the initial 1 tenth in 7 miles didn't keep progressing that way.

Nice to see that the odometer isn't as far off as the speedometer. :D

[NormanPCN]

Seems strange only because you assume GPS is consistent, which it is not. Entirely dependent on satelites in view and their geometry at a given time. Heck I have been on PCH away from urban areas and the coastal mountains caused me to lose reception as not enough were overhead or towards the ocean. Even when reception is not lost in that situation where you only have one half view due to mountains the geometry will be bad. Dilution of precision, DOP, is the more technical term.

[IntruderCruiser]

guess I will have to check it on a valley slab to see how it compares

[nurseBob]

guess I will have to check it on a valley slab to see how it compares
I ran a check last summer against the GPS where I had a couple hundred miles of good reception. My recollection is that the odometer and the GPS only varied by about 1.2% (odo said a couple more miles). Of course, the speedo is off by about 9% (reads fast).

Given NormanPCN's comments about the GPS, it's hard to know if either the odometer or the GPS is more accurate than the other. My recollection on the DOP subject is that if the GPS loses its signal for awhile, when it reacquires a signal it will calculate straight-line distances between the last-known and new position, introducing error... There are other errors introduced if you're hiking and not moving quickly enough, but I don't hike while riding.

[dond]

Doesn't GPS measure diSTance "as the crow flies"?

[nurseBob]

Doesn't GPS measure diSTance "as the crow flies"?
Yes. However, the sampling rate plays a big role. If it's sampling every few seconds it builds an internal map of each segment. However, if it's been off or out of contact with the satellites, it'll go in a straight line from the last known point to its current position.

[STArnie13]

I have found my odometer to be off by right at 1% without a GPS. All you have to do is check your miles againt the mile markers on at least some interstates if not all these days. These days many are marked every 2 tenths of a mile. All you have to do is check the difference for ten miles and the difference will be the percentage.:03biker:

[Rob Hephner]

This is the exact reason that I did not put a speedo healer on my bike. I always thought that the speedo and the odo would be off by the same amount. Turns out that is not true on the ST, in fact you can either have the odo accurate or the speedo accurate, not both. Since the odo was not off by an amout that would show more than true milage I decided not to worry about the speedo (that and 150 looks cool even though you are not going that fast.)

Turns out that the FJR has zero difference in accuracy between the odo and speedo and you can have both accurate. I say this so that it is understood that it is a bike to bike thing, not a universal statement about motorcycles.

[NormanPCN]

This is the exact reason that I did not put a speedo healer on my bike. I always thought that the speedo and the odo would be off by the same amount. Turns out that is not true on the ST, in fact you can either have the odo accurate or the speedo accurate, not both. Since the odo was not off by an amout that would show more than true milage I decided not to worry about the speedo (that and 150 looks cool even though you are not going that fast.)

Turns out that the FJR has zero difference in accuracy between the odo and speedo and you can have both accurate. I say this so that it is understood that it is a bike to bike thing, not a universal statement about motorcycles.

Very variable. On the ST the speedo and odo a driven on a speed sensor which is on the output shaft of the transmission. It is a simple pulse generator. Given a known final drive ratio and tire size you know the distance travelled for a pulse from the speed sensor. The odo simply counts total pulses. The speedo counts pulses in a given time period. The odo and speedo will read different with a new verses completly worn tire. Some models of tire can be slightly different is size. When someone measures distance over a long ride this adds up.

The speedo on the ST is accurate. Meaning it reads exactly what Honda wants it to read. Which is to say... Faster than true. The odo on the ST is accurate. Which is to say... close to actual given variations in tire size due to wear, model variations and air pressure and weight loading on the rear tire.

[Rob Hephner]

Thanks for re-stating what I said, in a different way.

I still have the same opinion, no speedo healer for me. ;)

[NormanPCN]

I still have the same opinion, no speedo healer for me. ;)

Same here.

[IntruderCruiser]

in my ride this morning all on flat land, straight ride and no obstacles to block the satellites,
In 25 miles there was a difference of 3 tenths. GPS said I went 3 tenths further
as for as the speedo, at 75 on the speedometer -- GPS shows 71.
the error appears to be very close to 6 percent at any indicated speed on the speedo as compared to the GPS

[NormanPCN]

The most common number I have seen reported on the net is 7%. That is about what my estimate came to. One thing to remember is that most GPS models will smooth speed readings so it lags behind in acceleration. With the power to weight ration of a bike it is pretty tough to get a constant speed. For me at least.

[IntruderCruiser]

I agree there is a lag in the GPS on acceleration and I think part of that is the sampling time allowrance since it is not connected directly to the drive on the bike -- I do have pretty good feel for keeping a constant speed - I will keep researching this and check the other speeds vrs gps more closely -- hey -- it gives me something to do on that boring slab in the morning

[STnAV8R]

Yes. However, the sampling rate plays a big role. If it's sampling every few seconds it builds an internal map of each segment. However, if it's been off or out of contact with the satellites, it'll go in a straight line from the last known point to its current position.

I have no experience with street use GPS units, but on my aircraft GPS "distance" is straight line distance between the aircraft and a "from" and/or "to" point, i.e. your departure point and destination. If you make constant S turns on your way from A to B that does not increase your distance traveled as far as the GPS is concerned.

It does do a position update once per second but that's not relevant.

Could be street units sum a large number of periodic distance measurements, in recognition of the fact that cars don't travel in straight lines like airplanes usually do.

[NormanPCN]

All GPS units that have "odometer" readouts use a sum of the fix to fix distances and not where you started to where you end. After all an odometer knows nothing about start or end points. The GPS fix update interval may not be known. Garmin typically says, "1/second, continuous".

When you ask how far is point B from here, then you get a direct bearing distance.

[STnAV8R]

That odometer feature is not on my aviation unit (Honeywell Bendix King) but sounds very useful on street units.