Speedo/Odometer on the bike v GPS Speedo/Odometer

[Chip]

Mitch's Speedo/GPS thread prompted this one.

 

Just completed a SaddleSore 1000 ride from Valentine, NE to Houston, TX (hold your applause please...it took me ~22 hours

 

Interesting observations

The Bike odometer showed 1,081 miles

The Garmin Zumo/550 odometer showed 1,093 miles

The Bike speedometer consistantly indicated I was going a bit *faster* than the GPS speedometer indicated. In other words, across all speeds, the Garmin indicated I was going a bit *slower* than the bike's speedometer.

 

Example: Spose the speedometer on the bike indicated 72mph and the Garmin indicated 70mph. You would think that, after 1 hour of riding, the GPS odometer would read 70 miles. And it did. You would also think that the bike odometer would read 72 miles. But it didn't. It read 69 miles. What was going on?

 

At first, I couldn't figure it out. Then it hit me. The Bike Speedometer reads *analog* (i.e., you see the needle pointing to a number on the dial). The bike odometer is digital and, of course, the GPS speedo/odo are digital.

 

What I think this means is that, if the actual speedometer guage (behind the dial) could be rotated counter-clockwise (i.e., needle toward a lower speed), then the digital odometer and speedometer would be synched.

 

If the Garmin is the most accurate of the instruments, then my bike odometer is showing fewer miles than it actually has (a good thing).

 

Note: The Garmin was always on, I didn't park it under cover, and it never indicated it had trouble finding satellites.

 

Anyone else notice such behaviour among their Bike instruments and GPS?

 

 

 

[Albert]

If you approached the speed of light during this trip (or even a significant fraction of the speed of light), you probably experienced a doppler time shift. This will, of course, have a greater negative impact on your analog devices. Have you felt any younger since the ride?

[Danny caddyshack Noonan]

You're on the right track. The GPS is using digital data. To report velocity, it takes position data and integrates the differences between reported positions. It then takes those integrations and averages them over an unknown to me number of points. That is why the GPS takes a few seconds to "settle" in to a constant velocity even though the bike isn't changing its speed. Depending upon the number or frequency of position readings, it is possible that on a curvy road, the GPS does a central difference calculation for the distance thereby drawing straight lines between points. That would give a shorter distance.

[John Ferris]

You're on the right track. The GPS is using digital data. To report velocity, it takes position data and integrates the differences between reported positions. It then takes those integrations and averages them over an unknown to me number of points. That is why the GPS takes a few seconds to "settle" in to a constant velocity even though the bike isn't changing its speed. Depending upon the number or frequency of position readings, it is possible that on a curvy road, the GPS does a central difference calculation for the distance thereby drawing straight lines between points. That would give a shorter distance.

 

But his GPS gave a longer distance.

 

The Bike odometer showed 1,081 miles

The Garmin Zumo/550 odometer showed 1,093 miles

 

[Sliplake]

...Have you felt any younger since the ride?

 

Every instance I manage to squeeze in time to ride, I always feel younger. That's why I live in MI. The winter counteracts this effect. Otherwise, after a few years, I probably couldn't qualify for a license

[Gene#3]

a public "non-certified" speedo is apt to underestimate your speed just a bit... it saves you $$$ in the long run e.g. tickets!

[szurszewski]

I haven't taken apart a "modern" speedo, but on airheads the indicated speed and indicated mileage are sort of two separate systems; both are spun by the same cable, but one is mechanical gears going around (odo) and the other has some sort of funky magnet thing that people here probably know the name of. Basically what I'm saying is those two systems could fail independently or show completely disparate readings over a given distance.

[Starfighter]

On most Bikes the speedometer tends to read fast...

BMW does make an "authorities" speedometer that is +/- 1 mph.

So they can do it but ???????

Now for your case...............

Usually, the odometer reads fast also (farther than you actually went) but as in your case the odometer read less................

I have heard it said on other sites that the speedometer and odometer are not necessarily always in agreement..................

[GRB60]

I did a trip of similar length a couple months ago. The Garmin Zumo 550 said 1083 miles and the bike's trip odo said 1064. Only about a 2% difference and off the same way as yours.

[macx]

I noticed something a couple weeks ago -

 

I set my cruise on 75, then set the trip computer to mph

and reset it. It showed 71.2.

 

So even on the same bike, the trip computer reads differently

'than the speedometer.

 

I know on the 1150R's the speedo is fast, too. Have read

that is because of the gearing in the speedo head, and there

aren't any gears available that would make it correct.

 

Same thing probly applies here.

 

But, why? Certainly BMW could get the speedo to read correctly?!

 

Or maybe they're just trying to keep everybody out of trouble?

 

 

[moshe_levy]

MCN always reports "reported vs. actual" speed in its tests of motorcycles, and BMW's speedos are generally one of the most inaccurate. There is a certain amount of slop allowed by law - forgot what the figure is, actually - and they're usually right at the limit. The GPS is far more accurate.

 

-MKL

 

PS - From what I recall reading MCN over the years, Harley's speedos are usually the most accurate.

[Skywagon]

I have a Zumo 550 on my 2005 1150rt. The bike always shows faster by as much as 5mph in the upper zone and always shows further on the trip. When I fuel at 200 miles according to Zumo, the bike will say 210miles. I have tested the Zumo in the car and the car and the Zumo match perfectly.

[Chip]

I have a Zumo 550 on my 2005 1150rt. The bike always shows faster by as much as 5mph in the upper zone and always shows further on the trip. When I fuel at 200 miles according to Zumo, the bike will say 210miles. I have tested the Zumo in the car and the car and the Zumo match perfectly.

 

"Upper Zone." I like that phrase.

 

My '10 is the opposite. The bike odometer shows fewer miles than the Zumo...even though the Speedo shows the bike going faster than the Zumo.

[DiggerJim]

Anyone else notice such behaviour among their Bike instruments and GPS?

Happens in cars too. The reason is "liability". In our society getting a ticket for speeding when the speedo showed you going slower than "true" speed will get the mfg sued. Likewise getting stiffed on a warranty claim because the odo was showing more distance than actual would also cause lawsuits.

 

So, the intelligent mfg makes speedos show a bit faster than reality & odos that are either dead nuts on (with the mfg recommended tires) or slightly low in miles shown vs real miles.

 

Making either precisely measure speeds or distance only invites litigation as normal mfg errors or user alterations (like different tires) will impact the precision & increase the error from an acceptable level to a litigable (is that a word?) level.

 

[jaytee]

One of my personal pet peeves- of *course* BMW and the other bike manufacturers can make accurate speedos. After all, their odometers are nearly dead on with factory tires. They choose not to.

 

As to your GPS speed, likely far more accurate than the bikes speedo. You have many factors that make the speedo read incorrect, the mist significant being tires.

 

On my S, pilot roads read 8% high but BT-016s read 15% high!!!

 

JT

[BerndM]

MCN always reports "reported vs. actual" speed in its tests of motorcycles, and BMW's speedos are generally one of the most inaccurate. There is a certain amount of slop allowed by law - forgot what the figure is, actually - and they're usually right at the limit. The GPS is far more accurate.

 

-MKL

 

PS - From what I recall reading MCN over the years, Harley's speedos are usually the most accurate.

 

On my 05RT, the speedo is always withing +2 mph of the GPS, regardless of speed, up to 80 anyway. I guess I got a good one!

 

Also, with regards to the GPS's accuracy, while the speed might be very accurate, the odo function (GPS's) may NOT be. This depends on the refresh rate of your particular GPS. The older units weren't as fast and there was the possibility that some distance got lost because it measures each refresh from where it was at the last point to where it is now in a straight line. So it could easily cut corners off when measuring.

 

 

[bobbybob]

I think the GPS data update rate is once per second for Garmins and probably most others. So a curve would be divided into "1-second-long" straight lines. If traveling 60mph, each of those straight lines would be 88 ft. long. I doubt there could be much loss unless you traveled exclusively on a continuous series of back-to-back super-tight twisties or switchbacks. And even then...

[KDeline]

Two bikes, two GPS's, same trip, four different mileages, got to love technology.

[tallman]

I've been behind you two.

One of you, not mentioning any names but they ride a GSA, wanders around more than the other so your mileage will vary.

 

i just know it always takes me longer to get there when I look at my speed/odo meters and I'm never going as fast or as slow as I should be.