Has anyone checked speedo accuracy?

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SouthBayDon

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I just recently purchased a 06 and am still getting familiar with it. One thing I've noticed is that the speedo seems to read fast. I live in SoCal and traffic is either stopped or 75 mph or so. It seems that when I'm rolling with traffic (not the stooped) my speedo is reading 80 or more. Just curious.

 
Many of us ride with a GPS. Ever consider getting one for the bike? They're pretty cheap these days, and will answer your question in a hurry.

Gary

 
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Mine was 7.7 % off (Typical of most Yami's) Mine is a 2010 (Gen II)

I installed a Healtech Speed - o - Healer, with good results.

I have been running them on all my bikes for years with zero issues

 
Speedometers almost always read optimistically. The amount they "over-read" is variable but usually in the 2 to 4% range. It is better that they are optimistic, as under-reading could well lead to more tickets in these zero-tolerance days.

Odometers are usually more accurate than that. In the last odo check, mine was 0.75% over. They can vary depending mainly on the tires fitted as the overall diameter is not identical on tire brands of the same size. Even over-inflation can have a small effect.

The GPS advice was good advice. They let you assess more accurately that you are running just over the posted limit, rather than just over plus 4%, which could be problematic.

 
Same error for my '09. 70 indicated shows as 65 on my NUvi 1490. I like the little cushion provided by the optimistic speedometer, my only wish would be that it was a constant 5 mph fast instead of varying with speed. For me everything else about the FJR is so good that my expectations are a little high for every detail..

 
For the sake of science I felt compelled to test the 14's today. 102 on the speedo registered as 99 on the gps.

 
Comparing speed on a GPS vs the speedo is reasonable as long as the GPS limitations are understood. A speedometer bases its displayed value on something related to wheel turns which has the ability for truly accurate measurement (assuming tires are not especially worn and are not spinning in a low-traction environment). Let's say it can yield excellent precision but accuracy is a function of (factory) calibration and actual tire diameter.

A GPS does not measure speed. It does a calculation of speed based upon two sets of XY coordinate positions and the elapsed time between the position measurements. The accuracy of the coordinates and the update frequency (and averaging algorithms) will dictate the accuracy of the speed display. The comparison is likely to be valid going in a straight line on level surface and at a rate of speed that is large in comparison to the update frequency. The GPS does point-to-point distance divided by time so in a curve, it will be clipping the circumference to a number of straight-line segments on the inside of the curve resulting in a measurement bias on the low side - the extent of the error will be a function of the radius of curvature and update frequency. (This is theoretical and I don't know if the update frequency is low enough to result in a measureable difference under normal conditions.)

If you are moving very slowly, the positional error of GPS measurement can come into play. For example, lets say that the absolute error in any direction is 2 meters. If you cover 10 meters between GPS measurement points, there is a potential error of +/- 2 meters divided by the time interval (20% in each direction; 40% total for a single measurement). If you cover 100 meters in the same time, the error is only 2%. this would result in a random error, not a bias as in the curvature situation. As I mentioned above, the extent of the error is dependent upon update frequency and how the GPS averages subsequent measurements.

Elevation changes can be an issue with GPS measurement of speed (or distance). The speedo is measuring the actual meters of pavement that passes under the wheel in a given time interval and the GPS measures the distance between points on an XY grid without (as far as I know) taking elevation changes into account. Lets consider an extreme case of a 45° slope which can be expressed as a right triangle. If the base of the triangle (and the elevation) are each one kilometer, then the hypotenuse (actual distance travelled) is 1.414 kilometers (Thanks, Pythagoras). The GPS measures the distance travelled as 1 kilometer- a whopping 41% error. As I said, the 45°slope is extreme but serves to illustrate the low bias that will result if you are riding in mountainous terrain.

Having said all of this, the GPS probably is more accurate than the speedo. The speedometer may have a built-in error which should be a constant percentage that can be corrected using a "fudge factor". Tire wear and factory calibration are the only factors. The GPS does not have a "built-in" bias but will exhibit a negative bias in curves and a negative bias with elevation changes (as discussed). There may also be a random error at low(er) speeds.

 
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We tested my speedo on a dead-straight, flat piece of 110KPH-posted highway, so I'm reasonably confident.

Having said that... my mate's BMW read slightly different speeds (we were connected by Sena SMH10s and had set our bikes on cruise control) at the same constant speeds side-by-side.

 
Odometers are fairly accurate, speedometers usually have a speed offset. Both use the same signal source for input, if the speedometer is off, it is intentional. In the FJR both values are digital calculations so there should be no errors in normal operation. I do find my FJR has a small odometer error, just like every vehicle I've owned. There are some aftermarket solutions that will let you tweak the speed readings.

 
I am going to put duct tape over my speedometer and GPS. I no longer trust either of them.
Isn't it horrible that the FJR road speed and travel distance indicators are crude automotive misinformation displays instead of precisely developed scientific lab grade instrumentation, accurate to at least 2 decimal points. It sure spoils the experience for me. Man, I'll never know my true speed and never know how far I've traveled. After all this time I've thought I was ripping it up driving 75 mph and now it may will turn out that I was only going 69 mph, whatta buzz kill. Fred W's 6,500 mile trip may turn out to only be 6,440 miles, what a downer.
wink.png


 
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Gen I & II are generally 5 mph off at 70 indicated....Gen III is about 2 mph off at the same indicated speed.
My Gen III is also off about 2 mph at indicated 70. I won't dispute the GPS description of RossKean above but I was always under the impression that a GPS is receiving multiple satellite signals at one time thereby giving you a very accurate reading of speed. I almost exclusively look at my GPS for speed instead of my speedo.

 
Gen I & II are generally 5 mph off at 70 indicated....Gen III is about 2 mph off at the same indicated speed.
My Gen III is also off about 2 mph at indicated 70. I won't dispute the GPS description of RossKean above but I was always under the impression that a GPS is receiving multiple satellite signals at one time thereby giving you a very accurate reading of speed. I almost exclusively look at my GPS for speed instead of my speedo.
I was just describing issues that CAN reduce the accuracy of a GPS measurement of speed or distance. Other than extreme elevation changes or travel at less than 10 mph, I doubt that any of the errors are particularly significant; especially compared to a speedometer that has been documented to be off by 5 to 10%. Personally, I tend to trust the GPS over the speedo; especially if the "real" vs "displayed" value has not been established for the particular speedo in question.

 
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