Speedometer is, in-fact, right on...

[LilBeaver]

While I realize there have been about a zillion and a half threads about people 'realizing' that their speedometer calibration is off, using a speed-o-healer, ignoring it all together, etc etc. I had an interesting realization last night while I was avoiding being pulled over puttering back across Vancouver.

 

Last Christmas I received a Ram-Mount for one of the GPSes I have. I have now used it twice (both times in Vancouver) Partially because I have it so why not, and partially to make sure that I know how fast I am going in the appropriate units.

 

As we all [most likely] know, the MPH side of the speedometer (on U.S. models at least) is off by 5-10% depending on who you ask, where you check it and most importantly whether you are using your turbo attachment or not.

 

All kidding aside, I have found now through a speed range of ~10 to ~60 mph (10 to 100 kph) checked at intervals that were distinctly marked (ie 20, 40, 60, 80, 100) I found that with the speed held reasonably constant the GPS read a speed that matched exactly (obviously within the error of the width of the needle).

 

Now this had me curious. Upon closer inspection I found (on my bike, which I presume is the same on others) that the MPH scale is not aligned properly with the KPH scale which contributes to the incorrect speed indication. Now before you say 'oh that guy is an idiot, of course it isn't perfectly aligned because the needle has to point towards the numbers and being that the radii of the curvatures of the speed range indicators are diffrent and the 'needle' points radially outward but is not a perfect , of course they wouldn't look visually aligned'. I solved this issue by looking at how fast my indicated speed was in KPH both on GPS and on the speedometer and verified that while the reading in KPH was correct, the needle was pointing to the wrong point on the MPH scale.

 

So, how is that for a fun fact?

 

By the way, I am in Canada where they use KPH -- which is why I have the GPS set to KPH and am paying attention to that.

 

Anyhoot, just thought I would share my findings while I try to wake up enough to get some work done.

[Formerfuzz]

Interesting observation. I may check mine the next time I am use the GPS.

[eagleeye]

LilBeaver,

 

That is a very fun fact. Can't wait to get on the bike and check that out.

 

Steve

[Guest tx2sturgis]

Its an optical illusion. Once you get north of the 49th parallel, calibrations change. Money looks different, folks talk different, and signs might be in English, might not. And late night consumption of dairy treats has been known to cause brain-freeze, which can mimic speed-o-healer correction.

 

Once you get back to Texas, it will be all katty-wompus like normal.

 

 

 

[V7Goose]

Well sir, I find your observations quite interesting, and I do not doubt that you saw what you saw, but it does not tell the whole story.

 

While I have never compared my GPS with the speedo in KMH, I do constantly compare it in MPH (I keep the speed displayed on the main screen), and I know that my 05 has a linear difference in displayed speed to actual GPS reported speed of around 9% (meaning the % off is the same over the full range).

 

I just went out and looked at the speedo display to try and see if the KPH and MPH scales were properly matched. They seem to be very very close, with a slight mismatch at the low end only. Of course it is a bit difficult to see exactly where the marks of one scale match to the other, but I think you can be reasonably accurate if you look closely. Below are the speeds in KPH, where I think they match to the numbers in MPH, and then the actual calculated MPH:

 

40 KPH = display 27 MPH = actual 24.85 MPH (displays 3.15 MPH high)

60 KPH = display 40 MPH = actual 37.28 MPH (displays 2.72 MPH high)

80 KPH = display 52 MPH = actual 49.71 MPH (displays 2.29 MPH high)

100 KPH = display 62 MPH = actual 62.14 MPH (virtually dead on)

120 KPH = display 75 MPH = actual 74.56 MPH (virtually dead on)

160 KPH = display 100 MPH = actual 99.42 MPH (virtually dead on)

180 KPH = display 112 MPH = actual 111.85 MPH (virtually dead on)

 

So based on those calculations, it seems to me that your observations are correct for speeds below 40 MPH (on the low end of the scale, the KPH numbers do look to be accurate), but at any speed above 60 MPH, the KPH and MPH numbers are both off by the same amount. Note that when I use the word "actual" in the chart above, I mean the actual calculation to convert kilometers to miles, NOT the actual speed you will be going when the speedo points to that number! When my RSV shows that I am traveling at 75 MPH, the GPS shows that I am really traveling about 68 MPH.

 

Goose

Edited July 21, 2010 by V7Goose

[LilBeaver]

Its an optical illusion. Once you get north of the 49th parallel, calibrations change. Money looks different, folks talk different, and signs might be in English, might not. And late night consumption of dairy treats has been known to cause brain-freeze, which can mimic speed-o-healer correction.

 

Once you get back to Texas, it will be all katty-wompus like normal.

 

 

 

 

Ah, well thanks for clearing that up for me!

 

I'll bet next you'll explain that anytime I drive north my fuel economy suffers because I'm driving 'up' the globe?

[SilvrT]

So, what's really being said here is .... I'd better SLOW DOWN when riding 100 km/h or over!!!

[Mel]

To compare the accuracy of mph to kph on our speedometers we can simply pick a speed in either category, do the math and then closely look at the speedometer and check the accuracy. Maybe they didn't print the scales accurately??? Here are the formulas:

 

 

Step 1

Multiply the speed in mph by 1.609344 km per mile. For example, 55 mph is 88.51392 kph. For most practical intents, multiplying by 1.6 is accurate enough.

 

Step 2

Convert from kph to mph by dividing the speed in kph by 1.609344. For example, in many countries the highway speed limit is 90 kph. This equals 55.92 mph. In most cases, dividing by 1.6 is accurate enough

 

 

Read more: How to Convert MPH to KPH | eHow.com http://www.ehow.com/how_2306811_convert-mph-kph.html#ixzz0uKOxMTuQ

[elmicko]

When my RSV shows that I am traveling at 75 MPH, the GPS shows that I am really traveling about 68 MPH.

 

Goose

 

That is actually part of the engineering design to make a 2nd Gen seem faster.:whistling:What??? You knew somebody was going say it.

[SilvrT]

I always use the rule of " 5/8 " .... 50 mph = 80 kmh

[LilBeaver]

So, what's really being said here is .... I'd better SLOW DOWN when riding 100 km/h or over!!!

 

Actually Rick - I have no idea, since ya'll speedometers are weird and only give KPH. I do not recall reading what the metric folks have had to say about their speedo's accuracy.

 

To compare the accuracy of mph to kph on our speedometers we can simply pick a speed in either category, do the math and then closely look at the speedometer and check the accuracy. Maybe they didn't print the scales accurately??? ....

 

This was my initial thought last night. Now, I have not done extensive testing, by any means, other than entertaining myself on my ride back to where I am staying and holding as steady as I could and reading the GPS as well as my speedometer (at the marks that I knew what they were) while driving and trying to stay awake.

 

Let us not forget that the resolution limitation of these speedometers are on the order of 2-3 mph.

[massey130]

To compare the accuracy of mph to kph on our speedometers we can simply pick a speed in either category, do the math and then closely look at the speedometer and check the accuracy. Maybe they didn't print the scales accurately??? Here are the formulas:

 

 

Step 1

Multiply the speed in mph by 1.609344 km per mile. For example, 55 mph is 88.51392 kph. For most practical intents, multiplying by 1.6 is accurate enough.

 

Step 2

Convert from kph to mph by dividing the speed in kph by 1.609344. For example, in many countries the highway speed limit is 90 kph. This equals 55.92 mph. In most cases, dividing by 1.6 is accurate enough

 

 

Read more: How to Convert MPH to KPH | eHow.com http://www.ehow.com/how_2306811_convert-mph-kph.html#ixzz0uKOxMTuQ

~~~~~~~~~~~~~~~~~~~

do you want this done while driving the motorcycle, or just pick a speed while sitting at the computer with pen and paper at hand?

[Mel]

Quote:

do you want this done while driving the motorcycle, or just pick a speed while sitting at the computer with pen and paper at hand?

 

 

I guess you could just turn on the speedmeter display and pick a speed to calculate or you could pick your nose or maybe go to the movies and pick your seat. Whatever floats your boat.

[Guest tx2sturgis]

.... Here are the formulas:

 

 

Step 1

Multiply the speed in mph by 1.609344 km per mile. For example, 55 mph is 88.51392 kph. For most practical intents, multiplying by 1.6 is accurate enough.

 

Step 2

Convert from kph to mph by dividing the speed in kph by 1.609344. For example, in many countries the highway speed limit is 90 kph. This equals 55.92 mph. In most cases, dividing by 1.6 is accurate enough

 

 

 

OK...nobody warned me there was gonna be a math assignment on the internet today!

 

[LilBeaver]

~~~~~~~~~~~~~~~~~~~

do you want this done while driving the motorcycle...

 

 

I don't see the problem with this?

 

Well sir, I find your observations quite interesting, and I do not doubt that you saw what you saw, but it does not tell the whole story.

 

While I have never compared my GPS with the speedo in KMH, I do constantly compare it in MPH (I keep the speed displayed on the main screen), and I know that my 05 has a linear difference in displayed speed to actual GPS reported speed of around 9% (meaning the % off is the same over the full range).

 

I just went out and looked at the speedo display to try and see if the KPH and MPH scales were properly matched. They seem to be very very close, with a slight mismatch at the low end only. Of course it is a bit difficult to see exactly where the marks of one scale match to the other, but I think you can be reasonably accurate if you look closely. Below are the speeds in KPH, where I think they match to the numbers in MPH, and then the actual calculated MPH:

 

40 KPH = display 27 MPH = actual 24.85 MPH (displays 3.15 MPH high)

60 KPH = display 40 MPH = actual 37.28 MPH (displays 2.72 MPH high)

80 KPH = display 52 MPH = actual 49.71 MPH (displays 2.29 MPH high)

100 KPH = display 62 MPH = actual 62.14 MPH (virtually dead on)

120 KPH = display 75 MPH = actual 74.56 MPH (virtually dead on)

160 KPH = display 100 MPH = actual 99.42 MPH (virtually dead on)

180 KPH = display 112 MPH = actual 111.85 MPH (virtually dead on)

 

So based on those calculations, it seems to me that your observations are correct for speeds below 40 MPH (on the low end of the scale, the KPH numbers do look to be accurate), but at any speed above 60 MPH, the KPH and MPH numbers are both off by the same amount. Note that when I use the word "actual" in the chart above, I mean the actual calculation to convert kilometers to miles, NOT the actual speed you will be going when the speedo points to that number! When my RSV shows that I am traveling at 75 MPH, the GPS shows that I am really traveling about 68 MPH.

 

Goose

 

First of all, I did not mean to imply that since my readings for 20-100 KPH were correct that I had assumed that the rest of the speedometer was going to be correct as well - this is why I specifically stated the speeds of which I checked. So, my apologies if I misled anyone.

 

I guess I could have been a little more thorough in my explanation of what I did when I had the 'hey those two speeds match' moment that prompted a slightly more systematic check to satisfy my curiosity. I hardly call it anything other than a quick 'eyeball' type measurement but I'll elaborate slightly now.

 

I only 'checked' the respective speeds at 100 kph ONE time during my ride yesterday, which is hardly enough to claim verification of anything other than the bike can go approximately 100 kph, which we already know. I only did this at 100KPH once mainly because of the speed limits on the roads that I was on were never at or above 100 kph except for one stretch of road that particular limit lasted for, at best, 2 miles. That particular stretch also happens to be quite scenic so I was rather distracted by the large bright white moon over the ocean that peeked through the trees on occasion, then remembered to check the speeds. So, I suppose I really should not have included the 100 kph in my original statement, although, I thought I was quite clear that I was just stating my observations were simply that - basic observations.

Also in the original post I did indicate the range of which I observed the particular speeds (20, 40, 60, 80 and 100 kph). I suppose I could have been a little more clear about how many times I sampled each speed which I will go ahead and comment on here since there is some interest. (I sort of assumed that there wouldn't be a whole lot of interest in this, which is why I labeled it as a 'fun fact' more than anything and decided to skimp out on some of the writing... That and when I made the post, I had a few minutes of downtime on my real experiment).

 

I verified 40 and 60 several times (probably somewhere in the neighborhood of at least 8 or 9 times - once I held it for approximately 1/2 mile stretch) throughout the 15-20 mile trip (every time I accelerated through these speeds I paused and held the speed for 5-10 seconds, long enough to get the gps speed to stabilize +/- 1 KPH. In order to try to be reasonably consistent, I stayed focused on my bike's speedometer until the needle covered the tick mark on the KPH indicator -- a few times I went a little over then back under (ie oscillated about the speed) then settled back down to it, to make sure I was close at least - THEN, checked the GPS and went back and forth from the GPS to the speedometer and saw when both were more or less steady, I made my 'mental note' of how close they were - again, I was finding +/- 1 KPH. Note that the speed readout I was going on read in WHOLE number increments and the resolution on the speedometer is actually a bit worse off than that.

 

 

As I was just reporting my observations and did no further analysis (other than the error analysis I did while I was riding -- yeah, I'm a nerd), I made no attempt to even hint at extrapolation beyond my observations.

 

With that said, the additional information that you [Goose] included about how well the tic marks appear to line up is quite good to know. That is something that I had not yet done.

Edited July 21, 2010 by LilBeaver

[Bubber]

Sounds to me that the artwork on the speedo is wrong.

I told a cop in Wisconsin my speedo was off by 10% and he informed me that by law it HAS to be within 6%.

Call YAMMA-MOMMA and tell her tht the speedo if off and oh ya the cassette needs a tune-up too.

Don't hold your breath! :crying:

[1 Canuck]

When my RSV shows that I am traveling at 75 MPH, the GPS shows that I am really traveling about 68 MPH.

 

Goose

 

Sweet! Due to my speedo error; if my speedo odometer shows 7,500 miles, I actually have less than that amount mileage accumulated.

This would also be relative to fuel economy; actual lower mileage per gal due to odometer error? Not sweet.

[Eugene]

Any of you guys have time to actually just ride????

:stirthepot:

[SilvrT]

Any of you guys have time to actually just ride????

 

:stirthepot:

 

we're too wrapped up in details... speedo error, fuel mileage, clutch basket whine, shock air pressure, tire air pressure, tire type/brand/mgf date, engine temp, brake squeal, rear drive lube, oil brand/weight, etc, etc.... how could we possibly have time to ride? It's no wonder there's so many low mileage bikes ...

 

[LilBeaver]

Any of you guys have time to actually just ride????

 

:stirthepot:

 

 

Nope... That's why I'm in Vancouver, BC with my bike and the little deal in the corner of all my posts has a Texas flag.... :rasberry:

 

 

:Venture: