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Discussion Starter #1
For those of you using Gearing Commander, keep in mind that it is reading wrong for our CBR250R right now. I have to change the tire circumference to 74 inches to get the same rpm vs mph that I have verified on my bike. On my bike 7000 rpm is 66 mph indicated, 67 actual. Dead on. Gearing Commander is a great tool but the stated circumference, or the stated gearing, one or the other, isn't matching up to the real world performance of our bikes.
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Gearing Commander: Motorcycle Speed, RPM, Chain & Sprockets Calculator
 

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Sorry if this is stating the obvious, but please make sure to contact them (if you haven't already) about this so they can correct it. There's a drop-down under contact for submitting an error report.
 

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Discussion Starter #3
Contact

Sorry if this is stating the obvious, but please make sure to contact them (if you haven't already) about this so they can correct it. There's a drop-down under contact for submitting an error report.
I talked to him about it already and he blamed it on my tach and was quite defensive. It's not a big deal. This is a great tool (he doesn't like it if you call it a spread sheet). It works great and some variation between the predicted and the actual should be expected. I don't think there is any error in my tach. I think it is just a simple matter of the computed tire circumference based on tire size, not adding up to what the tire is actually doing in real life. Different brands of tires may run big or small depending on what wheel they are mounted on. I will do a 10 revolution tire circumference measurement one of these days but it is not high on my priority list since I know what the real rpm vs speed of my bike is and how to correct it in Gearing Commander by setting the circumference manually to 74 inches. I offer this just as a word of caution to others, before you put blind faith in Gearing Commander and think that you want to gear 5% lower, because it is already off by that much based on my findings. Maybe other CBR250R owners can verify my rpm vs actual (not indicated) speed at 7000 rpm/ 67 mph.
 

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Gearing Commander not wrong

For those of you using Gearing Commander, keep in mind that it is reading wrong for our CBR250R right now. I have to change the tire circumference to 74 inches to get the same rpm vs mph that I have verified on my bike. On my bike 7000 rpm is 66 mph indicated, 67 actual. Dead on. Gearing Commander is a great tool but the stated circumference, or the stated gearing, one or the other, isn't matching up to the real world performance of our bikes.
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Gearing Commander: Motorcycle Speed, RPM, Chain & Sprockets Calculator
Stating this assumes your reading is correct which will not be the case as both the speed and RPM indicated will be off.
It just uses a calculated circumference based on the tire sizes which you yourself can adapt to the actual one (as that will be slightly off 2).
See GC: Tire Size and Circumference

Regards, ItalFreak
 

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Discussion Starter #5
Exactly

Stating this assumes your reading is correct which will not be the case as both the speed and RPM indicated will be off.
It just uses a calculated circumference based on the tire sizes which you yourself can adapt to the actual one (as that will be slightly off 2).
See GC: Tire Size and Circumference

Regards, ItalFreak
Exactly. The calculated tire circumference in Gearing Commander based on tire size is wrong for our CBR250R. We must adapt the value to 74 inches to get good agreement with my verified speed (indicated plus 1.4%) and soon to be verified RPM (as simple as a trip to Honda to plug in the scan tool).
 

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Exactly. The calculated tire circumference in Gearing Commander based on tire size is wrong for our CBR250R. We must adapt the value to 74 inches to get good agreement with my verified speed (indicated plus 1.4%) and soon to be verified RPM (as simple as a trip to Honda to plug in the scan tool).
How can a calculated circumference be wrong when using the actual indicated tire sizes ? I have checked several sites an the calculated circumference of 77.65" for a default CBR250R 140-70-17 tire is correct.

Maybe the actual circumference of your tire is different then it should be theoretically (calculated) but that is why you can change it manually. Also you should still doubt your speed and RPM readings as these are always off by default.
So it will be a combination of things that gives you different results.

Regards, ItalFreak
 

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Discussion Starter #7 (Edited)
Typo

There is an important typo in the link.

  • Mark the rear tire exactly in the middle of the tire with a tiny drop of paint.
  • .
  • (or just use the valve stem as the reference point and mark with chalk)
  • .

  • Now put the bike on its wheels and move it forward IN A STRAIGHT LINE at least 3 tire rotations
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  • (while sitting on the bike to place a normal load on it)
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  • Put the bike on the jiffy and measure the exact distance between the first and the 3rd paint dots on your floor
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  • (what's a jiffy? The stand?)
  • .
  • Divide this by 2 and you have your actual tire circumference !
  • .
  • (divide by the number of tire rotations that you used which in this case would be 3, not 2. The more, the better out to the longest measuring tape that you have, to minimize error)
 

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Tachs on bikes are typically pretty inaccurate. See the '17,500 RPM Redlining R6' fiasco for a BIG example of this. Note, a scan tool won't give an accurate RPM either as it's typically just reading the same value fed to the tach in the first place. Toss the bike on the dyno and have it read the spark pulses directly to get a more realistic read.
 

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Discussion Starter #10
Scan tool

Tachs on bikes are typically pretty inaccurate. See the '17,500 RPM Redlining R6' fiasco for a BIG example of this. Note, a scan tool won't give an accurate RPM either as it's typically just reading the same value fed to the tach in the first place. Toss the bike on the dyno and have it read the spark pulses directly to get a more realistic read.
A factory scan tool reads the crank sensor of a fuel injected engine down to several pulses per per revolution in order to compute injection duration. Very accurate down to single digits. It is possible that the gauge needle doesn't sweep exactly the same value though so I will have to verify that.
 

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A factory scan tool reads the crank sensor of a fuel injected engine down to several pulses per per revolution in order to compute injection duration. Very accurate down to single digits. It is possible that the gauge needle doesn't sweep exactly the same value though so I will have to verify that.
Right, but it's (the ECU) idea of a second doesn't have to be spot on accurate as no one will be setting their clock by it, it just has to be consistent. That's where the error comes from. The tach is servo operated, you tell it to read 9523 RPM it will, spot on.
 

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Discussion Starter #12
Clock

Right, but it's (the ECU) idea of a second doesn't have to be spot on accurate as no one will be setting their clock by it, it just has to be consistent. That's where the error comes from. The tach is servo operated, you tell it to read 9523 RPM it will, spot on.
It had never crossed my mind that the clock in the ecu could cause a tach inaccuracy. Crystal wrist watches are accurate to a few seconds per year. Why wouldn't the clock in the ecu be as good as a $10 watch. I could see the gauge being out of calibration and that will be easy to check with the scan tool. Some inductive timing lights have a tach read out so I could scrounge up one of those but why should we believe that, or the tach on a dyno?
 

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As a disclaimer, I should note that part of my job is maintaining time across a state wide network, and I'm a raging nerd so any excuse to play with NTP more is all I need to dig in further...

The reason I trust the tach on the dyno is IF the machine has public internet access and running Windows, it'll be running NTP and as a result it's clock will be disciplined. It won't be accurate enough to meet the standards I have to maintain, but it'll be reined in much better than a loose oscillator stuck inside an ECU or Tach. Based on that trust I've seen lots of bikes with both traditional 'add on' inductive, add on servo operated and ecu driven servo/digital readout tachs on bikes show up to 10% errors with what the dyno is seeing from it's inductive pickup. I've not had the opportunity to compare against a scan tool while on the dyno though so I don't know where the error is generated, but in at least one case (Yamaha R6) it came out that the MFG purposely set the system up to read high.
 

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Discussion Starter #14
Gearing Commander right - Tach wrong

It's my tach. 4.3% high. I scanned my fuel computer at Honda today and my tach needle does indeed read high compared to the actual values screen. 7000 rpm on the tach needle corresponds to 6700 on the scan tool. With the corrected value for the tach, my data from the road shows 66.9 mph at 6700 rpm. This is in agreement with Gearing Comander for the CBR250R with the tire circumference set to 77.25 inches which is very close to the calculated 77.65.
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I also did a rolling measurement of my rear tire circumference. 10 rotations measured 770.5 inches which makes my tire 77.05 while rolling at duck walk speed. Again, very close to the observed 77.25 at highway speed.
Excellent work by the designer of Gearing Commander to get data for so many bikes entered and ready for use.
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I am still anxiously waiting for my 16T front sprocket to be shipped so I can drop my 68 mph cruise rpm from 6800 to 6000 to improve the fuel economy of my commute further. With the sprocket and a loud pipe I should break 90 mpg US.
 

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For those of you using Gearing Commander, keep in mind that it is reading wrong for our CBR250R right now. I have to change the tire circumference to 74 inches to get the same rpm vs mph that I have verified on my bike. On my bike 7000 rpm is 66 mph indicated, 67 actual. Dead on. Gearing Commander is a great tool but the stated circumference, or the stated gearing, one or the other, isn't matching up to the real world performance of our bikes.
.
Gearing Commander: Motorcycle Speed, RPM, Chain & Sprockets Calculator

It matches my speed within a few KPH, given the requirement that the speedometer indicate faster than true speed. For example, 168KPH indicated at 10,200RPM (163 on GC.com). And cruising at 7000 gives me about 115 indicated. That's close enough for me.
 

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Discussion Starter #16
speedo error?

It matches my speed within a few KPH, given the requirement that the speedometer indicate faster than true speed. For example, 168KPH indicated at 10,200RPM (163 on GC.com). And cruising at 7000 gives me about 115 indicated. That's close enough for me.
Do you know the % of your speedo error?
 

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It's my tach. 4.3% high. I scanned my fuel computer at Honda today and my tach needle does indeed read high compared to the actual values screen.
So, Honda is sending a tach/ignition pulse to the cluster and letting it deduce RPM on it's own... that means wiring in a replacement dash (Ala Koso?) down the road will be that much easier. :D

Nice detective work chasing that down.
 

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I just got my speedo calibrated at the shop while the bike was already on the dyno. When I was out on the road holding 75mph, I noticed there's a 400 RPM discrepancy between the tachometer and what's calculated on gearing commander. Many of you probably already know this, but I just wanted to confirm it. I didn't know tachometers also tended to be inaccurate either until a few months ago. Interesting piece of information to have when calculating gearing.
 

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The shop didn't say because the guy at the counter wasn't the one who did it. What I did notice is that since the speedo has been recalibrated, it's reading 400rpms higher than what gearing commander is showing again, just like it did when the bike was stock. This leads me to believe I'm seeing the same thing noticed on this thread.
 
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