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Sprocket change

16K views 24 replies 12 participants last post by  xorbe 
#1 ·
Since I compared the speedometer with a GPS based one, we are at about 7% off. 100 kmh speed in the dash = 93 kmh gps. So I decided to match the sprocket to the true speed by taking 2 teeth off.

Ordered a JT rear steel sprocket, JTR486.41 If you look up on their website they will work for R3. Dunno about the weight compare to our sunstar one but I will weight it when I get it. Taking 2 teeth off = makes speedo overread 1% higher which is good enough for me.
 
#2 ·
Very Accurate!

My speedo is dead on from the factory.

See post #9
http://www.r3-forums.com/forum/554-...e-speedo-off-any-way-calibrate.html#post78721


"Just verified my R3 speedo on the way to work via my Garmin. Completely stock - I am no more than 1 MPH off at any given speed/gear. Most of the time I was within .5 MPH.


Speedo Read 25 - Garmin flickered between 24 & 25
Speedo Read 45 - Garmin flickered between 44 & 45
Speedo Read 50 - Garmin flickered between 49 & 50
Speedo Read 70 - Garmin flickered between 69 & 70


This is the most accurate speedo I have ever seen from the factory."


Good luck with yours!
 
#17 · (Edited)
OEM gearing is 43/14 = 3.07

If you go -1 front and +3 rear, that will become 46/13 = 3.54

So the new gearing is 3.54/3.07 = 1.153 what the old gearing was. (i.e. 15.3% numerically higher)

That will make your speedometer read 1.153 times as high as it did AT ANY GIVEN SPEED than it did before, but the "before" was incorrect to begin with, right?

So, it will read 1.153 times what it did before, but you now have to correct that false reading to reflect your ORIGINAL speedo error.

So, FOR EXAMPLE ONLY, if your original speed error was that it read 1.07 times the real speed (i.e. it read 7% higher than reality, then it would now read 1.153 x1.07 = 1.23 of actual speed (23% too high).

It's a brain twister, especially if the original and the modded are off in opposite directions. In those cases, it is easy to do the math ratios "backwards" and be wrong. I did this as a small business for a while years ago, and the math can suck. Don't ask me to do another example. :)

By the way, with the 46/13 gearing, your engine rpm at a TRUE (not indicated) 60 mph would be about 7000 rpm.

Jim G
 
#20 ·
Good information, Jim. I appreciate your responses.

I did see the GPS discussion that occurred on this forum as well. I understand that it’s not the end-all be-all for speed calculation.

Again, thanks for the detailed responses. It’s great information!

Shiny side up!


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