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Discussion Starter · #1 ·
On the way to work today I hit the interstate - got a clear stretch of road and I let 'er rip. It was downhill and I had the tach at about 13,700rpm and the speedo was pushin 167!! Wooooohooooo! Thats my new top speed. I never thought the needle would get that close to 170 before... My previous highest was 160 even. Unfortunately I ran out of room before I redlined in 6th -- I might try again tomorrow... /images/icons/smile.gif
Sorry, but I just had to share /images/icons/cool.gif

 

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Now you just gotta figure out what the speedometer error is at those speeds../images/icons/smile.gif

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on average about 5-6%

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5-6% at speeds under 100 or so..most speedos have a higher error percentage at really high speeds..

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is this a systematic error? meaning its generally off in the same direction by the same amount for a given year/model? or does it randomly vary from bike to bike?

I always wondered that.

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probably true enough blue, i was more or less speaking personally, on my ride at an indicated 160, my gps said 151, so i was just putting out a general presumption.

"Excuses are like Buttholes, everybody's got one and they all STINK!!!"
 

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I don't know what speed I was going but have seen my speedo needle at 170 mph b4 on my 2000 6R (only with the wind though) . Love it!!!!
 

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<blockquote><font size=1>In reply to:</font><hr>

I might try again tomorrow...

<hr></blockquote>

Have fun, but be carefull bro..

 

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yeah the error is more like 8% once over about 110-130.....

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The only way to really know is to do the calculations. The formula for speed includes Gear Ratios, Tire Diameter, and RPM.

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Ok, did the calculations. I got the exact gears ratios from Kawasaki's website for a 1998 ZX6R. The stock tire is a 170/60/17, and a brand new tire would have a diameter of 25.00". I did the calculation for two diameters, 24.5" (worn tire) and 25.0" (new tire). If the tachometer is correct, with 6th gear the results are below:

**This does not take into consideration Tire Growth at high speeds, which would result in slightly higher speeds**

24.5" Tire @ 13700rpm's = 158.38mph
25.0" Tire @ 13700rpm's = 160.58mph

24.5" Tire @ 13000rpm's = 150.29mph
25.0" Tire @ 13000rpm's = 153.35mph

24.5" Tire @ 12000rpm's = 138.73mph
25.0" Tire @ 12000rpm's = 141.56mph

I'm Not a Pessimist, I'm a Realist!
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Just from my own personal testing, with a new set of Bridgetstone 010's on my ZX-9R (with -1T sprocket in front)

Speedo GPS
10mph 10mph
20mph 20mph
30mph 28mph
40mph 37mph
50mph 45mph
60mph 52mph
70mph 61mph
80mph 70mph
100mph 90mph

(little jump here)
188mph 165mph


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ya gotta go with the GPS.

Tachometers on bikes are frequently wrong.
We know the Speedo's are wrong, and it has been tested by peeps on this site, like Blue, that it actually gets WORSE the faster you go. (surprises me! I thought it was constant)
Then you got Tire Growth at that speed too! How ya gonna figure that one out!??

Buncha crap if you ask me, you'd think they could do a better job for $8k - $10k.




BA
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Did everything that you could see turn to little white lights and start flying towards you? If so, you were at warp speed!!!!!
 

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uh, i dont know about that. that assumes a lot of things that probably arent true. the tach is probably off a bit too. not to mention some of the other variables mentioned earlier in this thread which will certainly be a factor. thermal expansion and deformation would change the ratios and velocity numbers.

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Discussion Starter · #18 ·
Wouldn't thermal expansion (as well as expansion due to centrifugal forces) cause the tire to become larger, therefore lowering the number of revolutions that the wheel has to make to attain a given speed? Considering that, it seems that the actual possible top speed would be even higher.. Or am I missing something? Granted the tach is probably off by a little bit, but the difference between 13,500 and 14,000 is only approx 6 mph. I doubt that the tach error is more than +/- 250 rpms, or am I mistaken? Even at 13,000rpms I'd still be at 167mph. Maybe the formulas that are used by that site are incorrect?

I don't honestly think that I was doing 175 either, but without knowing how they arrived at this number I can't really argue with it.

 

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i'm sure they are theoretically correct. there are a lot of factors though that cannot be fully taken into account. all i am saying is take the number with a grain of salt unless you can measure it directly. certainly the number will give a ball park figure.

(yes, you are right about bigger tires) lower angular velocity to achieve the same tangential velocity.

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KawRidr6R, look at my earlier post again, and you will see that I do the speed calculations with two tire diameters. Notice how the taller tire has a little more speed... Trust ME, these are the correct calculations below as I worked out above for you!

Engine RPM x Tire Diameter
_______________________________________ = MPH

Gear Ratio x Final Drive Ratio x 336


I'm Not a Pessimist, I'm a Realist!
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