Inaccurate Speedometer

[mpatch]

Don’t feel like doing math now.  
Figure if the tire were at their wear marks you lose about 1/2” of diameter… not sure what that equates to in percentage of circumference but wouldn’t surprise me if it’s close to the speedo discrepancy vs new tires. 
 

my rough calculations say around 5-7% difference between  worn out to new

Edited September 1, 2021 by mpatch

[whisperquiet]

My T7 speedo is 1-2 mph fast depending on the speed.  OK by me

[M_Johnson]

Nice!! I would be more than happy if my T7 speedo had the same error..not the 10-13% I am experiencing.

4 hours ago, whisperquiet said:

My T7 speedo is 1-2 mph fast depending on the speed.  OK by me

 

[M_Johnson]

5 hours ago, mpatch said:

Don’t feel like doing math now.  
Figure if the tire were at their wear marks you lose about 1/2” of diameter… not sure what that equates to in percentage of circumference but wouldn’t surprise me if it’s close to the speedo discrepancy vs new tires. 
 

my rough calculations say around 5-7% difference between  worn out to new

I took the liberty of doing the math. I just went out and measured my rear wheel which has the factory stock Pirelli 150/70-18 on it. I have an odometer reading of 9245 km. The tread remaining is 4.5mm measured at the centre of the tire as per normal industry procedure. A new tire starts with 9mm of tread. A worn out tire has 1mm of tread remaining as per the T7 operating manual. Thus we find ourselves with three scenarios and three corresponding equations:

 Circumference of a circle equation is 2pi r.

 

i) Radius of my tire is exactly 324mm.

ii) Radius of a new tire will be 328.5mm

iii) Radius of a worn out tire will be 320.5mm

 

Therefore:

 

i) Circumference of my tire is (2x3.141592) x324=2035.76mm

ii) Circumference of a new tire is (2x3.141592)x328.5 =2064.03mm

iii) Circumference of a worn out tire is (2x3.141592)x320.5=2013.77mm

 

If I do 100kph with a new tire then:

i) Speed with my tire which is just over half worn will be 98.6kph. Difference of 1.4%

ii) Speed of a worn out tire will be 97.6kph. Difference of 2.4%.

Therefore the speed difference between a brand new tire and a totally worn out tire is 2.4%. That difference is within an acceptable tolerance. Surely then a difference of 10-13% is far beyond reasonable. This needs to be corrected by Yamaha.

 

 

 

 

 

Edited September 2, 2021 by M_Johnson

[marktbike]

On 9/1/2021 at 8:50 PM, M_Johnson said:

Thank you for your opinion. If you carry this same logic over to the other end of the spectrum would you not be putting yourself in danger by adhering to the indicated speed? You will be holding up traffic and no doubt someone will try to pass you in a less than ideal spot putting you, him and his passengers, and oncoming traffic in extreme danger with possible fatal results.  With the speedo over reading the producer (manufacturer ) is still at fault.

Just trying to guess what the reason for the error could be. Actually, it isn't something that spoils my pleasure in riding the bike in any way, so my contribution to the thread was just pure speculation. Of course an accurate speedo would be welcome, but it wouldn't change my life. Anyway, I hope you'll find satisfaction as well...

[Xpt007]

Thanks for the post MJohnson.  The explanation you got from the company was fun to read.  It’s good to see the engineering thinking and the legal minds combined.  Now I know I can go 8-10% faster.  Haha

[Aleksandar13]

On 9/2/2021 at 3:26 AM, M_Johnson said:

Nice!! I would be more than happy if my T7 speedo had the same error..not the 10-13% I am experiencing.

 

You can always get a pair of rings from myself to correct this and not have the hassle ... Then you can enjoy the bike and not have to think about it anymore .. I mean it's a great solution and doesn't cost and arm and a leg...