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Old Wing Nut
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A larger rear tire is such an insignificant change the difference in speedometer reading will be practically nil.

Face it...motorcycle speedos are almost always optimistic and inaccurate. Fact of life. Helps keep you from getting speeding tickets.
 

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As far as I am aware the 10% optimistic readings are built in by default by all manufacturers it is a margin for error built in when calibrating the speedos so they cannot be held responsible by owners who get speeding tickets.
 

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Similar in Oz. ADR rules do not allow for any tolerance on the low side. The speedo on my car is optimistic, and yet the trip meter is bang on.
 

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It's odd because I changed the rear tire size and don't see any change in the speed display. Normally a larger tire will have a small effect. Could be the stock tire was on the larger side of a 180, and the replacement was on the smaller size of 190.
I understand that. A larger tire usually has a larger circumference. That should effect speedo
Well, not necessarily.

Looking at aspect ratio, for typical motorcycle tires, the tire height is determined by multiplying the tire width by the aspect number.

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These are typical 17 inch tire aspect ratios for current motorcycles:

For a 180/55 tire, that is = 99mm
For a 190/55 tire, that is = 104.5mm
For a 180/60 tire, that is = 108mm
For a 190/60 tire, that is = 114mm
For a 190/55 tire, that is = 95mm

The max difference in circumference for those listed aspect rations is (114-95) x 2 = 38mm = 1.49606 inches. (That's a 0.74803" difference in rear ride height!)

Sounds significant; when compared to the total tire diameter, the difference in both tire's total circumference is essentially the same.

Not exactly significant.

This is why aspect ration has such an effect on handling, but doesn't effect speed indication.
 

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They were in the not-so-distant past. A solid 10%.
 
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