Lego wheel diameter size question

[Mechbuilds]

How does the size actually work in lego? 

When you compare: Wheel 75 x 17mm (88517) 
to a Wheel 81.6 x 15 Motorcycle (2903)

The 88517 is obviously bigger than the 2903.. 

What do the numbers mean? 
 

[dmcc0]

The numbers are diameter x thickness (in mm).  According to brickset 2903 is actually 61.6 x 13.6.  Where does the 81.6 x 15 come from?  Is that the tyre size maybe?

 

Edit: Answering my own question.  It seems 2903 is the rim size and 81.6 x 15 is indeed the size of the tyre that fits it - part 87911

Edited March 22, 2019 by dmcc0

[Mechbuilds]

17 minutes ago, dmcc0 said:

The numbers are diameter x thickness (in mm).  According to brickset 2903 is actually 61.6 x 13.6.  Where does the 81.6 x 15 come from?  Is that the tyre size maybe?

 

Edit: Answering my own question.  It seems 2903 is the rim size and 81.6 x 15 is indeed the size of the tyre that fits it - part 87911

It shows 81,6 as the size of wheel on brickowl.. https://www.brickowl.com/catalog/lego-wheel-81-6-x-15-motorcycle-2903
It seems that it's a problem on brickowl. 

[dmcc0]

Bricklink showing the same size as Brickset - 61.6 x 13.6

Clicking through that Brickowl link, it does actually show the correct size under the title (see screengrab below)

[Mechbuilds]

Oh.. The title is different than the part name.. Man that threw me off hard. 

[1gor]

It is the same with wheels 32004a and 32004b; they are named upon tires that are used with them...