30mm vs 30.4mm Wheels & Tires

[tom alphin]

There are a number of wheels with an outer diameter of around 30mm, but they are very inconsistently described as 30mm or 30.4mm across different websites.  I noticed that LDraw is the most consistent here, using 30 (mm) for all four of these parts.  (Within MLCad they appear to be exactly the same width.)

1. Are they actually the same size, or are they actually measurably different?
2. Even if they are ever-so-slightly different, is 0.4mm so small that almost nobody cares?
3. Does LDraw prefer technically accurate measurements, or is it generally preferred to nudge measurements ever-so-slightly to make the system work better together?

I would like to make my part naming both technically correct and consistent.
---Tom

LINK TO RELEVANT PARTS: https://brickarchitect.com/parts/categor...artstyle=1

P.S. Here are dumps of the part names across the relevant sites:

56904 (14mm wide)

• Brick Architect Long: 30mm D. × 14mm Wheel w/ Technic Axle Hole
  
• Brick Architect Short: 30mm D. × 14mm, Axle
  
• Bricklink: Wheel 30mm D. x 14mm
  
• Rebrickable: Wheel 30 x 14
  
• Brickset / LEGO.com: RIM NARROW Ø 30/14 W/ CROSS
  
• LDraw: Wheel Rim 14 x 30 with 6 Spokes and No Pegholes
  

42716 (20mm wide)

• Brick Architect Long: 30.4mm D. × 20mm Wheel w/ Technic Axle Hole, 5-Angled Spokes
  
• Brick Architect Short: 30.4mm D. × 20mm, Axle, 5-Spokes
  
• Bricklink: Wheel 30.4mm D. x 20mm with No Pin Holes and 5 Large Spokes
  
• Rebrickable: Wheel 30 x 20 5-Spoke
  
• Brickset / LEGO.com: RIM WIDE DIA.30X20 W/ CROSS HOLE, NO. 1
  
• LDraw: Wheel Rim 20 x 30 "Torq Thrust" with 5 Spokes and External Ribs
  

66155

• Brick Architect Long: 30.4mm D. × 20mm Wheel w/ Technic Axle Hole, Angled Spokes
  
• Brick Architect Short: 30.4mm D. × 20mm, Axle
  
• Bricklink: Wheel 30.4mm D. x 20mm with Center Axle Holes Motorcycle
  
• Rebrickable: Wheel 30 x 20 with Center Axle Holes
  
• Brickset / LEGO.com: RIM, WIDE, DIA. 30X20, W/ CROSS HOLE
  
• LDraw: Wheel Rim 30 x 20 with 3 Dual Angled Spokes and 4L Hub
  

[Rene Rechthaler]

your rim list of that family misses 44292 (oldest one), 7301 (newest one, Shelby Cobra), 54087 (variant of 56145) and 5650 (Countach)
tyre list misses 44309 (old) and 6620 (Bronco, not on the PT yet)

[Magnus Forsberg]

It looks like our description of 66155 is wrong. The dimensions are reversed. 
Rims should always have it in (width) x (diameter) in millimeter. Not including the hub/centre.

[Magnus Forsberg]

It looks like our description of 66155 is wrong. The dimensions are reversed. 

Updated, corrected files uploaded to the tracker.

[tom alphin]

your rim list of that family misses 44292 (oldest one), 7301 (newest one, Shelby Cobra), 54087 (variant of 56145) and 5650 (Countach)
tyre list misses 44309 (old) and 6620 (Bronco, not on the PT yet)

Rene,
My guide doesn't aim to be a complete catalogue, but rather to provide an easy-to-use visual guide to the most common current parts (top 2000 current parts).  I am also aiming to include the top 2000+ parts of all time, which includes a bunch of common retired parts.  (They are hidden by default though.)

That said, I am adding parts to the database all the time, and less common parts show up eventually if they are used in multiple sets.

---tom

[tom alphin]

It looks like our description of 66155 is wrong. The dimensions are reversed. 
Rims should always have it in (width) x (diameter) in millimeter. Not including the hub/centre.

I'm glad this thread helped the group in an unexpected way, too :-)

[tom alphin]

Does anyone happen to have any insight into the original question - whether both "30mm" and "30.4mm" wheels are actually the same diameter or not?

[Magnus Forsberg]

Does anyone happen to have any insight into the original question - whether both "30mm" and "30.4mm" wheels are actually the same diameter or not?

I think one reason might come from how we choose to describe these.

All ldraw description should be the same. Rim (width) x (diameter) in millimeter. The rim and the tyre share a common size, here it is "30". 
They belong together.
If I check all of these in ldu they all measure 76 ldu, which translate to 30.4 mm
And then we round it of to the nearest integer. No decimals,  30mm

[tom alphin]

I think one reason might come from how we choose to describe these.

All ldraw description should be the same. Rim (width) x (diameter) in millimeter. The rim and the tyre share a common size, here it is "30". 
They belong together.
If I check all of these in ldu they all measure 76 ldu, which translate to 30.4 mm
And then we round it of to the nearest integer. No decimals,  30mm

That's good enough confirmation for me, thanks.