Tip: measuring scale wheels

I've noticed people complaining about scale wheel sizes not being correct, so I decided to share some of my knowledge regarding this subject.
There are numerous articles on the internet about wheel size measurements. I would recommend everyone who is into modeling to read into the subject.
I would also recommend this awesome calculator that will give precise measurements on the wheels based on their standard size.

http://www.rimsntires.com/rt_specs.jsp

Let's get into some of the issues with determining correct wheel scale.

1. Rims:

Rim standard diameter and width are measured in inches. These measurements are taken at the bead seat, where the tire is fitted to the rim.
Actual diameter and width also contain the lips, those small sidewalls or lips that keep the tire in place. I'll use the word lip from here forward.

http://img203.imageshack.us/img203/9381/techwheels2.jpg

One very common mistake is for people to presume standard diameter and actual diameter are the same.
Let me give you an example: my real car has 17" rims. I measured the actual diameter (with lips), they had 18.5". Actual diameter will depend on wheel model. There is some variation for lip size, but between the same type of rims it should be negligible. I'm not familiar with off-road tires, but my guess would tell me that they have bigger lip to keep the chunkier tires in place. Don't quote me on the off-road tires though, need to research more.
This is very important to scale models, because almost always, the scale rims are bald, and therefore, when measuring them, you measure the actual diameter, not the standard one.
Here's an example on this set of 17" Fujimi BBS wheels:

http://img714.imageshack.us/img714/4361/dsc00661lr.jpg

I measured them with calipers. I used millimeters 'cuz I'm euro-trash :wink:

http://img715.imageshack.us/img715/4113/dsc00711ek.jpg

So:
Measured 1/24 rim diameter: 18.86 mm | 0.736 in
Equivalent 1/1 rim diameter: 452.64 mm | 17.66 in
Actual 17" rim diameter: 470.00 mm | 18.50 in (as measured on my real world car)

As you can see, these scale rims are smaller than real-world 17", they're more like 16". :(

2. Tires:

For tires, I'll measure tire diameter (while mounted on rims) and width and compare it to the measurements obtained by entering the marking on the sidewalls in the wheel calculator above.
The tires on the BBS Fujimi wheels are supposed to be 245/40 R17.

a. Diameter:
Measured 1/24 diameter : 25.25 mm
Equivalent 1/1 diameter : 606 mm
Real tire 1/1 diameter : 628 mm

b. Width:
Measured 1/24 width : 11 mm
Equivalent 1/1 width : 264 mm
Real tire 1/1 width : 245 mm

The tires are smaller in diameter but wider than their real world equivalent.

Conclusion for these wheels: Not to scale! :(

One sentence recap:

When measuring rim diameter, be aware that when converting the measurement to 1/1 scale, you get the actual diameter of the wheel, lip included, which is always about 1.5" larger that the standard defined diameter!

Thanks for the tips, very helpful!!

Thanks. This was informative.