Measuring for new track wheels

[funbnme]

I want to get a set of the 15" track use steel wheels.

Here's what I have on the car now on the rear:

Boyd Junkyard Dog wheel:
20x8.5"
Tire is 26 1/2 tall
5" Backspace (from mounting surface to back lip of wheel)

In the wheel well, from the mounting surface to the upper fender lip, I have 4 3/4" of room. From the surface to the inside edge of the wheel well at the front, I have 6"
https://static1.pt-content.com/images/pt/2009/03/DSCF3092-1.jpg


Would this size wheel fit?
15x10" wheel with 5" OFFSET

Is offset from the front of the lip to the mounting surface of the tire?

If I want the tire to sit inside the wheel well a bit, do I want a larger or smaller offset?


Do I need to measure anything else to make sure I get the order right the first time?

Any vendors here sell this type of racing wheel?
http://www.daymotorsports.com/product/1309/BART-10%22-MEDIUM-WEIGHT-BLACK-WHEELS

[Apogee]

Offset is the distance from the wheel mounting surface to the centerline of the wheel. That said, a 10" wide wheel with 5" offset would have the mounting surface flush with the outer bead. Also, offset is typically measured in millimeters, so I'm guessing that the 15x10 you mentioned has 5" of backspacing, not offset.

I'm sure the wheel guys can hook you up with what you need based on your application as it appears close to stock.

Tobin
KORE3