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... I've never tried a circular mask and used to (in the old days) just paint the rubber after shooting the wheel and rim colors. This is how Mason did his SAS Jeep and the Tiger 1...and is in the midst of doing on his Ausf. F1. Do you have a recommendation?
Put the road wheels on toothpicks with a small bit of reversed masking tape for ease of handling.
Then run a line of black ink wash (I use Citadel minatures Black Ink Wash) around the tire-rim separation lines. This will establish a nice sharp line around the tires. As you put the ink wash on, you can load the brush and flow the wash around the rim almost like a regular wash using capillary action to follow sharply along the line.
After the ink wash dries, you can then easily paint the rubber with your favorite scale rubber color following the wider black line already there. (I use medium gray color mixed from black and white with a touch of flesh or tan.)
For even more detail, before you paint the tires, run a silver pencil between the wheels to highlight the steel "wear rims" between the each pair of road wheels.
Painting Pz IV road wheels is an armor-builders "right of passage"
Serously, though, mounting the wheels on toothpicks does make them much easier to handle and paint, Using the toothpick, with some practice, you can "roll" the wheel a little as you brush and get nice, precise results fairly quickly. Besides, the wheel masks won't help you with the inside tire lines if you've already glued the wheel pairs together.
Anyways, this is how I do 'em.
"Wheel masks? Wheel masks!? We don't need no stinkin' wheel masks..."