The ERA wheels are 16" dia. and 12" width, when I got my Mk1 wide body it came with Hoosier slicks and I thought it would be really hard to source 16" tires so I went and had a custom set of Vintage Wheels made up in 10" and 12" widths to the ERA offsets. They are still in the boxes, unused thus far as I've been pretty successful in finding 16" tires.
The 15" is pretty tight clearance and would not allow for large brake calipers.