The term "offset" is still used but a more common term I've used is
backspacing.
View attachment 1414755
It refers to the distance from the back side of the rim edge to the flat part of the wheel that mounts to the brake. From there, you do some simple math to determine what I call "frontspacing".
Wheel width is measured on the inside of the mounting surface of the wheel where the tire fits. This means that the actual width of any wheel will be wider than the published number. All aluminum wheels have thicker flanges than steel so an 8 inch steel wheel will be about 8 1/2" wide while the 8 inch aluminum wheel is closer to 9 inches wide.
Example....
My 18 x 10 rear wheels are actually 11" wide on the outside and 10" between the mounting surfaces for the tires. Back spacing is 4 3/4" from the outer inside edge to the mounting surface where the wheel attaches to the axle/brake. This leaves 6 1/4" front spacing. The wheels are 3/4"
offset from center with that offset to the outside.
If you want to fit these tires within the confines of the body, you need at least 4 3/4" backspacing for a 10" wide wheel. I took multiple measurements before I ordered my wheels because they were custom made to order and NON refundable.