While size designations for tires (i.e., 245/40R18) and wheels (18x8) confirm matching (18-inch) bead seat diameters, measuring a wheel's overall diameter from the top edge of one flange to the other 180-degrees away will reveal that the wheel, including its flanges, is actually about 1.4" larger (19.4-inches) than designated.
Tire beads feature high-strength steel wire concentrically wound at the exact diameter specified for the tire size. Covered in rubber, the steel bead wire provides an anchor for the tire's casing plies and resisting centrifugal forces to hold the tire on the wheel.
In order to mount a tire on a wheel, the tire's beads have to slide over a 7/10"-tall wheel flange on one side of the bead seat. Since the tire's steel bead wires don't stretch, this requires the wheel to also feature about a 7/10"-deep tire well/drop center around the wheel adjacent to the other side of the bead seat. This helps avoid damaging the tire's bead by allowing it to maintain its manufactured diameter when being mounted.
Most wheel designs feature the tire well/drop center on the outboard side of the wheel, requiring mounting the tire with the front of the wheel's design/spokes facing up when installed on the tire mounting machine.
Mounting begins with positioning the tire's bead in the wheel well on one side, followed by the mounting machine carefully slipping the bead over the wheel flange on the opposite side. Once the first bead is mounted, the procedure is repeated to mount the second bead.
However, wheel designs featuring deep-dish or wide-lip styling must typically be mounted from the rear/back, requiring their tire well/drop location be adjacent to the inboard side of the wheel. The technician needs to recognize the wheel has a reverse-mount design, and carefully secure the wheel in the mounting machine with the front of the wheel's design/spokes facing down.
Warning: Reverse mount wheels must always have tires mounted/dismounted from the back of the wheel.
If a shop technician attempts to mount or dismount a tire on a reverse-mount wheel from the front, the distance from the wheel flange to the tire well/drop center will exceed the tire's bead diameter. The tire mounting machine's hi-torque turntable (850 ft.-lbs.) can apply excessive tension to the tire bead assembly, tearing the rubber covering and/or damaging the casing cords by compressing them against the wheel or the tire machine's mounting head. Overstretching the tire bead may also exceed its breaking point, fracturing the steel wires and causing them to fail as the tire is inflated.
Note: Wheels should always be clean and free of rust or corrosion. It is important that the shop technician apply a commercially available tire mounting lubricant to the tire bead and the wheel safety hump, bead seat and flange to assist bead seating.