The same principles apply...
The same principles apply whether you are using a four-post lift or are doing the measurements on your garage floor. Use a level surface, take careful measurements before removing the wheels, and be patient as it will likely take several attempts to ensure the suspension is leveled properly.
3 Rim Width - The measurement of the rim width is not across the entire lip-to-lip surface of the wheel, but instead across the rim from the bead seat to bead seat. In general, the rim lip (flange) will be approximately a half inch on each side of the rim, so a rim width of 8 inches makes the overall width of the wheel from lip-to-lip 9 inches. Note thatthe 18x10-inch YearOne wheel has a 10-inch rim width and an overall width of 11 inches.
4 Bolt Pattern - Bolt pattern is the number of lug-nut holes and their spacing. Pontiac has typically used a five-lug pattern on all of its performance cars, and other than the B-body and C-body Big-Cars that had a 5-on-5-inch lug pattern, the standard is 5 on 4 3/4 inches. Five-lug patterns are measured from the centerline of one hole to the back edge (farthest) of the hole across from it.
5 Offset - This measurement isn't required to place a custom wheel order. It is the location of the mounting flange relative to the wheel's centerline. Offsets are referred to in positive or negative numbers and expressed in millimeters. If the flange is more toward the outer face of the wheel, the offset is positive. If it's more toward back of the wheel, the offset is negative.
After leveling the car, the...
After leveling the car, the Wheelrite is installed and the telescoping arm is set to the 18-inch wheel diameter. Note the silver mark on the tool-it was painted on by DuSold Enterprises to always have a visual reference mark in case the tool gets bumped during fitment.
A wheel with an actual rim width of 11 inches (a 10-inch bead-to-bead rim width) and 6 inches of backspacing has its offset determined like this:Actual width / 2 - Backspacing = Offset11 inches / 2 - 6 inches = -0.5 inch
Despite the fact that the 0.5 is a negative number, if the backspace is greater than the centerline measurement, as is the case here (5.5-inch centerline versus a 6-inch backspace), the 0.5 inch is considered a "positive" offset since the flange is closer to the face of the wheel. The opposite would be true to have a negative offset.
Offset in inches x 25.4 = offset in mm0.5 x 25.4 = 12.7 mm
Tire Considerations
Before purchasing tires for your Pontiac, make sure you visit the tire manufacturer's or distributors' website for complete specifications on those tires that interest you. Pay careful attention to the listed tire section width, as it plays a critical role in whether the tire will fit.
The tire section width is the measurement of a tire from the inner sidewall to the outer sidewall at the widest point (minus any decorations or raised white letters). This is supposed to coincide with the first numeral in the tire designation (for example, 285mm for a 285/40ZR18-sized tire).
The listed section width is determined using a recommended and measured rim size from the tire manufacturer. In the chart are three different manufacturers' tires with the listed section widths noted. The industry standard is that for every 0.5-inch deviance from the measured (recommended) rim size, the section width will change by 0.2 inch (5.08 mm).
For example, with the 295/40ZR18 tire that has a listed section width of 295 mm with a 10.5-inch rim, you would reduce the section width by 0.2-inch (5.08 mm) if you used a 10-inch rim instead. In this example, the section width would be 289.92 mm based on the actual section width.
Now that you know rim-width changes will affect section width, there is another variable about which you need to be aware. On the 285/40ZR18 Goodyear Eagle F1 Supercar tires, the listed section width is 11.4 inches (289.56 mm) versus the 11.22 inches that would match its 285mm sidewall designation. Although manufacturers typically round the inch calculation to the nearest decimal, this calculates out to 289.56 mm, or a full 4.56 mm (0.18 inches) greater than the tire's designated section width. This is why you really need to check all of the tire specifications carefully, as an equal-sized tire from two different manufacturers may be slightly larger or smaller than one another.
Percy's tire simulator wire...
Percy's tire simulator wire is then carefully measured with a millimeter tape and adjusted to equal the 285mm cross section (11.22 inches). Using a tape measure, a reading is taken from the center of the hub to the top of the simulated cross section of wire. Since the tire dimensions were 26.97 inches (approximately 26 31/32), half of that diameter is 13.485 inches (approximately 13 15/32). Minor adjustments for height were then performed on the wire, and the cross section and diameter of the tire was checked a second time, whereby the dimensions were verified.
The wheel-width mounting pad...
The wheel-width mounting pad is then snapped into position, and the two wheel-width measuring sections are assembled. A tire-width starting point is calculated by measuring from the closest section of the rear frame to the inside of the fender lip. This friction fit (largest allowable tire width) of 12.75 inches, minus a reasonable clearance of a half inch on each side, defines the largest possible tire width to be 11.75 inches or 298.45 mm. The tire simulator wire is then placed into the grooves and set up.
The tool is adjusted so that...
The tool is adjusted so that the tire-wire simulator is roughly in the middle of the wheelwell, and a 10-inch-wide rim is set up by sliding the bars on the tool. The Wheelrite is then rotated, first toward the back opening of the wheelwell and then to the front of the wheelwell (door side). Both wheelwell measurements were taken, but the front of the wheelwell is much tighter (2.75 inches) than the very generous rear of the wheelwell.