Whether your allegiance is to the classic Pontiac muscle of the early '60s or the newest generation of GTO and G8, you have likely noticed the trend of mounting larger wheels and tires on just about anything that rolls. For most considering such an option, one universal question always pops up: How do I determine the largest wheel-and-tire package that will fit?
Often owners rely on the experience of others with a similar model to help them, but this is not foolproof. And what if you have a non-traditional Pontiac and you don't know anyone who has stuffed big wheels and tires under one like yours?
Percy's High Performance has addressed the problem by offering an affordable measuring device, the "Wheelrite." The innovative tool was a past winner of the prestigious SEMA International Product Award, and is quickly becoming the standard tool of professionals in the wheel-fitment industry.
The Percy's Wheelrite (PN 01201) is available directly from the company for $99. Capable of measuring wheel diameters between 15 and 30 inches for four and five-lug applications, it can support wheel-width measurements of 6-11-plus inches, and identify the proper backspacing to allow an enthusiast to confidently order custom wheels.
According to Ryan Alder of Percy's, "We developed this product to address the issues an enthusiast faces when he tries to figure out what wheel-and-tire package will fit on his car. The inspiration came from projects where we made costly mistakes because we couldn't accurately determine how the proposed wheels and tires would fit into the front and rear wheelwells without actually buying and mounting them.
"The market has really responded well to the Wheelrite, and we now offer a plastic version for the enthusiast and a metal version for the professional wheel-and-tire service provider."
Before you begin measuring, determine your build goals. Is the wheel/tire package just cosmetic, or are you including suspension and brake upgrades to maximize performance? If not, then the job is considerably easier, as the variables of whether the upgraded brake package will fit inside the wheel becomes a non-factor. Keep in mind that aftermarket suspension systems typically improve upon the front suspension geometry, and sometimes provide additional clearance for larger wheels.
To simulate the tire size that you desire, either visit a local tire dealer that will allow you to bend the tire simulator wire over a tire, or find a candidate with a similar tire profile. Here David DuSold of DuSold Enterprises is bending the wire over the rear tire of an '09 Ford Shelby GT500, which runs 9.5-inch wheels and 285/40ZR18 Goodyear Eagle F1 Supercar tires in the rear (shown) and 255/45ZR18s in the front.
Follow along as we utilize a Percy's Wheelrite to take you through the procedure to measure and simulate a wheel-and-tire package that is being engineered for aggressive street driving and road-racing with a '77 Can Am.
The wheel of choice is the YearOne Billet Snowflake in an 18-inch diameter. This size was required to ensure adequate front-caliper clearance for a KORE3 Big Brake Kit, which is based on C6 Corvette Z51 Performance Package production parts. The suspension is a G-Plus System from Global West.
Stories on these products will follow, but they are important to note for this article because affect the decision-making process, as compared to a Pontiac fitted with stock suspension and brakes.
The Wheelrite is packaged with everything necessary to measure wheel width and backspacing. It includes a mount for the bolt pattern, a telescoping arm to lock in the wheel diameter, a set of horizontal bars, and a simulator wire. Instructions make it a snap to assemble and adjust. Note the wheel-width gauge (top), and the hub and wheel diameter assembly (bottom).
Wheel And Tire Measurements
Provide the parameters numbered 1-4 below to your retailer or wheel manufacturer. For example, our YearOne Billet aluminum Snowflake rear wheels measure 18x10 inches with 6 inches of backspacing for the rear, as shown, while the fronts are 18x10 with 5.5 inches of backspacing.
1 Backspacing - This is measured from the hub-mounting flange to the lip of the rim. It's easer to measure without a tire mounted, using a straight edge across the rim. Note here that the 18x10-inch YearOne billet-aluminum Snowflake for the rear was made with 6 inches of backspacing. An original Pontiac 15x8-inch Snowflake has 4.5 inches of backspacing.
2 Rim Diameter - This is the measurement of the diameter of the wheel from where the tire bead sits if you are viewing the wheel from the front. Rim dimensions are stated generally in inches (such as 16, 17, and 18) except in certain special applications.
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 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.
Next, the Wheelrite is rotated around the circumference of the wheelwell to check for clearance issues relative to the inner fender lip. A torpedo level is placed against the fender lip as a reference point to measure the clearance from the lip to the wire (9/16 inch). A measurement is then taken from the frame to the inner portion of the wire simulator. Steve DuSold recorded 15/16 inch.
The YearOne wheels were ordered to allow for 0.5-inch of clearance in the front and rear based on a maximum section width of 298.45 mm. Anything below that figure provides additional clearance. Conversely, a section width that exceeds it reduces clearance and increases the potential for wheelwell/lip or frame contact when under hard-cornering or braking conditions.
MANUFACTURERS’ TIRE SPECIFICATIONS Size 285/40ZR18 295/40ZR18 295/45ZR18 Diameter 26.97 27.29 28.45 Rim Size Range 9.5-11 10-11.5 9.5-11 Recommended Rim Size 10.00 10.50 10.00 Section Width (mm) 285 295 295Tools And Supplies
Percy's Wheelrite
Tape measures-inches and millimeters
Torpedo Level
Jacks and jackstands
Standard sockets and socket wrenches
Conclusion
Given the vast choices, wheel and tire fitment has never been more challenging than it is today. Thankfully, Percy's Wheelrite has answered the call for an easier method by which a hobbyist can science out a wheel/tire combo that will fit properly before he places an order.
Whether your car is a classic GTO or Firebird, a brand new G8 GT or Solstice, or any model in between, using this tool to make the calculations means you no longer have to seek out an enthusiast with a proven wheel/tire combination to copy in order to get it right the first time.
