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.
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.
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.
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.
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