Fat Back Fitness
Measuring rear wheel room is a little easier than measuring the front. Remember that most outer wheelwells tend to move inboard as they taper toward the front of the car.
Stuffing fat tires with short, stiff sidewalls under the rearend is a little easier. One big reason is that the sidewalls on these 16-, 17-, and 18-inch tires tend to stay put, unlike tall, two-ply wrinkle-wall drag tires that move around a bunch. We'll deal with both styles here. Always measure both sides of the car when determining maximum tire size. Often, the body does not sit squarely on the chassis, reducing outer wheelwell clearance on one side and inner wheelwell clearance on the opposite side. This is extremely common with older musclecars. If the car is more than 1/2 to 3/4 inch offset, consider loosening all the body-mount bolts (on a full-frame car) and shifting the body to make it more square over the chassis. Our experience with GM cars is that the rear axle tends to be offset to the passenger side. This creates a tight clearance on the outboard of the passenger side and inboard on the driver side. On earlier unibody cars like Camaros, Novas, Mustangs, and Challenger/'Cudas, you may have to live with the offset unless there are minitubs in the car's future.
When measuring rear tire clearance, minimizing body roll will allow you to run a larger rear tire with tighter clearance. The biggest hurdle is most rear axles tend to articulate, which occurs when you enter a steep driveway at an angle and one tire angles into the rear wheelwell more than the other. This generally demands more clearance if this is a problem.
Tire-clearance limitations are often traced to the leaf spring, which intrudes into the wheelwell area. Several companies offer offset-shackle kits for specific cars. For early Camaros and '68-'74 Novas, Detroit Speed offers an offset-shackle kit (PN 040901 for $275) that uses billet aluminum pieces to move the springs to accommodate up to a 335/ 40R17 or 18-inch tire with a 13.5-inch section width. This kit works in conjunction with minitubs that add additional clearance. Mopar Performance sells kits for A-Bodies (PN 4876558AB, $119.95 from summitracing.com) and B/E-Bodies (PN 4120665AB, $99.95 from summitracing.com). These conversions require some fabrication work to optimize tire clearance, so they cannot be considered a basic bolt-on.
In this early Camaro application, the lower shock absorber is the limiting factor to moving the wheel more inboard. We adjusted the mount to create enough clearance, which is pretty tight. The best fix is to cut the shock mount and move it inboard.
Fitting fat, rear drag tires involves a little more work because you must be concerned with taller-sidewall tires and much more lateral sidewall movement. This will require much more clearance than short-sidewall tires. With DOT-legal drag tires, you need to be careful when researching rear tire dimensions. The best process is to actually mount a friend's tire-and-wheel combination before you buy. If not, then using our tire-and-wheel tool can be helpful. Remember to always check the clearances at ride height, not with the car sitting on jackstands under the frame and the suspension in full extension.
If you decide to trim the inner fender lip to improve tire clearance, it can be worth as much as 3/4 inch. But move carefully here; if you need to remove most of the lip area for clearance, carefully trim it back rather than rolling the lip under. The difference between rolling and cutting is an additional 1/8 to 1/4 inch. Don't cut all the way out to the edge because many cars use a pinch weld to hold the inner fenderwell and outer fender together. If you cut through this, you'll have to carefully weld them back together. If the car is already painted, this will cause problems. Ask us how we know. END
Keep in mind that choosing a sticky rear tire involves more than just fitting the tire. Tread and section widths do not always follow what's embossed into the sidewall. For example, a Mickey Thompson 26x11.5x16 tire mounted on an 8-inch wheel has a tread width of 9 3/4 inches. A smaller 26x10.5x15 M/T has a tread width of a little more than 8 3/4 inches. Both tires are within 1/8 inch of the same diameter of 26 1/4 inches. | 
For coil-spring cars like A-body GM cars, such as Chevelles and Olds Cutlasses, Fox-body Mustangs, and third-generation Camaros, a tubular lower control arm like this one from Global West or Hotchkis will stabilize lateral rear-suspension movement. Those spindly stamped control arms bend more than you think. | 
Taller rear tires put down a larger footprint, which improves traction but also reduces the effective rear-gear ratio. For example, if you've got 3.50 axle gears and 26-inch-tall tires, the engine will spin 3,166 rpm at 70 mph assuming no overdrive or slippage. Changing to 28-inch tires would reduce the rpm to 2,940, which is about the same effect as if you'd kept your 26-inch tires and swapped to 3.25 axle gears. |