Understanding Wheels and Tires
Wheel dimensions are all taken from where the tire bead seats on the rim although the wheel lip can extend out an inch or more. Therefore, actual wheel widths are slightly larger than the published specification. Similarly, the actual wheel diameter is slightly larger than published specification since measurements are made from the flat seating area, not the extended lips.
Caliper clearance
Caliper clearance
Caliper clearance indicates the maximum brake size that will fit inside the wheel rim.
Our build will use Team III Halibrand replica 17×8 front and 17×10.5 wheels that can accommodate the 11” front and the 11.65” rear brakes (caliper clearance in excess of 11.65”). The brake rotors are too large to fit 15-inch wheels without modifying the caliper bleeder or adding a hub spacer, both non-optimal fixes.
Offset
Wheel offset
Offset, whether positive or negative, is measured from the centerline of the wheel to the wheel mounting face or pad where the wheel bolts to the hub flange of the vehicle.
Positive offset, where the wheel mounting face is outboard of the wheel centerline, is common on passenger vehicles today.
Negative offset, where the wheel mounting face is inboard of the wheel center line so it creates a sucked in deep dish effect, is popular for sport and classic cars.
Rear axle width determines the extent of deep dish that can applied. A narrower pin drive IRS provides extra space for negative offset, and thus deeper dish wheels.
Exceeding offset limits will place undue strain on the wheel bearings which may impact steering and handling characteristics adversely.
Backspace
Wheel backspace
Wheel backspace, which is easier to measure than offset, is more frequently used when specifying wheels. Adapters for pin drive 5-lug will alter backspace dimensions.
The properly fitted wheel will ride comfortably in the wheel arch without rubbing the bodywork or suspension.
Another terminology for indicating wheel specification is to separate backspace (or back side) and front side dimensions: the distance from the back edge of the wheel (brake drum side) to the hub, and the distance from the front edge of the wheel (fender side) to the hub, respectively.
Front and back side dimensions added together will be approximately 1” wider than the designated wheel width, since wheel width, as discussed above, is measured at the tire bead seat and does not include the flanges or lips where balance weights clip on.
The narrower the back side, the greater the possible front side deep dish effect.
For Team III 17-inch wheels, the front\back specification is:
| Wheel size | Front side | Back side |
|---|---|---|
| 17 x 8 front | 3.750 | 5.250 |
| 17 x 10.5 rear | 4.875 | 6.625 |
The FFR Manual 
183 specifies the following based on 1987-93 spindles:
| Wheel size | Max tire size | Max back clearance |
|---|---|---|
| 17 x 8 front | 245/45 R17 | 5.720 |
| 17 x 10.5 rear | 315/35 R17 | 6.800 |
The backspace measurement is made without pin drive adapters.
Tires
Basic tire dimensions are indicated by three numbers: the width in millimeters (X), the aspect ratio of the height to width in percent (y), and the wheel diameter in inches (Z), all compressed in the X/Y Z format.
The FFR recommended tire height is 26” with a 4.5” bottom clearance (ride height), but the Mk3 wheel well will not accept a height above 27” without rubbing. Thus the practical range of tires heights is 25.50 to 26.25 inches. The following table summarizes possible tire sizes for the 17×8 and 17×10.5 wheels (in inches).
| Wheel size | Tire size | Height | Rim range |
|---|---|---|---|
| 17×8 | 245/45 R17 | 25.7 | 7.5 - 9.0 |
| 17×8 | 255/45 R17 | 25.9 | 8.0 - 9.5 |
| 17×10.5 | 275/40 R17 | 25.6 | 9.0 - 11.0 |
| 17×10.5 | 285/40 R17 | 26.0 | 9.5 - 11.0 |
| 17×10.5 | 295/40 R17 | 26.1 | 10.0 - 11.5 |
| 17×10.5 | 315/35 R17 | 25.7 | 10.0 - 12.0 |
To avoid a forward- or backward-tilting vehicle, the tire heights should match as closely as possible to provide a level attitude which is most appropriate for a street Cobra. Since only two front tire sizes match the wheel rim specification, the desired rear tire height is either 25.7” or 25.9”. There is no 25.9” exact match rear option; the closest match is 26.0”. Thus, there are two reasonable tire alternatives:
- front 245/45 R17 matched to rear 315/35 R17 with a 25.7” tire height
- front 255/45 R17 matched to rear 285/40 R17 with a height differential of 0.1” (slight forward tilt)
Tires are grouped by performance characteristics, and the types most appropriate for the street version Cobra are ultra high, max, or extreme performance summer tires. Summer tires are not intended to be driven in snow or on ice, and feature a low profile design for responsive handling and crisp steering response in both wet and dry conditions. These types provide a good blend of dry and wet street performance for light weight sports cars like the Cobra.
The rear larger tire sizes are typically the most difficult to locate. As of April 2008, Tire Rack listed 8 different summer performance brand options for 285/40 R17 sizes and 4 summer performance brand options for 315/35 R17 sizes (plus 8 competition grade 315/15 R17 options).
