* Backspacing: This is the distance from the mounting surface of the wheel to the inner edge of the wheel. Too little backspacing and the wheel will hit the suspension or brake calipers. Too much and the wheel will sit too far inside the wheel well. This is the *most critical* factor.
* Wheel Width: Wider wheels allow for wider tires, but also increase the chances of rubbing against the fenders, suspension, or body.
* Tire Diameter: The overall diameter of the tire (including the sidewall) will determine how much the wheel and tire combination will stick out and how much ground clearance you have. A larger diameter wheel often means a smaller diameter tire to keep the overall diameter similar to the original (preventing speedometer errors and affecting gearing).
* Suspension Modifications: If you've lowered the car or have any other suspension modifications, this drastically affects what will fit. Lift kits will allow for larger wheels and tires.
* Fender Modifications: Rolled or flared fenders can significantly increase the space available for larger wheels and tires.
In short: You could potentially fit very large wheels (perhaps up to 18" or even 20" in some cases), but it would require careful selection of backspacing, tire size, and potentially modifications to the car's suspension or fenders. You'll likely need to do some research specific to the exact year and model of your '66 Mustang (coupe, fastback, convertible, etc.) and consult wheel and tire fitment guides or forums dedicated to classic Mustangs. Many online resources and Mustang clubs can help determine the right fitment for your specific car. Improper wheel and tire combinations can lead to safety issues.
