* Wheel well clearance: The 1991 DeVille was designed for much smaller wheels. 20-inch rims, with tires of appropriate size, would likely rub against the fenders, suspension components, or bodywork.
* Suspension and braking system: The suspension and braking system weren't designed to handle the extra weight and stress of larger wheels and tires. This could lead to premature wear and tear or even failure.
* Offset and backspacing: The offset and backspacing of 20-inch rims designed for modern cars are almost certainly different from what the 1991 DeVille's wheel wells can accommodate. This would necessitate custom-made wheels, which are expensive.
While some aftermarket parts might claim compatibility, you'd almost certainly need extensive modifications like:
* Fender rolling/flair: Modifying the fenders to widen them and prevent rubbing.
* Suspension adjustments: Lowering the car (possibly even requiring an aftermarket suspension kit) or using spacers (not recommended for safety reasons).
Even with modifications, the ride quality would likely be significantly harsher, and the handling might not improve as the suspension would be working outside of its intended parameters.
In short, while technically *possible* with significant expense and modification, it's generally not practical or recommended to put 20-inch rims on a 1991 Cadillac Sedan DeVille. Sticking to wheels within the manufacturer's specifications or slightly larger sizes with appropriate offset is the safer and more sensible approach.
