I just did the tire calculator on Discount Tire site and the difference is:

Overall Dia:
20" => 31.91
17" => 31.61

Speed:
20" => 65 mph
17" => 64.39 mph

The 20" is the 275/55/20
and the 17" is the 265/70/17

The reason the '05 went to the 17" spare is they decided to do a bigger brake option JF7, 17" brakes.

Hope this helps