I'm confused. You stated 17" was OEM, but you are stuck with 20"? Did a former owner switch to 20's? In that case, choose the tire that yields the closest overall diameter/circumference & rpms/mile to the original tires. That should put you near the optimum gearing. From what you stated, it sounds as though the 275/55 is very near the OEM size (632 vs 638 revs/mile). If it's really that close, no need to worry about 17" wheels in the future.