I have the 20's that are forced on most every premium package around here. I want move to 18" rims, either the stock FE rim or another rim I can find with a sizable offset (not a big poke fan). So will probably run 265 or 275 70/18's on a rim with at least +40 offset. The rig came equipped with a full size spare on the stock 20" rim. The 60R20's and the 70R18's have very nearly the same diameter. Any reason to buy five new rims/tires ? Or do I just keep the original spare and buy four? (Spare will only be used in emergencies and not for long distances).Thx