+1 Not only that but your ABS maybe affected as well. More than likely with today's standard cars, we should have wheel speed sensors by changing it very small upgrade should not affect it so much, now when you put swampers or 20" gangsta wheels, your speedometer is offset dramatically, yet alone your killing your mpg.
205 is a decent size, but I would recommend that size for a 16" wheel, then 215 for a 17" wheel, etc.
2009 Cobalt LS XFE U74
Mods: K&N Typhoon Intake, Option Cat-Back Exhaust, Cosmo Short shifter, Maxxim Ahead 17X7's, MPx shorty antenna, FE5 Strut/shocks, Tein S Techs, Moog endlinks, Moog spring seaters, ZZP shorty header, LSJ downpipe, LNF front grille