Changing wheel size will not increase speed. Probably actually slow you down, effectively, as whatever 15 inch wheels you bolted on to the car will likely weigh more than 14 inch bundts. Your speedo is probably off.

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1982 240 D, 308,000 - 321,127 miles (sold)
1982 300 TD,166,500 - 226,000 miles
1998 E 320, 120,000 - 144,000 miles
2005 C 230 K, 26,000 - 77,000 miles (sold)