What's the rpm difference at 65 mph in ML320 and ML430? ML320 vs ML230     ML320    ML430   A  5th gear 0.830      0.831  B  axle ratio   3.70   3.46    final ratio = AxB  3.071   2.875 Final gear ratio means the tires turn x times less than the engine crank shaft.   miles per minute   1.08333     1.08333  Here are 65 miles per hour converted to x miles per minute  Millimeters per minute  1.744.167  1.744.167 Since all tire circumfrences are in Millimeters we have to convert the miles to Millimeters  tire size   255/65R16   275/55R17    tire circumfrence in mm   2 251  2 243  wheel rpm @ 65 mph   774.84   777.60 Note that the 17 inch tire on the ML430 is slightly smaller than the ML320 tire !! Dividing miles per minute by the tire circumfrence gives us the revolutions per minute of the wheel engine rpm   2 380  2 236 Revolutions of the wheel per minute times the final ratio gives us the engine rpm Difference between ML320 and ML430 is 144 rpm @ 65 mph © 1998-99 Harold Pietschmann