There should be no difference for those speeds. If car A is a base 3.6L and car B is a base 2.8l and both have a 3.42 diff and the 5 speed auto then the crusing rpm for 60 MPH should be a tad less than 2100 RPM for both. Tire diameter is going to be the main contributor to differences. The above calculation assumes a 225/55R16 Goodyear Eagle RS-A for both cars(25.8" diameter). A couple of years ago I upgraded to 18" wheels with 225/45R18 tires, diameter is now 26.00" instead of 25.8" so my RPM at 60 is only slightly lower (using GPS for comparison not the indicated speed since it would be slightly off now)