I doubt the Xterra's speed governor is based purely on RPM, though fundamentally it is since the (electronic) speed sensor is just counting RPMs at the transmission output or further down the line. You could do 100 MPH in third gear if you wanted to. For the computer to read 100 MPH w/ a 31" diamter tire, 4.90 gears, and 3rd gear's 1:1 ratio, you would be turning 5310 RPM which is still (just a bit) under the redline...
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