2 things here you are putting too much faith in as to their accuracy! Rev counters and speedo's in these shonky old Fords are rarely accurate and will differ from car to car. Also wheel size/tyre size if different between the 2 cars will affect gearing and so give different readings anyway. Can't remeber what the ratio is for 5th on either the 2wd or 4wd but this would also make a difference if of course they are not the same...