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Your best bet is to swap the rear to a different diff completely. You will never be able to get the kind of ratios you will need in a Mazda diff, let alone the fact that the torque from a diesel will most likely shred any differential set up for a rotary. Depending on the diesel you will want your cruising rpm at 75 to be about 1600 - 1700 rpm. This will require around a 2:1 final drive ratio with a 25 inch tire. Depending upon the overdrive ratio in the transmission you use, it is still going to leave you looking for 2.73:1 or higher gears. You might look into the Ford 9 inch as there are standalone center sections made that can be adapted to work with your independent rear suspension and a lot of gear sets made for them. Any way you look at it, it will require a fair amount of money and fabrication to make it work. Other than that the auto tranny FD gears are probably the best you're going to get, but you aren't going to be happy with the results. They won't even get close to altering the rpms enough for a diesel engine.
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