Differential Ratio for 9 Speed Direct Transmission

I’m rebuilding a 2000 single axle Freightliner that has a Series 60 Detroit with a 9 speed direct transmission. The differential is a Meritor/Rockwell 3.91 ratio Model A3200P1706. The transmission is in great shape with a new clutch, but the truck tops out at 55 MPH. I’d like to install new gears in the differential so the truck can run 75 MPH. The truck will be used to pull horse trailers. May I have your all’s help choosing the correct gear ratio for the rear end.

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