The first few years the Focus had two engines with different axle gears. A SOHC and the DOHC Zetec. The SOHC had 3.65 gears while the Zetec has 3.82 gears.
The 2005 ZX3 had taller tires(865RPM@60) too, so I get 2750RPM at 65mph for stock gears and tires. I'm guessing someone installed 4.1 gears. Either way, a set of 3.65 gears from the base engined cars would be what you want.
2000-2004 Focus LX gears
2000 Focus ZX3
Then in 2005 they installed the Duratec in all the Foci. Transmission gearing changed but the axle gears were the same numbers as the previous Zetec.
2005 Focus
And Miller88, you are incorrect about the MTX75, it is infact an OD gearbox.