If the speedo gear drives the driven gear one revolution every time the differential spins once, then the gear ratio would be irrelevant.
For example, you change final drive ratio to the ZETEC FD of 3.86:1 from 4.06:1
Intuitively you think that the diff will spin slower. i.e. it only spins 3.86 times per 1 input revolution versus over 4 times per one revolution for the other FD.
Well, the speedo drive gear turns the same speed as the diff so its going slower too. Then, figure that the tires are also going slower because they are hooked directly to the diff without any other gear changes.
What will throw your speedometer off is changing tire sizes.
Now, don't get this confused with changing a final drive on a rear wheel drive vehicle! The rear wheel drive vehicle has the final drive in the back, AFTER the speedo gear measures the transmission output shaft speed. In this condition the output shaft speed remains constant while the wheels will spin at different speeds from the changed final drive and this change will not be reflected in the speedo. That is why some companies offer changing the speedo calibration in the computer to reflect gear changes. Those computers that read the sensor will have a different multiplier put in to compensate for the different final drive.
Again, you should have to do nothing except move your diff from the one transmission to the other along with the bearing races, and re-measure the shim thickness. While you are at it, upgrade those shift forks since they are easy.
You can do the shift tower later if money is an issue as it can easily unbolt while the trans is in the car.
Former owner of '99 CSVT - Silver #222/2760
356/334 wHP/TQ at 10psi on pump gas!
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'05 Volvo S40 Turbo 5 AWD with 6spd, Passion Red
'06 Mazda5 Touring, 5spd,MTX, Black