Raymond,

TH325 gear ratios 1st 2.74 2nd 1.57 3rd 1.0 OD .67 (TH 325-4L)

TH425 gear ratios 1st 2.48 2nd 1.48 3rd 1.0

If you're NOT shifting it at all, then I don't see any reason why you'd have to modify the trans. If you DO break it, then see what goes, ... Login to read more.