For years the standard rear end for an OD was 4.11. You can fudge on this down to 3.73 if no load hauling or trailer pulling is in the picture. T5's had several OD ratios from about 18% to near 30% so you need to find out what you have. If you have a 30% then take 0.7 x 3.08 and you get 2.16 for a final drive ratio in OD. You didn't mention your engine so if it's a 502 big block with twin turbos then it can handle it.
