Here are all the R-21 ratios. The R21 turbo is the one you want to start as a base for modification. If you do the 1 piece input shaft you can change the 1st gear to a more suitable ratio. Off the top of my head, it was about 2.6. While you are at it install a Qauife type TBD. Both my gearboxes have Quaife Torque biasing differentials in them. HUGE improvement!
UN1 gear ratios:
............ .21 Turbo ... Esprit GT3 ... Esprit SE, S4, S4s, V8
........... .UN1-013 ... UN1-028 .. ... UN1-016 026 & 027
1st ...... 3.364 .......... 3.364 ........... 3.364
2nd ..... 2.059 .......... 2.059 ........... 2.059
3rd ...... 1.381....... .... 1.41 ?? ........ 1.381
4th ...... 1.03 ............. 1.037 .......... 1.037
5th ...... 0.8205 ......... 0.76 ............ . 0.8205 ... (V8 = 0.76)
Final ... 3.44 ............ . 4.11 ............ . 3.89
1st, 2nd & 4th gear ratios are the same between the GT3 and Renault 21 Turbo versions, but 3rd, 5th & the final drive ratios are different. The final drive is very different, and that would make a huge difference in all the overall ratios (individual ratio X final drive ratio = overall ratio)
1st Gear ......... Overall
3.364 x 3.44 = 11.5722 21 Turbo
3.364 x 3.89 = 13.0860 Esprit SE, S4, S4s, V8
3.364 x 4.11 = 13.8260 GT3
3th Gear ......... Overall
1.381 x 3.44 = 4.7506 21 Turbo
1.381 x 3.89 = 5.3721 Esprit SE, S4, S4s, V8
1.41 x 4.11 = 5.7951 GT3
5th Gear ........... Overall
0.8205 x 3.44 = 2.8225 21 Turbo
0.7600 x 4.11 = 3.1236 GT3
0.8205 x 3.89 = 3.1917 Esprit SE, S4, S4s, V8
I copied the above data from an old thread. You can find just about everything known about Renault Transaxles If you search "Renault UN1 transaxle, everything"