Facts and figures
| Component | 10 speed | 11 speed |
|---|---|---|
| Cog spacing | 3.95 | 3.9 |
| Shifter cable pull | 3.4 | 3.6 |
| Pull ratio | 1.2 | 1.1 |
Calculations
Each click of a 10 speed shifter moves the 10 speed rear derailleur 3.95mm using 3.4mm of cable pull. An 11 speed shifter pulls 3.6mm per click and to use a 10 speed rear derailleur will need a modification to the pull ratio so it moves 3.9mm (the 11 speed cog spacing). So we need to move the cable mounting point 3.6/3.4×3.95/3.9=1.07. In other words, we need to put a spacer under the 10 speed cable mounting point that will make the lever arm 7% longer. Arm length is about 31mm, so a 7% increasing spacer would be about 2.1mm thick.
Conclusions
An extra 7% longer is not a huge amount, so at least on paper it would seem possible to adapt a 10 speed derailleur for use with 11 speed shifters.