Condensed Answer: Only a 9-speed XT derailleur can operate with Ultegra shifters (10-speed models) by default. If the goal is to use 10 or 11-speed derailleurs, an adapter will be needed.
Cable Pull and Rear Shift Ratio
To understand the criteria for compatibility between a shifter and a derailleur one has to become familiar with two terms – cable pull and rear shift ratio.
The cable pull of a shifter indicates how much cable the shifter pulls or releases during each click/shift. The value depends on the number of speeds at the back and the shifter type (road or MTB).
The cable pull of indexed shifters is pre-determined and non-changeable so that each click of the shifter is identical.
The rear shift ratio describes how much a derailleur moves per 1mm of cable pulled or released by the shifter.
For example, if the shift ratio is 1.7, then the rear derailleur moves 1.7mm per 1mm pulled or released by the shifter. Just like the cable pull, the rear shift ratio changes with speed increase/decrease and bike type.
The different rear shift ratios and cable pulls could make certain shifter + derailleur combinations non-workable.
The table below contains the rear shift ratio of Shimano’s derailleurs:
| Brand | Number of Speeds | Rear Shift Ratio (MTB) | Rear Shift Ratio (Road) |
| Shimano | 8 | 1.7 | 1.7 |
| Shimano | 9 | 1.7 | 1.7 |
| Shimano | 10 | 1.2 | 1.7 |
| Shimano | 11 | 1.2 | 1.4 |
| Shimano | 12 | 1.2 | 1.4 |
Ultegra shifters come in 10/11/12 speeds. XT derailleurs cover the same range but there is also a 9-speed unit.
As you can see in the table, 10-speed Shimano road derailleurs have the same rear shift ratio as 9-speed MTB derailleurs, namely 1.7.
This means that a 9-speed MTB derailleur can be integrated into a 10-speed road drivetrain with road shifters. The only XT derailleur designed for 9-speeds that I could find is Deore XT RD-M772-GS. Unfortunately, that derailleur is old and hard to source, especially locally.
The vast majority of XT derailleurs, however, are designed for 10/11/12-speed and have a different rear shift ratio than their road equivalents. Consequently, those cannot be used with Ultegra shifters.
Conclusion: Only 9-speed XT derailleurs can work with Ultegra shifters (the 10-speed models).
Alternative Solution
Another option is to install a product known as Wolf Tanpan. This is an adapter that changes the pull of 10 and 11-speed road shifters so that they can work with 10 and 11-speed MTB derailleurs. The primary goal is to get a derailleur that can cover a larger cassette and acquire MTB gearing on a road bike.
