the 11t has to be used with the 15t as they suit the difference in size between the 39 and 43t steel spurs - ie a difference of 4 teeth between 1st and 2nd gear, on both the spurs and pinions.

important bit: nearly forgot - due to the spurs being smaller, the engine needs to be moved closer to get a good mesh. this results in the flywheel hitting the brake seat, so a fair chunk needs to be ground off the seat. also the engine mount adjuster slots need to be extended on the engine base plate.

i have this setup in my truck, but 2nd is proving a prob as i am struggling to get the 15t gear. the truck has lost a little acceleration, and 2nd gear isnt really needed, but its just what i needed for the beach here - very long and flat...

some pics - hope that clears it all up for u