What is the "underspeed pulley" mod?

[Topless]

Originally Posted by Steve Tinker

Topless - I fitted my Raby underspeed pulley 3 years ago, and most of my normal driving is between 2-3,5k rpm - no problems as yet.
When I installed the pulley, I temporarily connected my multi meter into the car to get some idea of the voltage output of the alternator. Above 1250 rpm, the voltage was back to standard @ 14.3v, so as long as your alternator is in good condition & you're not continually in heavy traffic with a high electrical load, you should be OK....

Good to know, thanks for the meter read. Now go find a nice tight canyon road and wind her up!