Been looking at swapping the diff ratio to 3.7's in my Falcon. Its a 4 speed Auto.

Will i have to do anything with the speedo? Or will it self learn etc?

Has anyone else done this on theirs?

Cheers.

If your BF has ABS (which they all do unless they're a ute I think?) then the speedo will be fine as the reading for the speedo is taken from the ABS\wheel speed sensor and the diff ratio change will be before that. You might have transmission shifting issues if they shift depending on speed as opposed to RPM and throttle position? Although it might self learn with some driving. I know of someone who went from 3.45 to 2.73 in a BA and didn't have any issues.