Aussie Falcon Enthusiast
Join Date: Feb 2001
Location: Sydney, NSW, Australia
A friend with an auto EL2 XR8 put 3.9 gears in and wasn't overly impressed with the difference but admittedly his car was basically stock.
Generally auto cars will benefit more from shorter gearing but even a stock manual car will apreciate something from 3.5 to 3.7 for an improvement in acceleration. Note though that good traction is important to get the most benefit from it off the line, especially at the strip.