The LS1 computer uses the speed input from the T56 & 4L60e transmissions for a variety of reasons including speed signal output to the speedometer, cruise control, lean cruise, idle strategies and anti stall strategies. If you've ever been in an LS swapped vehicle that stalls when coming to a stop, chances are that's due to a lack of speed signal input. The older GM boxes such as the TH400, TH350 & Powerglide units contain an output for a mechanical, cable driven speedometer but no provisions for an electronic signal. This device simply screws into the existing cable output and plugs into your LS harness.
There are two types, pass thru and standard. Pass thru allows you to retain the cable driven speedometer, great for if you've implanted an LS into an older vehicle that uses a cable driven speedometer.
Standard is a slimline version with no output for cable speedometer, perfect for more modern vehicles which use an electronic speedo or if you've TH400/350/Powerglide converted your VT-VZ & VE-VF commodore.
Recalibration of the speedometer is required which can be done through a dyno tuner.