GM Speedometer frequencies:

2K PPM VSS

15mph 8 hz
30 mph 16 hz
60 mph 32 hz
120 mph 64 hz
240 mph 128 hz
255 mph 136 hz 

4K PPM VSS

15mph 16 hz
30 mph 32 hz
60 mph 64 hz
120 mph 128 hz
240 mph 255 hz

Convert to 0-5v signal for megasquirt