I have A 74 gran torino with a 460 and c6 The transmission shifts 1-2-3 rd gear before 20 mph and was wondering what the problem was thanks

 

Check the vacuum lines to the modulator, the line starts at the back of the intake. It will have a few inches of rubber line, to about 3 feet of steel and back to rubber and hooks to the pass. side of the tranny at the rear. Rich