HELP! I dont know how fast im goin...

Any of you masters of knowledge know where I went wrong here: I bought a speedocable from auto zone and it was 6.99, supposed to be 15.99(or so) it looked similar so what the hell. I took it home and it's not the correct size at the tranny hook-up. It measures about 3/4 (diam) and the one on my tranny measures almost an inch (diam). Is my trans from a different year maybe? or was my first mistake calling autozone? :P