In desparate need of early model 71 dart fuel cap

Either I dont understand how a micrometer works or that measurement isnt correct. .635 in my understanding is a little more than a tenth of an inch over a half inch. Dont take this as sarcastic as I rarely use micrometers but might this be one inch .635 which would sound right for an 1.25 or 1.50 tube?
Your right i forgot the 1, should be 1.635 .