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Oh, too short, not too long. I wouldn't have expected that. By how much?
You don't mention the design. Does it screw into the door frame, or is it one of these: [link]?
If the latter, maybe you could buy some pipe at the hardware store to extend the bar a little on each end, or build out a spot on the door frame to make it narrower where it needs to be.
weird, I get a lot of relevant results using “tension pull up bar”. Here’s one from amazon though: