You'd be surprised how much speed variation there can be within a single swipe.

When I had to write a bar code decoder I started with what you suggested but quickly found that very few scans are of such a uniform velocity to make that approach work. Turns out you have to calculate the average length of bars and spaces many times throughout the scan. What you end up with is a sliding average that represents the average width of the previous and next 5 (or some other magic number) bars and spaces. And even then you have to be ready to handle minor printing errors that distort the bars, etc.

Regards,
Craig Steiner