Originally Posted by HillRider

The ODs of road bars, beyond the center bulge, are all the same. The ID can vary slightly since lighter bars have thinner walls and will be a bit bigger where the bar ends go. The Velox plugs are adjustable enough to work over the entire range.

BTW, Trek dealers have "Bontrager" branded bar plugs that have circular rings around the stub that goes into the bars and these have a pretty firm grip on all of the bars I've used them in. They are very cheap or even free if the dealer likes you.