I plan to upgrade my brakes levers to Tekro Aero plus buy and install Tektro interrupter (cyclocross style) brake lever(s) (http://www.tektro.com/_english/01_pr...r&sort=1&fid=3) on my French made mid 1980’s Motobecane Grand Tourer with Weinmann center pull brakes. The bike is stock and has alloy drop bars. This upgrade is to replace the stock Weinmann brake levers and rubber handlebar grip (not grip tape). The grip is an 'imitation leather' wrap (including stitches) made of rubber, looks like it was almost 'shrink-wrapped' onto the handlebar & brake hoods, can't be removed without being destroyed, and the entire cockpit is anti-ergonomic: I have to move my hands a good distance to grab some brake. As is, no adjustment to the brake lever position is possible without cutting off the rubber grip.

I've read about the utility and ergonomic benefits of upgrading to Aero brakes (ride/brake from the hoods), it's not prohibitively expensive, and I'm sold on it (the bike is a tad big for me and I'd also like to replace the stem for one with a shorter reach). As long as I'm going to do the Aero swap and put on bar tape (and hopefully swap the stem), I plan on putting on the interrupter brake lever(s). Most my riding is playing in NYC traffic and my thinking is I really only need the interrupter lever for the front brake (left side) because if I 'm going fast enough to need the rear brake I'll probably be in the drops anyway: 90% of my NCY riding is on the bar tops (can't really grip the stock Weinman hoods due to anti-ergonomics).

The Tektro Interrupter brake levers are sold in three sizes intended for different (outside diameter) handlebar sizes:

RL720 - For use with 24mm OD handlebars
RL721 - For use with 31.8 mm OD handlebars
RL726 - For use with 26.0 mm OD handlebars

I measured my handlebars’ OD with calipers at the ‘top grip area’, where the rubber grip ends and the bare handlebar is exposed: I get a measurement of about 23.62mm. However, I notice that the handlebar’s OD is slightly, but noticeably, larger at its center point where it’s held by, and makes contact with, the stem (I didn’t unscrew the handlebar from the stem and measure). Which of the two dimensions, grip area or stem area, is the one which designates handlebar size?

Further, looking over standard handlebar sizing charts is somewhat confusing and some measurements differ by just half a millimeter: what size handlebar would mine be classified as?

Thanks a bunch.