go with a 13-27, it provides a bail-out gear for the really steep stuff. how often do you really spin in 53 x 11 anyway?

depends on your fitness and course, ie how low a gear you'll need at race pace. but try a 12-25 or 13-27 rear cassette, 12-25 with a 39/53 front will get you through most courses but don't be scared to go for the 13-27 or maybe a switch to compact cranks might be in order. I personally only run 12-25 if the course has some straight up "walls" in it, usually train on a 12-25 and race a 11-23, about an hour 40k in a race

I ride on the flat, spin 90-100, and average 21-23mph for a half ironman.

I roll on 53x39 up front and a 13-25 in the back. I'm not sure of the math, but I don't spin out until I'm in the mid-30's, and that's just fast enough.

My personal opinion is that most people are on way more gear than they need.

what cassette and chain rings do you run now?

If you need more lower gears (and sounds like you probably run Shimano 9) and if you dont already run a 12-27 (or 12-28 BBB cassette which Shimano compatible) give that a go first as this is probably cheapest option before going to a compact front. If campy there is a 13-26 and 29 option

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Simple Simon
Where's the Fried Chicken??

If you do a lot of group riding this won't help, but if you ride by yourself I would recommend going to a smaller chainring combo. I have ridden 46/36 (110mm) before and also 48/38 (130mm). Frankly, most people don't need a 50 or bigger chainring unless they ride with other people because either:

1) They can't push 53 x anything on the flats at a decent rpm or

2) They use the big ring to go really fast on the downhills which is a waste of energy (observed from riding with a power meter).

A ten-tooth jump will give you a nice quick shift and the 36 or 38 will combine with a 23 or 25 to give you a relatively decent low gear. If the hill are really steap or long then try a 48/34 with the 25.

Chad

It really depends on the course. And changing cassettes is more typical (and a lot easier) than doing chainrings. I run the standard 53/39 up front (700c wheels) and an 11-23 out back for most cases. If it's a hillier course I'd do a 12-25. Gearing also depends on your pedaling style and strength. If you're a spinner, you'll need lower gears than if you mash. I typically TT at about 100 cadence, so I'd obviously not need as high of gears at the same speed as someone doing an 85 cadence. I have only spun out my 53x11 on two occasions, and neither was during a race. It is better to err on the conservative side- if you happen to spin out your highest gear, just consider that your time to rest for the run. A couple weeks ago at the Eagle Creek Sprint Tri, the highest gear I needed was my 53x12 on a downhill section. I was kind of suprised- It is rare that I get so close to my 53x11. I did get the fastest bike split of the day, though (not trying to brag- but your strength/speed plays into your gearing... a BOP rider won't need near the gears of a FOP rider).