I'm only 5'11 compared to you that is short ;-) But I do have a saddle height of 81cm! The P3 does have a longish top tube, but I ride 80 degree angle. That puts me on a 58cm frame... perfect. I also ride a 2300 TREK road bike (58cm) with a 320mm seatpost to give me the extra hight. So if your torso is longer (which seems to be the case compared to me) you will fit on a 58cm BUT I'm not so sure you will get the cockpit "right". I use a 6 degree stem (12cm) which is the same as my TREK. I guess what I'm saying is a 61 would seem to be the logical choice as you want the "same" as your road bike but also remember the head tube is shorter than you TREK (14cm) which will give you an immediate more aggressive aero position. Read Dan's article on head tube issues on Cervelo's site (technical articles).

Hope that helps. If in doubt go and get fitted.

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For one who has no objective, nothing is relevant. (Confucius)
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I'm 6'0" and I ride a 58cm Trek 5200 with a saddle height of 77.2cm. I just bought a 700c 55cm P3, and it fits great. The easy answer would be to size down, but that makes the assumption that you will be riding steeper on the tri bike than the road bike. If you plan to ride the P3 at a similar seat tube angle as your Trek then you will probably need a similar top tube length. Correct answer- get measured and fitted.

"If in doubt go and get fitted."

Cervelo claims that their bikes are designed to usually fit your same size road bike. That works for me, but not for my wife who rides her P2K one size smaller than her road bike.

This is too expensive a purchase to guess.

Its best to get fit, you may size down slightly, but be careful you will get advice from many here that will end up having you on a 49.