tri bike fit compared to road bike fit

How tall are you and what's your inseam?

I ride a 51 or 52 road bike and a small tri bike which is usually a 51. I rode the P3 in 51 and it was a little to long for me. I could make it work but I do better on different style of tri bike shorter and taller.

I'm a tad over 5'5".

HI Kathy!
I am 5'6'' with a approx inseam (from what I could measure) 77cm.
Is the geometry of the P2 and P3 different? What type of tri bike do you ride? Thanks for your help!

You can look here to compare different bike's geometry.

Poke around and you can learn a lot about bike fit there.

P2 and P3 in 51 have same geometry.

I have an Orbea Dama road bike and my tri bikes are Kuota K Factor in small and Orbea Ordu in small as well. You can look up the geometry on same link at I posted.

 Competitive Cyclist has a fit calculator on their website.

My computer is acting up so I am having tough time typing...sorry for short answers.

I ride a 56cm Madone and I ride a 54cm P2. So the one size down rule works good for me. But it would be a good idea to get a pre fit before you buy the bike. The last thing you want is a tri bike that is too long and low for you or one size off. Or if you plan on getting the bike no matter what it would be wise to get a fit from the bike store after you purchase it.

Close call on the step down in frame size.  I dropped from 56 CAAD 9 to a 54 P3 and it definitely feels like the right size.  While I was able to dial in the road bike quickly, the P3 is a different story.  I love the Cervelo, but I can tell it will take some time to get it to the point where I have my road bike.  I had the initial setup on the P3 set to a pretty neutral position and figured I would work backwards. 

How aggressive is the setup on the P2?

Something I have noticed men typically get a size down going from road to tri bike but not so much women.

I looked at the measurements of my road and tri bike since we are similar in height.

From center of the crank up tube to top of middle of my saddle: road bike-69.2 and tri bike 70.2

Back of saddle to middle of the base bar: road bike 74.9 and tri bike 73.4

My point is there are slight difference to road and tri positions so you can see if you think the bike might work for you. For me the Cervelo reach was the issue in the 51.

Have you considered asking the person if you could test ride it and see if it can be fit to you before you buy it? They shop you had your road bike fit may be willing to help you for free or a small fee to set it up for you and let you test ride it before you buy it.

I have the bike currently since the owner lives in NYC and her mom (who lives here) is the one selling it for her. So they let me take it home and I have it on my trainer so I could play with the positions. I can really keep it for as long as I need but I don't want to abuse that privilege since they do want to sell it, so need to figure it out. I haven't had time to get out on the road with it...hopefully this weekend. I will give the bike fitter who did my road fit a call and see what he can do since I will be bringing it back to him if I do indeed buy it.

Calling your previous fitter is probably a good idea as they may still have your measurements/dimensions on file.  Every bike and body is really different though.  Both my road bike and my tri bike are the same size (48), but if I'd gone with Felt for my tri bike - I'd have gone up to a 51!   So 'rule of thumb' is often broken...

Well, first set the seat position.  You would want the same distance from the BB to the center-top of the seat as on your road bike (unless crank lengths are different).
On a TT bike, the seat will not be set back as far behind the bottom bracket.   I suggest starting with the seat oriented fore/aft with the front of the saddle's nose directly above the bottom bracket.  You need to make seat fore/aft and height changes together as changing one changes the other.

Once the seat is there (remember it's just a starting point), you assess the distance to the arm rests and their height.  If you are in the ball park, but find you would need a new stem to make the fit then you should definitely still buy the bike. 

Can you have someone snap some photos to show us here how it fits?
I could tell you whether the bike is close enough to buy and then have your fitter take it the rest of the way...or whether it's not going to be right and you should pass.