I have a Cervelo P2C with original components. I am looking at making some changes this winter, but have no idea what I can or can’t do and not a lot of expert tri people in my area. My biggest question is on gearing. I’d like to have a few more climbing gears, but am not sure what is the best way to obtain this without compromising the bike in any way. I’m also trying to decide on the best wheel set upgrade to make without having to get into a situation where I am switching out wheels for different purposes. I have 650s so that limits things a bit. Any advice would be great!
It would be helpful to know if the bike currently has a standard or compact crank-set.
Standard. It has all original components other than general maintenance stuff.
What year P2C?
I really depends on how strong of rider you are and on what type of terrain.
I ride a 48 P2 and use a 53/39 up front on a standard crank. I’m a fairly strong rider and rarely run out of gear in my area that is really flat. Typically for training I use an 11-23 in the back. If there are a lot of climbs I have an 11-28 that I will put on.
Now my GF rides 650’s too and she is riding a compact crank 50/34. She is a fairly strong rider for her age and still never runs out of gear. While doing intervals she will get up to near 25-27mph and races near 21mph. The 34 is a really low gear that will get her up pretty steep hills.
She’s done a really hilly Half (Little Smokies) and has plenty of gear using a 12-27.
How sure are you that your cranks are “standard” (in the strictest sense of the word)? When i worked at a store that sold Cervelo’s, they came with compact cranksets. In the sport of cycling “standard” refers to a specific size rather than “what came with the bike.” I had my compact crankset swapped for a standard one so it is obviously possible that you did too, but based on your posts I’m just concerned that you don’t know what you have on your bike (which could cost you money).
If you’re using 650c wheels and you have a standard 53/39 crankset (or thereabouts), that’s a combination that’s already equivalent to someone on a 700c bike using a compact crank because of your smaller wheels. Your first move should simply be to get a 11x27 or 12x27 cassette on your rear wheel. I don’t know what components came standard on your bike, so I’m not entirely sure what cassettes will fit your rear derailleur, but odds are you’ll be able to find something in that gear range that will work. (You’ll need to be sure your chain doesn’t need to be replaced with a bit longer one to handle it … your LBS can help you with the cassette change and the chain, if necessary.)
Try that first. If that’s not enough to handle the hills you’re dealing with, only then would I consider going to a compact. Seriously, if you need a compact at that point, you need to go to the weight room or something and get stronger. 
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Thank you. I have read reports that Cervelos don’t handle compacts very well, so trying to avoid having to do a lot of tweaking. I’m pretty confident it is a 53/39. Tom Demerly sold me the bike at Bikesport in Michigan and we only modified the aerobars. I ride what I would consider moderately hilly terrain–Bloomington, IN, Nashville, and surrounding counties.
My fear is that since not a lot of people at my lbs work on tri bikes, they will try to put something on my bike that doesn’t fit properly. The new Cervelos have a different crank set, so I am wondering if their new set up would fit on mine. I also don’t know if I should just start with a different rear cassette or swap out an entirely different crank set. I wish I knew more, but I don’t.
Your older Cervelo will have a pretty standard bottom bracket. I wouldn’t mess with your crankset right now. Your easiest and cheapest route is just to get a new and larger cassette on your rear wheel. Do you know if you have Shimano components or SRAM? I’d bet Shimano in which case you should be able to get a 27 tooth cassette on the rear. A 27x11 cassette would be ideal, cuz that would also give you the best top-end for the downhills.
I’m an IU grad. I know your terrain well. I was in the stands for the crowd scenes at the filming of Breaking Away. FWIW.