Calling on Leegocrap and other bike experts - Help with choosing a rear cog

Just want to make sure I am interpreting this correctly.  On your tri bike, you mostly ride in the small chain ring up front, and the smaller cogs (the faster ones) in the rear, thus causing the cross chaining?  And if you shift to the big ring up front instead, you are forced to use the larger cogs (the slower ones) which causes worse cross chaining?

What are the chain ring sizes on your crank?  Is it 50/34?  Do you mostly ride in flat areas or do you often ride hills?  Do you find that you want more gears when riding hills?  

Good question and I'll have to double check that shortly when I get home.  Tri is a 2014 P2 so FSA Gossamer BBright 50/34 , can't remember length.  It wasn't swapped out on the 3 fit sessions I had.

Road is Tiagra guessing 172.5 or 175.  I just looked at both not too long ago, just can't be sure off the top of head.

Yes you got it and correct 50/34.  Pretty much all the races I do are flat.  I train in a hilly area part time but typically use road bike for that.  If I was on tri where I live I would be looking for more gears.  I got used to the triple on hills.

Drop the big ring down a few teeth. I'm running 46/34 on my gravel bike, and when I put it in road mode, I use a 12-23 10-speed cluster for nice tight gearing but still able to take decent hills on the 34. On my road bike I've got 50/34 and a 12-25, and it seems I'm running just a bit left of center on the cluster most of the time in the big ring (which I rarely shift out of on most regular rides).

Looking at gear calculators, I think your current setup should be okay so long as you are shifting properly.  50/23 and 34/12 seems to be the gears you want to ride, but they are cross chaining.  However, they are not nearly the same gear.  

At 85 rpms, 50/23 should get you 14.6 mph and 34/12 should get you 19.1 mph.  

If you want to simulate the 34/12 gear (19.1 mph) without cross chaining, then switch to the 50/17 or 50/19 gear which is right in the middle of the cassette.  You may have to change you cadence slightly to match the same 19.1 mph speed, but it's still a good option rather than riding 34/12.

If you want to simulate the 50/23 gear without cross chaining, then switch to the 34/15 or 34/16 gear which is also right in the middle of the cassette.

Now if you want more gears for climbing, then obviously choose a cassette with easier gears.  Maybe a 12-28.  But for flat riding, I think you should be okay with 11-25.

This guy's page is pretty helpful for comparing cassettes:

http://home.earthlink.net/~mike.sherman/shift.html

Have fun.  It really depends on your strength and terrain.  

Thanks I'll read that tonight

Now reading this I'm starting to understand better how to talk about this, thanks.  So if I figure out hat the gear combination is on road bike can that be converted?

Based on this stick with same cassette and just combing ear combinations?  I was thinking a different cassette was answer but that was a guess

Thanks Shane

I realized this morning I could get the same cog I'm running now for $25 on ebay.  Last time I was in LBS they wanted over $100 so I didn't want to experiment too much at that time.  But you're right, going to try another one out also.