My road bike has 110mm stem and 55cm (effective) TT.

If I were to get a TT bike...how does this relate?

General opinion is to go 'one size smaller'.

I use 650 total top tube (54+110) for my road bikes, and like ~630 for my TT bike; but I slam the saddle all the way forward to open up my hip angle.

Do TT frames have an inherently shorter top tube at the same seat tube length?

Never checked. I'd think it'd be the other way around, though, because the head tube should be ultra-short for a TT bike. You want to rotate your entire body forward with the bottom bracket as the 'pivot' point.

Remember that the 'handlebars' are much longer on TT bikes than road bikes.

Here is a post I made regarding a similar post in the tri forum. Now, things are a little different if you have to adhere to UCI fit rules, but I think it's still relevant:

"I think that there is somewhat of a confusion on the exact nature of road to tri sizing. Many companies have now adapted size naming so that a person who rides a 54cm road bike will also ride a 54cm tri bike, however, the top tube of the two bikes will be different, the road bike being longer.

For instance Cervelo 54cm:
P2(tri) at 78* = 53cm top tube
S2(road) at 73* = 54.5 cm top tube

Felt 54cm:
B2(tri) at 78* = 52.8 cm top tube
F3(road) at 74.5* = 54.5cm top tube

So in effect your top tube will be shorter on your tri bike, but in many cases the size name will be the same. Obviously if your proportions are different than most people or your riding style is different than your bike selection may be different. One clear example is that if you are riding in UCI time trials, then you may need to size down a frame size because your seat will be farther back then if you're riding triathlons.

At ~6' you could probably be anywhere from a 54-58cm depending on the company and their fit characteristics."

Thanks for the input. FWIW, the "size name" is usually the seat tube measurement, not the top tube.

This is my biggest hold-up. I'm almost not even sure how close to 55cm (top tube) i should be given the fact that my elbows are where the 110mm stem would start.

My road racing bike (Ridley Noah XS) has an effective top tube length of 52cm. With a 100cm stem it fits me rather well at a 10cm saddle-bar-drop.

My TT bike (Slice Hi-Mod 51) has a top tube length of 50.5cm. I use a one-piece aerobar setup from Vision for which I cannot remember the specs offhand, but the saddle-bar drop is 13cm or more. I am able to extend the seat post higher and get a more compact forward position. This gets me a slightly better CdA than my old time trial frame which was larger. The results have been around 1mph increase in TT speeds (at same power) since I got the new frame.

Basically, what I am saying is that my anecdotal evidence backs up the common practice of going "one size smaller".

Geometry on different bikes, and how the Manufacturers measure, is going to play into this a lot. I have a 59cm Merlin Extrlaight, a 23" (58.4cm) Schwinn Paramount, a M/L Giant TCR Team Advanced, and a 56cm Cervelo P2C TT bike.

And even though there is a 3 cm range, they all fit.

The significant thing is they all have almost the same length TT. The Merlin's TT is 57.5cm; The Giant's effective TT is 57cm; The Cervelo's effective TT in the 75 degree setup is 57.3cm, and IIRC the Schwinn is also57 cm.

All that said, my Cervelo is nominally one size smaller than my road bikes.