Frame Size

How tall is your girlfriend?

How long is the top tube on that 54cm frame?

Shes about 1.64m

and as for the top tube im not really sure.... but from what i can see on the pic its about 40mm. only the clamp and a tiny piece under it.... so the seat will be able to go quite low

You are over 6' tall and she's 5-4 and a half. There is no way you can share a bike that will be safe and enjoyable for both of you. Your bike should be in the 57-59 cm range, modern sizing, and hers in the 49-52 cm range if a women's specific frame, 48-50 if not.

That's over a 3-inch gap. If you try to split that difference you'll be uncomfortably cramped and she'll still be dangerously stretched out.

Quoted post said:

Originally Posted by DuToit [IMG]/img/forum/go_quote.gif[/IMG]

Shes about 1.64m

and as for the top tube im not really sure.... but from what i can see on the pic its about 40mm. only the clamp and a tiny piece under it.... so the seat will be able to go quite low

As oldbobcat has inferred, the ONE-bike-for-the-two-of-you is probably not a good idea ...

FYI. Having said that, depending on your flexibility, a 54cm frame may not be too small for you ...

• I know someone who is 6'1" (a CAT-something-or-other racer) who purportedly (I haven't seen it) rides a small GIANT Road frame ... at least, that was the situation a couple of seasons ago
  
• obviously (¿!?), he uses a 130mm stem
  
• plus, a seatpost with a lot of extension showing
  
  

Now, the greater problem would fitting the bike so that it would be comfortable for your girlfriend ... so, it is very important to know the size of the frame's top tube BEFORE you proceed ...

• basically, you might be able to get by with the ONE bike if you were willing to have TWO sets of handlebars-and-shifters + a separate saddle & seatpost for each of you ...
  
• TREK frames used to be measured C-T (maybe, they still are), so that means that a 54cm Trek probably does have a (virtual?) Top Tube which is about 54.5cm ... I reckon that you want the seatpost to handlebar/stem to be in the 68cm +/- range
  
• and, probably a 42cm-to-46cm wide handlebar
  

[*] and, for your girlfriend, the distance to the handlebar/stem junction from the seatpost should probably be about 59cm-to-60cm +/- range .

• you COULD use a DH (Downhill) stem for her handlebars
  
• BUT, anything longer than 60cm would probably be uncomfortable for her to ride UNLESS you outfitted the bike with a set of FLAT bars + short stem + appropriate shifters
  

If you really want your girlfriend to ride with you more than a couple of times, then you should strongly consider fitting the bike to her; and, that probably means a bike with a Top Tube which is closer to 52cm in length paired with a relatively short stem.

In other words, you should probably abandon the idea of sharing the same bike with your girlfriend ...

OR, I suppose you could get a taller girlfriend. Kidding!

All you may accomplish here is end up racing on a bike that doesn't fit you, or make your GF ride on an uncomfortably large bike. Best case your cockpit is cramped and you are too low on the bars (only really a problem if short on flexibility), while your lady will be too stretched out and her weight will not sit appropriately over the steering axis, which will likely (and drastically) reduce her safety factor while riding.