More Time Trial questions

[TxDoc]

For those of you who do time trials, how do your tempo ride times compare to your actual TT times?

Short answer, they don't.
Long answer, see below...

Your tempo zone pretty much spans from 76% of your FTP to 90% of your FTP. Almost everyone agrees that the most accurate way to measure FTP is to follow its definition and take the average power output over a 60 minutes effort as hard as you can. In other words, average power output over a 1 hour time trial.
In time trial you expect to go as hard as you can. This means, as hard as you can go over that course, using all you have, with a reasonable steady pace. It is a hard effort. You will probably want to fall off the bike at 20 minutes, curse the sport and the race at 30, and be about to throw up at 40 minutes. If you are happily maintaining a fast pace and feeling good, chances are you are not going as hard as you can go. Old school crono specialists used to say that if it does not hurt it is not time trial.

So let's say you do your 60 minutes TT and find out that your FTP is 250w. Now you know that your tempo zone is between 76 and 90% of that, so your tempo power output is between 190w and 225w. Just looking at these fictitious numbers you can see that if you ride your TT like a tempo interval - you are generating lower power than you could if you really rode as hard as you can.
Another way to say the same thing is that TT effort is usually in low-mid threshold zone, which is definitely higher than tempo zone.

I am working toward doing my first TT. I do 20 minute tempo intervals on a big loop near my house with 155 feet of elevation gain. The farthest I have gone is about 7 miles in 20 minutes (20.7 avg pace) and the first TT I do will be 12.5, so close to twice as far. And it actually might be exactly twice the elevation, depending on which TT I do first.
Assuming I did that one, a 12.5 mile TT with 300 feet of climbing, is it reasonable to assume I'd do it about the same average pace as my tempo ride?

No. Dangerous assumption. And, incorrect assumption.

Or might it be faster because of race day adrenaline ... or slower because it's a longer distance?

It will be different. And the only way to know whether it will be faster or slower is to test.
When you do a workout that includes tempo intervals, as you are describing, your workout includes one or more intervals in the tempo zone, i.e. one or more intervals with power output 76-90% FTP.
Your first time trial is 12.5 miles, so you are looking very likely a 30-40 minutes effort around 100% FTP.
The only way to accurately estimate what your pace will be in a 12.5miles time trial is to do a 12.5 miles time trial.
Most people find that their 30 minutes test is very similar to the FTP value, and that FTP is about 93-97% of the 20 minutes test. So, an approximation would be that your 12.5 TT output would be reasonably close to your FTP, and maybe a tad bit higher. So that, compared to tempo which is only up to 90% FTP - is definitely quite different.

I know wind and other things play a big part, but generally speaking how would you say your tempo rides compare to your TTs? I don't have a power meter and still working on getting my stupid HR monitor to work ... so I average pace is really my only method of analyzing how I'm doing.

One small recommendation first - start thinking about a power meter. You do not need one to race, but if you want to specialize in time trial, it becomes avery useful piece of equipment. That said, yes, there are a few things you can estimate with speed and/or HR - as long as you keep always in mind the variability of speed and HR and can take the fallacy of your measurement system into account.
For example, you could test and determine your HR for a threshold-like effort, and use that to establish your zones.
Which brings me to a question though... if you have no power and no HR measurements - how did you establish 'tempo'? In other words, you mentioned doing tempo intervals - but how do you know where your tempo is, without measurements? Or maybe are you just temporarily without power and HR, but you did establish your zones with it? I'm kind of confused...

Also, would you say it's important to do a full 12.5 mile mock TT before my first? Kind of makes sense, but I also know you don't have to ride 100 miles to train for a century or run 26 miles to train for a marathon.

You do not have to train 100 miles do complete a 100 mile ride - but to win a 100 mile race you may need to train 130. Likewise, you do not need to run 26 miles to complete a marathon - but to win or place your city marathon, you might.
There is a big difference between completing a century ride and racing one. The first one, you enter with the goal to complete; the second one, you enter with the goal to win or place. BIG difference.

So, back to your TT - there are a few things specific to time trial which is why the time trial specialists train time trial and often try to simulate race conditions to predict how things will go during the race. My recommendation would be:
1-yes, do a 12.5 miles TT, and do everything like you would on race day (same nutrition, setup, bike and wheels of course, same warmup, etc).
2-if possible do a recon on the race course and try a TT there (will help determine entry/exit lines around dificult corners or turnarounds, areas to watch for, any problems with the road, etc).
There's a couple of 'first TT' threads on the 33 on bike forums, good advice so go take a look.
Good luck!!!