Predicting your race finishing time..

I had two projections. The "If everything goes perfectly and I perform at top speed for each event" time and the "but here's realistically what I'll probably do because I'm racing three events back to back." I was within 3 minutes of my realistic time. I actually hit my ideal time goals for the swim and the run, but missed the bike by 15 minutes -- so I missed my "perfectly" time by . . . 15 minutes. :-( (Still kicking myself for that bike performance.) It would seem kind of weird to me to have absolutely no idea how I might run a race.

I really like this calculator I play around with it in the weeks leading up to a race and like the other poster I generally have a 'this is what I will be happy with' knowing it can go either way.

I also think the more you train and the more you race the closer your expectations can be. Throw in an injury or illness and all the numbers can go right out the window. 

I have only done sprints to date, but I find that I'm pretty good at estimating what my finish time will be. It's all based on my training times and what I know I'm capable of. I add a little bit of cushion to my goal time knowing that my times will not be as good as my standalone times for each leg.

For example, my goal for my last race was 1:30 and I finished in 1:31:54.

My predictions are kind of self fulfilling prophecies.  I take my training times for each individual event and come up with a number I think I can hit.  I calculate what pace I need to bike/run at in order to make that time and then I watch my GPS watch to make sure I'm on that pace the whole race.

I generally set my goals pretty aggressive so I'm not limiting myself.  This weekend I am hoping for/predicting a 2:30 Oly.  I know that I need to do the swim in 26-28 minutes, Bike at 21mph, and run at an 8 min. mile pace to hit that.

I came up with the numbers because standalone I can do the swim in that, I biked the course at 22mph standalone and usually run at a 7:30 min pace.  Also, 3rd place in my AG last year was 2:31.  lol

If you train on terrain similar to race it is easier to know from training how your race will go.

I base my predictions (race plan) off my training.  Even in a year like this one where my training is off due to life circumstances, I'm still usually pretty close.  This past Sunday at Musselman, I projected/planned 35 min swim, 3:00 bike, and 2:15-2:20 run.  I went 35:xx on the swim, 3:01 on the bike, then blew up on the run for a 2:30.  The blow up was partially because of not allowing enough extra time for the forecasted heat (97 degrees by the finish), and partly because my training volume has been so low this year, that I haven't had any training sessions long enough to be certain how I was going to hold up.  I hit the wall hard at mile 11, and struggled badly for those last 2 miles.

Normally, I have a "most likely" prediction, a "worst case", and a "best case".  Usually, I'm somewhere between "most likely" and "best case".  Last year in my first IM, my most likely was 13:00-13:30, best case of 12:30, and worst case of just finishing.  I felt like I executed my plan well, and finished in 12:34, but it was helped in part by cooler than normal temps.  In sprints and olys, I'm usually plus or minus 5 minutes of the prediction.

I just look up the course record and thats my prediction

Seriously though, very similar to you I try to formulate estimates based on training or recent race times. Typically I come out pretty close as well.

I usually can predict my splits reasonably well.  Again, from training I have a pretty decent idea of a 'ballpark'.  Challenge comes from translating to course/conditions, so I try never to think too hard about my 'goal' splits during a race.  And, if I race 'well' I don't worry about whether my predictions were right or not.  Similarly, if I race 'poorly', I wouldn't view meeting my goal as a success.

When I race a course I have done before, it's a bit easier to get a narrower (and more accurate) expected range.

I use this handy tool, it's been around for a couple of years, arithmetic.

Swim + T1 + Bike + T2 + Run = Finish Time