Did you know it is possible to race (either Bicycle or Triathlon) with absolutely nothing displayed on your Garmin Computer ? Within the Garmin menus (Under Training/Alerts) you can set up Alerts for Cadence ranges, Wattage Ranges, Heart Rate Ranges and more. Whilst technically possible this may be a tad boring but the Alert feature is a very useful one that I will talk about later.
(BTW - Did you know the Garmins have a cool feature that allow you to take screen shots using the Power Key)
There are really only two values that need to be displayed when racing although the Garmin 800 has a minimum of 3 fields. The two values are Energy Zone and Cadence. Before you go looking for Energy Zone this is a term I use for the value to measure energy output. For most people this is Heart Rate but for people with a Power Meter this would be Wattage
It is far easier to use Zone than actually Heart Rate. This is because trying to remember the actual numbers for your heart rate zone eg aerobic, threshold, lactate etc can be difficult. By entering these as zones mean you just need to keep the range in that zone. For me, I set Zone 4 as 159-166 BPM so as long as my Zone say 4.x I know I am working hard enough without overly tapping into Glycogen.
For triathlon it is important to maintain an even cadence (so as not to grind) in a range similar to your expected run cadence.
Using these two numbers together is simple. As mentioned I ride in Zone 4 and with a Cadence of 95. If, at that cadence, my HR starts rising I go to an easier gear - if it starts dropping I go to a harder gear. Simple.
On my bike I have a Power Meter. The concept is the same in terms of adjusting gears etc. I use the 10 second averaging as it is much easier to read (smoother)
I strongly recommend against having speed displayed. Speed has too many variables that are outside of your control specifically wind and course profile. Using speed into a head wind can mean pushing yourself out of your optimum zone and 'burning out' your legs for the run. Speed can also 'mess with the head' of some people as they feel they are riding slower than they should but are in fact riding better than expected relative to the rest of the field. Similarly having time displayed can cause similar issues.
I originally mentioned Alerts - this is a feature I use for my Nutrition Plan. By setting a Time Alert for every 20 mins I am reminded when to take on fuel. (I use the Distance Alert on my 910 for the equivalent reminder on the run). By using the Alert Feature it negates the need to have the mind-messing time displayed
As you can see the key is to keep it simple. You have enough to focus on whilst racing without the need to be performing calculations, remembering times, zones etc. By using the features of the Garmin you can focus on racing - and where the draft marshalls are.....