a lot of cycle computers have available rear wheel kits to pick up speed and tell you distance, etc. If your goal is to lose weight, that's not necessary, but it is nice to see progress. I use a Sigma Sport 1600, which also has cadence (how fast you're pedalling) with a rear wheel kit on one of my bikes. I also use a fairly cheap Polar HRM. I'm considering a combined unit, but the separate HRM is nice when I travel and use hotel fitness equipment.
The 'fat-burning' zones are where you a relatively high percentage of calories burned are from fat as opposed to carbs. More intense excercise will still burn at least as many fat colories. This has been beat to death here and over on www.cyclingforums.com
And doesn't matter all that much anyway.
The primary thing with losing weight is running a calory imbalance: burn more than you eat. It doesn't really matter if they're calories from fat or carbs, if you burn more than you eat, you'll lose weight. So you'd think that the way to lose maximum weight would be to go as hard as you can for the length of time you have available, right? Like if you have an hour available for excercise, you go as hard as you can for an hour. You'll burn the most calories that way and lose weight the fastest, right?
Well, there's a couple problems there:
1. More intense excercise tends to make you hungrier.
2. More intense excercise is harder on the body, and you need to make sure you eat enough of the right things to recover from it.
3. More intense excercise hurts, and many people who are trying to lose weight will be demoralized by this.
4. When you start an excercise program, your body is in no way ready for intense excercise. From my experience, your personal definition of 'intense' changes over the first year or two, edging higher as your mind and body get used to the rigor.
My personal advise, as someone who lost 50 pounds or so a couple years back:
1. Be realistic about how fast you should/can lose weight and maintain health (mental and physical!) a pound a week is frequently mentioned as a reasonable to high rate of loss. I know it's about as well as I can do for any length of time. That's about 500 calories a day deficite, or 1/4 of the fabled 2000 Calorie a day diet!
2. Consider changing your diet as well. Don't "go on a diet." Change your diet. There's a suble difference there - when you "go on a diet" your setting yourself up to "go off your diet" When you "Change your diet" you're acknowledging that you need to eat better than you have been and you're taking responsibility for eating better from now on.
3. Start slow with excercise. At first, you're likely to see a lot of benefits from shortish (1/2 hour) rides at low intensities. The first year I road my bike, I never went further than 10 miles (45 minutes or so). That year was the bulk of my weight loss. A heart rate monitor can be helpful here, as can a cycle computer, but don't be a 'slave to the numbers'. Your average heart rate or average speed aren't why you're doing this, they're how you measure progress.
4. Weigh daily, at the same time every day, with your bodily functions in the same state. Don't freak out over daily fluctuations - watch the trend.
Well, that was long, but I hope it was helpful.