Let's get all scientific on this one! I propose to calculate your base calorie burn rate (driving). To do this, calculate your base calorie burn in a 24 hour period (sleeping). It is necessary to know your age, weight and height for this. Enter those figures into a calculator here:
http://www.caloriesperhour.com/index_burn.phpFor example: I ended up with a BMR of 1776 (calories) and an RMR of 1716 (calories). I personally chose the RMR rate as it is likely to be more accurate.
So, taking 1716/24 = 71.5 calories per hour. I sleep 8 hours, so, I would be expected to burn 572 calories doing this.
You can then apply factors to estimate your burn rate doing other activities...for a sedentary lifestyle a factor of 1.2 (described as little or no excercise and a desk job) would suggest I burn 85.8 calories per hour in this state.
Thus; if I want to compare biking to work vs commuting I can then estimate the extra calories burned biking. It is estimated that I burn 290 calories in 25 minutes of biking (based on heart rate/gps data). Based on the sedentary factor above - driving 25 minutes would consume 36 calories. So, I need an extra 250 calories or so to fuel my morning commute.
I don't tend to eat more unless I'm hungry (i.e. don't eat more just because your are bike commuting). However, if you do need to eat more - you don't want to eat more than 250 calories worth (in my example) or you'll be overdoing it (and probably gaining weight).
Here are some calorie values for common breakfast items:
http://www.livestrong.com/article/290693-list-of-breakfast-foods-with-calories/My commute might then cost me 2 pieces of toast!? Wanna lose weight? Don't eat anything extra!