I recently purchased a Garmin 305 forerunner, a wrist device GPS. I inputted my personal info, age, weight, sex and it keeps track of my calories burned. I have found that when I walk, approximately 3-3.5 mph, it states I burn an average of about 85 calories per mile. However when I run, typically 7-7.5 mph, the calorie burn per mile jumps to about 130. This is pretty significant and I wondered if it was accurate. I would think a covered mile would burn the same amount of calories, regardless of how long it took so what am I missing here?

First, welcome!

Second, I have the Garmin 305 Forerunner also and while I like its features, estimating calories burned is a very rough estimate.

Third, running burns much, much more calories per hour and per distance. For example, let's say you walk 4 miles / hour for 1 hour. You may burn 200 kcals. Thus, each mile takes 50 kcals. If you run 6 miles / hour for 1 hour, you may burn 600 calories, so each mile takes 100 kcals.

It's not that accurate. Somewhat liek the treadmill telling you how many calories you've burned in an hour by simply reading your palm by infrared.

Most listings of calorie burn indicate that running and walking the same distance burns about the same number of calories. Of course, since running covers that distance faster, you burn more calories per amount of time by running than by walking. Being higher intensity, you may also get better training effects in the high aerobic or anaerobic intensity range by running, whereas walking is typically in the low aerobic range for most people other than very unfit beginners.

No, it's not just a fact of time, running does burn more calories than walking.

And secondly, i'm pretty sure calories are not calculated by infrared use on the palms. Instead, it is the revolutions per time that does.