How accurate is CDI fuel economy calculator?

[shertex]

Posed this over on Benzworld W211 forum...thought I'd check here has well.

For those of you who actually track your fuel economy, how accurate do you think the W211 fuel economy calculator is? Mine was reset 2700 miles ago and shows 36.6 mpg. Tracking via Fuelly over 2100 miles, I'm at 32.9. Assuming that my Fuelly measurements are essentially perfectly accurate (simply miles traveled and gallons used), and acknowledging that there are 600 miles unaccounted for in the Fuelly measurements, it's curious that the discrepancy is as large as it is. A couple of folks who track these things have found that the calculator is skewed about 1.5 mpg high. So I guess my expectation is that that will be my gap over time.