Hi Katie,
I would say that 6.5 miles is fairly short, and at least for the first half of that trip you will be driving at about half or two thirds the mileage your car can get once it's warmed up. Also, as mentioned, your mileage should improve once you have 1-2k miles under your rubber. I'd say, be patient, and track the mileage per tank of gas, to see how it develops. If you can. do a few longer trips on a separate tank of gas and calculate the mileage for that tank, to see what you get there.
There is also the learning curve. My mileage has improved somewhat - or at least gotten more consistent, since I bought my Prius. I also notice that loading the car up a lot decreases my mileage substantially.
The bottom line is still, whatever your actual mileage you are doing a lot better than without HSD.
Cheers,
M