That didn't seem like a big difference to me but it was enough for me to notice the 5mpg difference in our trip. And recently Consumer Reports tested speed vs. fuel economy found that there is a noticeable difference based on the speed that you drive.
The car's trunk and back seat were filled when driving to the east coast; but pretty empty going two states over and the tires were up to snuff on both road trips, the only difference that I can tell is the speed traveled and that the Crown Vic was a year older.
However, when it came to driving to the east coast we had no rush, where as the trip back from Iowa meant a few more hours of sleep before work the next day, so speed was of the essense. Some might say the time saved would be worth the loss of the miles per gallon, I just didn't realize the difference would be that noticeable. Personally I'm fine with any speed, but not to sacrifice safety in the process, and that has to be gauged by the person driving and the surroundings.
Have you found a difference in your mpg depending on speed, lightness of the car, tire pressure, etc?