In two words: fuel economy.  The energy used to overcome friction and air resistance is exponential with velocity. In simple words, this means that the energy needed to drive 50mph is far more than double that needed to drive at 25mph.  What that also means is that fuel economy drops off very quickly with increases in speed.  Just 10 miles per hour can mean a change of fuel economy of 25% when over 65mph.  Naturally, it depends on the cars exact, current, physical traits. However, that's quite a huge difference when you're paying $4.50 a gallon.

We recently moved to an area without any nearby major freeways or interstates. In our daily driving, we only have a small stretch that allows 50mph with the rest being 35-40 mph. How has this impacted our fuel economy in our Honda Accord Hybrid?

Interestingly, it's put it much closer to the EPA highway numbers, which test in similar conditions to our off-highway driving here. In real numbers, we've gone from getting 28-32 mpg to 35-40 mpg. If you look at the middle ground, that's 30 mpg to 37 mpg, or just about 25% increase in fuel economy.  That's a per mile cost reduction from 15 cents to 12 cents. On a 50 mile per day commute, that's an extra $1.50 for the morning espresso. And that's almost purely because of the different speed limits.

Strange how driving on the freeway where we used to live gave EPA city numbers and driving on slower, surface roads is giving EPA highway numbers, huh? That test really needs to be modernized.