Does the C get better mileage overall??

From The most fuel-efficient cars, Best & worst fuel economy and Best vehicles for city & highway mpg, the regular Prius (aka liftback) provides better highway and overall mileage than the c.

See last page of http://www.consumersunion.org/Oct_CR_Fuel_Economy.pdf for CR's tests. Their highway test is at 65 mph.

At 70-80 mph, I suspect the liftback will yield better mileage than the c due to its larger engine. Toyota gave fuel economy as justification as to why then went from the 1.5L 1NZ-FXE engine in the Gen 2 Prius to the larger 1.8L 2ZR-FXE in the Gen 3 liftback. Well, in CR's tests, the Gen 3 got better highway mileage but worse city mileage than the Gen 2 (ConsumerReports.org - Most fuel-efficient cars).

Prius c uses the 1.5L 1NZ-FXE, but supposedly has undergone significant redesign besides a whole bunch of other stuff that was added.

 

Look at how badly non-hybrids do in their tests.Please see above PDF about their tests. The short city drive is what's killing their mileage and that of ALL cars. There are 0 non-PHEVs/EVs that do better than the Prius c in their city test.

From Best & worst fuel economy, the Civic HF gets only 21 on that test whereas LX and EX versions only get 19 or 18 on that test. Are the high-mpg versions of the Chevrolet Cruze, Ford Focus, and Honda Civic worth the money? mentions some "40 mpg" (EPA highway rated) cars. For easier reading, this is what those 3 got in CR's city tests:
Civic HF: 21 mpg
Focus SFE: 21 mpg
Cruze Eco (automatic): 17 mpg

If you want to read about the EPA's tests, see Car and Driver: The Truth About EPA City / Highway MPG Estimates | PriusChat. Ratings are at Compare Cars Side-by-Side.

The highest average speed in ANY of the EPA's tests is only ~48 (!) mph.

 

I took a 500 mile trip last weekend in my C. Bucking a 25-30mph headwind, going 70-75, I got about 38 mpg. On the way back I got 50-ish going same speeds.

My daily commute is at least half highway at 60-65. My fill ups are approximately 50-51 mpg.

The C is pushed around a bit by the wind. Road noise is higher than a regular Prius. The bow wave of trucks pushes it around, too. This is about the same as every other smaller car I've ever driven.

The ride was smooth with 38/35 psi.

I've had worse, I've had better. The C won't fit everybody but it gets great mpgs and is easy to live with. After 4200 miles, I'm pleased.

 

With high speed highway driving my money is on the Liftback but in just about any other situation I believe the c would get the best mpg.

The problem is the two cars are so different. The c is a lot smaller and less refined. Some of the interior appointments feel/look even cheaper than the already cheap looking GenIII. That was the biggest turn off for me. If you are ok with that and want max mpg/efficiency then get the c. Fact is, a lot of Prius owners start slowing down once they see what speed does to their mpg. So your 70-80mph trips may go down to 60-70mph with time.

 

On a trip last year, summer time, round trip from Seaside to San Francisco, taking Interstate 5 and the coast route back we averaged 54 mpg calculated. This is with three people
luggage etc. gross vehicle weight of 3,850 lbs.

At 70 mph or so mpg's are around 50 -52 calculated. At 50-60 mph mpg's are 55-60 mpg calculated as was the speed on the way back along the coast up to Seaside.

 

At speed greater than 70mph, the regular Prius (2nd or 3rd generation) will get better fuel efficiency than a Prius c - due to the regular Prius' more aerodynamic kammback form. The Prius c lighter curb weight and smaller frontal surface should give it a fuel efficency advantage over the regular Prius at speeds under 45 mph. My 2010 Prius achieved 44 mpg @81 mph on a wet road(in the rain) at 70 degrees fahrenheit (tire pressure set at 38 psi front/36 psi rear) and has achieved as high as 67 mpg @50-58 mph on dry road at 70 degrees fahrenheit (tire pressure set 44 psi front/42 psi rear). Highway speed really causes the MPGs to drop...