What did I do wrong?

No no no. I enjoy your posts!
As a fellow OCD Prius MPG fanatic I completely understand the pit in the stomach feeling that a poor tank causes. The difference between you and me is that I can blame my wife.

 

My '84 Mustang's 2.3L engine was rated 88 HP. My Prius has 134 HP and a much more useful 'transmission'. Your 'c' has 99 HP, also has the much more useful 'transmission'.

Did the Mustang's looks make it more powerful?

 

Check them and fill to at least what the door sticker says. I personally do the sidewall max, it rides like a truck but helps mileage. My (used) Prius had 25-30psi in the tires when I got it home from the dealer, no good for mileage or safety.

 

A good place to begin with tire pressures is the door recommendation, but I started with 38psi front, 36psi rear. I have settled with 42psi front, 38 psi rear.

 

I remember the police dept here bought some of those low power mustangs. I believe they really could not catch up to someone speeding. If you are going like 80 you could just step on the gas and the police would not be able to catch up.

 

That must have been a local problem. I don't recall any police here buying the 2.3L-I4 that most of us civilians bought. Instead, they bought the 5.0L-V8 version.

It was a very common patrol car for a while. I did like being able to recognize the nearly unique Mustang taillight reflectors from a very long way off at night.

Were you referring to me, or to the OP?

If me, then no way. The low mpg was very clearly due to strong winds. I had to drive that way while learning in an old '63 Ford, to keep it from stalling while braking. But I quickly abandoned that mode after graduating to a '78, then owned only manual transmissions for a quarter century.

If the OP, then I wonder what the current Prius' brake-throttle override does to a two-foot driver. I haven't tested it.

 

The cold weather didn't help you out any. Colder air is denser which hurts the MPGs a bit. Not as much as rain or water on the road, but it does have a correlation.

 

Even when not using cruise control, I'm suspecting that the brake-throttle override will both reduce the excess drag, and provide more feedback to help retrain two-foot drivers back to one foot.

 

My understanding of ambient cold is that the major effect is on the lubricants which are higher viscosity until they warm up.

 

Our first tank of our C3 2012, was 50.03 mpg in December and January in Indiana. One year later @ 10,500 miles that tank was 51.30 both actual for the same time period. Yesterday's fill up for 401.4 miles took 6.982 gal for 57.49 mpg actual in May with air on occasionally, however we have learned to drive the C for better milage. The 0 to 10 mph is very important but after that you can slowly feed in more throttle. We also do a 10 to 60 mph full throttle to clean off the top of pistons and plugs once every tank full.

 

1. probably stopped at the wrong gas station on the wrong day and maybe even wrong hour
2. didn't drive like the EPA computer controlled sequence using gas like theirs
3. perhaps didn't accurately measure your mpg.
4. perhaps your tire pressures are a bit low.
5. head winds, window down, heavier weight?

 

We already established it was his cold weather.