Fuel is important - to a degree.
However, if you are going to "bug-out", that possibly means you will be more worried about dealing with getting around obstacles than how much fuel you will be using.
Every person's situation is different, but mine is such that I live at what I consider to be my destination for a "bug out". So any of my vehicles will get me there, with a full tank of fuel, from any place I am likely to be driving them.
I am not saying a vehicle like the Prius isn't good to have as a daily driver, but if SHTF, I wouldn't be driving anything on a daily basis, much less a Prius.
My daily driver, a BMW coupe, gets reasonable fuel mileage (during the summer, it is 28 to 32 MPG in mixed driving). I drive it because of that mileage, because it is dependable, because it is comfortable and easy to drive.
But if SHTF, I would go home and stay home. If I needed to go out some place, I would probably be driving one of my 4x4 trucks, depending on what I needed to do, the small one or the large one.
I wouldn't worry too much about fuel mileage because most of the time I would be staying home. I would anticipate that my usage of fuel would be about what it is now with those trucks, maybe at most a tank of fuel a year. I also have a 250 gallon diesel tank in my shop, and once I set it up, it will be full.
If SHTF, then fuel range (not mileage, range) will be an issue for some people if their bug out destination is quite a distance, but mine isn't. Even my secondary destination (my backup BOL) is not that far.
I recommend that a prepper think about the fact that they may very likely find themselves hoofing it to their BOL, in which case a really remote BOL may be unreachable without caches and places to shelter on the way, along all of the alternate routes.
I would estimate that if a person isn't injured or sick, and doesn't have small children or elderly in their group, and they are in decent shape, they have food and water, they can make about 10 miles a day on foot. That BOL 400 miles away suddenly becomes a 40+ day hike at best (you will need days of rest, and alternate routes can make your hike a lot longer).
Sure, there are people who can, under optimum conditions, hike more than 10 miles a day. But my estimate is conservative, and it could easily get worse if you are sick, injured, etc.
Still it all boils down to fuel, you have to fuel your body for the 400 mile walk. Odds are the walk would be impossible in a mad max time. Food and water for 40 days would be difficult for anyone unless you picked a route that had what you need and no one else is on it.