How to improve gas mileage in this situation?

Easy one: slow down. Even if you did and got the EPA hwy of 48mpg, you'd only save 10 gallons over the entire trip. Would it be worth the longer drive times and increased stress of constantly watching traffic approach rapidly behind you for over 2,600 miles? Your call.

We did a similar summer family trip in my CMax with sometimes even higher, legal speeds of 80-85 mph here in Texas and netted 38mpg for the entire trip. It was worth it to me.

 

What tires do you have? What pressure? My '05 will average mid-40s for a similar trip with Ecopias and 44PSI.

Any roof racks or other aftermarket stuff sticking out in the wind?

 

i think you're pretty much maxed out.

 

Can't offer data for the while trip, but can for part of it. I run my tires at 40 psi all around, only run 2-3 mph over the limit, use cruise control and the A/C is set to 74 degrees on Auto.

In August we filled up in Midwest City OK and the next stops were:
- 259 mi to Amarillo TX for 39.6 mpg
- 289 mi to Albuquerque NM for 42.3 mpg
- 140 mi to Gallup NM for 44.7 mpg
- 341 mi to Glendale AZ for 47.3 mpg

In March we filled up in Yukon OK and the next stops were:
- 352 mi to Tucumcari NM for 41.3 mpg
- 314 mi to Gallup NM for 39.8 mpg
- 322 mi to Glendale AZ for 49.8 mpg

In October 2014 we filled up in Amarillo TX and the next stops were:
- 253 mi to Moriarty NM for 37.4 mpg
- 178 mi to Gallup NM for 46.0 mpg
- 321 mi to Glendale AZ for 48.2 mpg

 

Yeah, Fotomoto is right. There are no simple tricks to get better fuel economy when you are traveling 75 MPH. Slowing down is the only thing you can do, but with a car load of people, you don't want to waste everyones time by driving under the speed limit. Avoid using the brakes as much as possible, which on a freeway is easy to do.

The hilly terrain shouldn't affect MPG much, it's mostly the speed that matters.

You choose the right car for the trip, and 40 MPG is great considering the speed.

 

Find a moving van and follow it about 200 yards. That way you can steer around road debris.

Bob Wilson

 

OP went to Disneyland which is a money pit. Here's what I found ONE large beer cost at the equivalent Universal Studios in LA:







Why worry about a few MPG's on the trip?

 

... and that is why flasks were invented. Or a person could exercise willpower. I know the latter option is going out of style.

 

+1

Doesn't have to be a van. Any car would do. Follow with a safe distance. You will notice a nice bump in MPG.

 

As others have pointed out, that is very good mileage for your speed, compared to nearly every other car on the road.

If you want better, you have three options: 1) slow down (already suggested, and not very appealing on a long road trip); 2) reduce weight (again, not very appealing for a long trip, where you will most likely be carrying luggage and extra passengers); 3) if you're feeling adventurous, improve the aerodynamic efficiency of the vehicle. The 3rd gen Prius has a Cd lower than most cars, and frontal area comparable to other mid-size cars. You can't really do anything about the frontal area, but it is possible to improve the drag coefficient. Removing the passenger mirror will help both; solid wheel covers gave me more than 1mpg at 55mph in testing; I run a grill block year-round (sending air around the car rather than through the "dirty" engine bay results in lower drag and less lift; use an OBD2 reader to keep an eye on coolant temperature); my car is lowered, which increases the length relative to the height and can improve drag; if you want to go crazy, wheel skirts and/or a boat tail can significantly reduce drag coefficient. These little things can add up; I don't drive quite as fast you, but my best freeway trip was a fall run from Pennsylvania to Illinois at 60-65mph, where the car returned 70.7 mpg. Ultimately, I'm shooting for 50mpg average at 70mph on a road trip from Illinois to Washington state next summer.

 

Hmm, not concerned about gas mileage, just want to do better...
Slow down, put you tires at 40-38 and relax.

 

I am not familiar with the tires mentioned.
Typical LRR tires we talk about around here are Bridgestone Ecopias, Mich Energy Saver A/S, GY Fuelmax. Not sure your climate there, but the LRR tires not always best in snow.

 

Speed kills mileage and I still don't understand why someone buys a Prius, wants to drive 75 - 80 mph on the highway, then wonders why their gas mileage is in the dumper.

1.) Set the cruise at no more than 65 mph and sit your butt in the right lane and enjoy the ride.
Don't get me started on the myth of speed and time. I drive a Volvo 960 from Pittsburgh to Naples Fl. 1170 miles and the difference between driving the speed limit and 65 mph maximum speed is 45 minutes longer and a 3 mpg gain.


At 65 mph a Prius will get you in the 53-54 mpg range on average, a savings of 15 gallons of fuel on your trip, a tank and a half?
A benefit of the slower speed other than less fuel stops is, you will find that you'll be less tired during the trip.
When I drove the speed limit (70 mph) or higher to Naples I couldn't make it past 13-14 hours with out having to stop for a three to four hour nap in a rest area. With the max speed of 65 mph I can drive straight through (1170 miles), stop at the grocery store, unload the car, unpack and open up the condo, and still go out for dinner.

Three people that have taken my advice on setting the cruise control at a maximum of 65 mph have thanked me after long trips.
My GM at work, she and her husband go on vacation every year to Myrtle Beach and they get between 42- 43 mpg in their 2010 Honda Civic when setting the cruise at a maximum of 65 mph.

2.) Tires should be between 42F and 44F, 40R and 42R

 

I get what you're saying, ALS, but aren't the speed limits on that route 65-70 to begin with, so you're not creating a traffic hazard by going 65? I think it would be hazardous for me to try that on our annual 2100 mile trek to the Savannah area where limits are 75, then 80 and finally 70 once we hit Big Muddy. A bigger problem though is that we're not going to a final destination where the arrival time doesn't matter. The distance between motels can mean that extra hour turns into an extra 2 hours or more because there are only a few cities with motels in West Texas. If we go past El Paso, we have only 3 real choices; Van Horn, Ft Stockton and San Antonio. At 65, San Antonio is out of the question and so is Ft Stockton because we no longer drive at night. Van Horn is doable, but if you've ever been to Van Horn, you'd understand why we stop early in El Paso and then take advantage of more motel options the next 2 nights. Even if we could get past Van Horn, motels prices in both Ft Stockton and short of San Antonio (Kerrville) completely crush any savings from higher mpg, especially with fuel so cheap right now.

That said, I have started taking the back roads between Phoenix and El Paso, mainly to avoid the horrendous Phoenix to Tucson commuters, but partly to avoid the 75-mph limit as long as possible because we have the time. I learned a long time ago how much more comfortable slowing down can be. I used to run up to 85 mph between San Francisco and Phoenix in the 70s and it was white-knuckle through LA back then. We started going slower and stopped once we got through LA. That made the drive so much nicer and we arrived refreshed instead of worn out. We also break other trips into shorter segments now where we can take more back roads and see the country, a side benefit of which is higher mileage. I completely agree with your assessment that 65 is the upper limit if mileage is a goal while still making decent time. 70 isn't too bad, but 75 and for sure 80 are real demons. In my van, we tended to get 24-25 mpg regardless of speed whereas the Prius is definitely more affected above 65. I'm not convinced abut tire pressure though. I drove one trip at 37 psi vs 40 and actually got better mileage. However, there are so many factors on long trips at different times of the year that it's hard to credit tire pressure.

 

Absolutely correct:

There is no 'road hazard' by using semi-trailer trucks as your pacing vehicles. The following traffic will see the truck and smoothly change lanes and pass. This is so easy and so true.

Bob Wilson

 

It may have already been said but don't use your cruise control! Do a search here and look for how to improve your gas mileage video. You need to learn how to pulse and glide, you can't do that when your in cruise control.

 

We will have to agree to disagree:

But then I don't mind running out of gas either.

For example, this July 2013 driving stunt:
Efficient driving for a 1,000 mile tank | PriusChat

Results:

• Cruise control and shifting to "N" on shallow grades
• Ran to fuel exhaustion
  • Another 1/4 miles on battery, and added 1 gal, spare can

There are options and the manual overhead of 'pulse and glide' is like chastity, its own reward . . . and punishment.

Bob Wilson

 

+1 Slow down. Follow based on the 8 second rule. Look 1000 yards down the road not 100 yards. Remember there's a very fragile egg between your foot and the pedals. Stay in the right lane while doing your 63-65mph. Enjoy the extra scenery that magically appears at 63 but is invisible at 73. Don't worry about the hare. Remember the tortoise was the winner.