I live in a relatively rural area with lots of highway miles. Traffic is generally light. My last fill-up told me I got 45.9 MPG. I turn the engine off whenever possible whether it's coasting downhill or coasting to a light that just turned red. Also, I take it easy on the accelerator. I cruise at 50 MPH and accelerate gently.
What else can I do to raise my MPG? The best I ever got was 48.3 MPG last September. The days were hot so my engine would warm up quickly.
You might be able to keep experimenting upwards with the tire pressure. I recommend against exceeding the maximum pressure stamped on the side of the tires, which is probably 44 or 51psi. OTOH, you may have already found the highest pressure that works for you.
A lot of strategies commonly used are not applicable to highway driving. For example, avoiding the brakes; every time you brake you waste energy that you already spent gas making. So, you would take curves with as little braking as possible, and try to catch the "green wave" of synchronized traffic lights. You could learn your car's DFCO behavior and be sure to DFCO when slowing/stopping. There's no braking on the highway.
However, for mostly highway driving there's fewer techniques to apply. You're already driving slower and Engine Off Coasting (EOC) when possible. It might be possible to Pulse & Glide (P&G) or even EOC+P&G on the highway, especially since you're patient enough to go 50mph.
You could try drafting, if you can find a tractor-trailer going 50mph. There's a good thread about drafting linked in my sig, but here's cliff's notes:
Find a clean full-length box trailer. Don't draft anything that might be a flatbed (even with sides attached), as rocks are likely to be falling from it.
Keep a safe 2 to 3 second distance, which is a little further than a common tailgater gets from such trucks. If you get too close the turbulence destroys the draft.
With summer coming, you will probably find better fuel economy on the highway using air conditioning than open windows. You might also try other cooling techniques, like a cold wet cloth draped on your neck. There was a thread about that.
I know there's got to be a couple techniques I'm forgetting that apply to highway driving.
So without further techniques to apply, we get into modification territory:
I am a little hesitant to do any engine modifications because I do not want to void my power train warranty, which is 10 years. The car is 2 years old now so I have quite a ways to go.
Since I started hypermiling, I stopped using the air conditioner. My commute is only 15 minutes so I can persevere through the summer heat with the windows up and make it home OK. I usually run the regular air vent and I roll down the windows when I am coasting to a red light. I never thought about a wet cloth. That's a good idea!
Drafting kind of scares me. I do not feel comfortable driving too closely behind a large vehicle that I have no control over. Besides, every large truck roars down the highways here at 70+ MPH. Makes for a nice whooosh!
Seeing that I spend a lot of time on the highway where my main enemy is air resistance, aero mods would seem to make the most sense. I'm thinking of clean and "easy" things like disc hub caps, lowering the suspension, etc. One of my co-workers said I should work on smoothing out the underside of my car. I don't know how well I could block the radiator grill and still make the car look nice.
The other hesitation with aero mods is the cost vs. payback. 14" Disc hub caps are $100. How many miles will I need to make up the cost?
DFCO is Deceleration Fuel Cut Off. It's commonly but incorrectly thought that your engine will not use any fuel, any time you're in gear and not on the gas pedal (aka "engine braking"). In reality, there's often a complex set of conditions that must be met for DFCO to engage. It will usually engage if you take your foot off the gas for a few seconds at highway speed.
With trucks going 70mph you're better off not drafting since you're willing to go 50mph. However, drafting doesn't need to be scary; a 2 to 3 second distance is a reasonably comfortable distance, not up close at all. It is close enough to get rocks kicked up though.
Disc hubcaps and lowered suspension will probably never pay for themselves in fuel economy gains; do them if you want them for other reasons, but don't expect them to save more money than they cost. It is possible to create decent DIY disc hubcaps from pizza pans.
That last picture reminds me of one I forgot, wheel skirts. However, they are ugly.
It is possible to block the grille on the inside so it's not visible, or if you're crafty you may be able to do it on the outside and have it look good. Otherwise you could just block the radiator itself, and that is very easy to do without being visible.
did you ever say whether you had a scangauge or not? I know it was discussed but if you don't have one, get one. it is a fun little toy but it does take a while to pay for itself.
a good outside grill block can be made of plexiglass (plastic sheeting at lowes) for around $15 or so and can be put on with black wire ties. it doesn't look factory but better than cardboard or other material.
Or, if you're concerned about making the vehicle look funny, you can always block the grille from behind like I did on one of my trucks... I used a flattened out piece of black plastic from a broken trashcan.