My girlfriend's Yaris has a leak between the engine and transmission; might be the main bearing is what I am thinking???
Comment 15:53 makes a great point. All cars get better gas mileage at the speed that is just high enough to keep the car going in top gear without lugging the engine.
I practice "hyper-miling" on many trips. That is where you keep your engine at as low an rpm range as possible in top gear, ease off on the gas going up hills to cut fuel consumption, and never downshift unless it's a must. My optimum speed for good fuel mileage is between 50 and 55 miles per hour. On the highway, my Ford Fusion I-4 will get 40-42 miles per gallon. My large SUV will get as high as 27. Above 55, you are dumping more fuel into the engine than is required to keep the car moving at minimum rpm in top gear.
I have always gotten a laugh out of folks who make arguments such as "Some cars get better gas mileage at 80mph than at 55". NOT possible!! That defies the laws of physics, just like the silly arguments that smaller cars can survive crashes better than larger cars.
48 MPG from a 2008 Honda Civic? I think not. Even the bloated Civic Hybrid can't approach 48 MPG.
I have a 2008 that was manufactured in November 2008. I had the same leak develop around 50k miles. It was taken care of by the dealer under warranty.
Let's see, in 7th gear my car is barely above idle at 65-70 MPH. Taking into account flat terrain, and wind and body aerodynamics, the speedometer has little bearing; it's low RPM, not the speed. Very slippery at speed. In turn, an econobox may struggle with higher RPM to achieve speed and running power robbing accessories such as A/C.
Ease off the gas climbing hills? Why not drive normally in a car that meets normal MPG requirements for your needs? Keep the traffic flow normal. I ride motorcycles with high MPG and don't care to run into you.
Easing off on the gas is a standard practice for "hyper-miling" (getting maximum MPG). It keeps fuel demand low. If you ease your speed up gently before coming to the hill, then you can ease off on the gas without slowing very much at all. I've used hype-miling in my I-4 non-hybrid Fusion, and gotten as high as 41 MPG on the highway at 50 mph. And anyone rear-ending me will get sued.
So if you are on a 50 MPH hill and clear the crest at 20 MPH and are in the blind spot, you can be rear ended. That's great, suing someone that may rear end you that may die due to your saving 50 cents. Why not drive normally? What's next, shutting off the engine and coasting down hill? In my state we have roadways clearly marked with minimum speed limits. They fine if you go too slow to create a hazard on the roadways. The only way I see your theory working on the steep hills near us, is to break the law at excessive speed at the base and coasting to the top to not be driving like a turtle past the crest. Just drive normally please or carpool.
"Hyper-miling" is a way to save a couple dimes. Many of the techniques are illegal in most states, and you can create an unnecessary hazard for others on the highway by driving much slower than everybody else. Just drive normally, and if gas prices are so bad for you, buy a hybrid or ride a bike.