That’s interesting you figured out where the most bang for the buck is on speed/mileage. As I said in a previous post the Civic will get 40 mpg when my wife is going along at 70, maybe 75, but it drops to 36-37 when I hover around 80. I have an Accord that was struggling to get 25 city but now it has limbered up and can get about 28. Highway mileage is similar to your Camry.
My wife’s 2006 FWD Ford Freestyle averages about 22.5 mpg given her non-freeway commute, but we clocked it at 32.165 mpg (measuring actual gas consumed vs. milage) on a roundtrip from Turlock in Stanislaus, County, California, to the San Jose airport and back. That was about 99% freeway milage using cruise-control at 65 mph under close to ideal conditions.
So if you use cruise-control on the freeway and stay within the “sweet spot” mph for your vehicle, you will get close to the maximum mpg for your vehicle.
For even more fun, try calculating how much additional time you will spend during a month of commuting at 55mph compared to 70mph. Then take your gas savings and divide by those extra hours to find out how much your free time is worth.
At 1,000 of freeway driving, people will spend an extra 3.5 hours per month driving if they limit themselves to 55mph rather than keeping up with traffic at 70mph. And they will save 5 gallons of gas by getting 25mpg instead of 22.5mpg at the higher speed.
So they are effectively pricing their free time at (5 gal * $3.50 per gal / 3.5 hours) = $5 per hour.
The higher-mileage vehicle means even less money saved due to driving slower, so your time becomes worth even less. Driving a Prius slowly might mean giving away your free time at a rate of $3 an hour.