Max MPG- slant vs 318/340

The engine struggles a little bit against the steep ratio. The engine rpm needs to be in the sweet spot where the most torque is made for best mileage. This is where the engine makes the most Efficent use of the fuel. How much fuel it needs vs the amount of power it needs to produce to turn the wheels is show in resistance and the gear ratio is the resistance. Along with the rest of the drive train and wind resistance.
The faster you go, the more air/wind you must overcome.
Same idea with the gears. This is how I explain it.

Now how much more fuel is required to propel the car @ speed in drive (A 1:1 ratio) vs. overdrive (A.69 in the auto trans) is a good question!