The real situation doesn't change, the only advantages are which is easier for the calculations used more often or what you want to emphasize.
"Only" a gallon more used by the 33mpg example vs. the 50mpg one is an attempt to hide the MAJOR difference of 3 vs. 2 - quite a large percentage improvement.
At the other end of the scale 1Mpg is a big deal, at 5mpg a truck would burn 200 gal. for 1,000 miles and that drops to 166 at 6mpg - 34 GALLONS savings. That's the same 17% improvement as the 14 vs 17mpg example that showed a 1 gal savings for 100 miles.
33% better in the car doesn't add up to much admittedly with lower miles driven. It's still more than $10 at a 300 mile fill up.
Look at the savings in the truck example at a common "fill up" mileage. $136 @$4/gal. is nice pocket change.
P.S. - it DOES kinda show how worry about a couple mpg more or less at around 40 hwy isn't worth much worry, one reason we always comment that exact figuring is needed to even tell any difference there.