The difference is that the more efficient the vehicle, the bigger the change (same percentage). In a Prius at 55mph you get about 55mpg, and jumping to 65 lowers it to 45mpg. That is a 10mpg difference. Leaf is equivalent to a 20mpg difference, and very noticeable because we have equivalent to a one gallon gas tank in the car.
Related to the above was a concept I kept ignoring until I found the article that I posted at http://priuschat.com/threads/car-and-dr ... nts.95536/
" onclick="window.open(this.href);return false; regarding how we should be measuring "gallonage" and the bad math (in a Consumer Reports test) that http://ecomodder.com/forum/showthread.p ... 10091.html
" onclick="window.open(this.href);return false; pointed out.
The "small" drop in mpg that the V8 Mercury Mountaineer (from 23.8 to 17.8 mpg) as a result of going from 55 to 75 mph actually resulted in a FAR greater delta in "gallonage" than the "large(st)" drop made by the Insight when it went from 51.9 mpg to 36.5 mpg.
Over 100 miles, the Insight with it's "large" drop would use 0.81 gallons more when going 75. The "small" drop Mountaineer would consume 1.42 gallons more.