YM Will V - Correct - A lesabre will show little difference, as you
noted, between 65 and 70. A 4 cyl car will show more difference at
those speeds. The smaller engines will need to work harder to maintain,
hence use more gas. Unfortunately its all relative. The lesabre mpg may
be, lets say, a difference of 20mpg to 23 mpg at 65mph and 60mph
respectively. the little 4cyl car may show a larger difference but may
be more like 22mpg to 28mpg at 65 and 60 respectively.

Not to knock you for having a large car though. You have every right to
drive what you want and spend your money how you want without
criticism. (I drive a Chevy Suburban and a Chevy Silverado!)