Not always. When I was still driving cross country in my Chevy sprint.. which by itself got 62mpg... I would draft trucks, and boost that up to about 80+... Long flat stretches, like west texas, I could cruise at 70mph, and the sucker wouldn't be more than 800rpms, a tad over a high idle.. Yeah... I know it was dangerous and all, but driving from San Diego to Jacksonville on less than $25 worth of gas, gave me much more per diem money to spend on more pleasureable items than gasoline...
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Hey, that's smart. How closely did you follow the truck?