the most direct way to do this is to be more fule-economical

to be 就表示一种途径联系,“是……”
become作系动词时,与be类似,但在翻译上有些微的差别,它更侧重于“变成”,有一定的改变与变化.例如,WIth the time going by,he becomes more older.但在这题上不存在变化,只是告诉你最直接的途径是什么而已.这是我的理解,希望对你有所帮助.