Word that means 'able to produce the same amount of output with less input"

If I improve a process to increase output per input, I can then do two things:

• increase total output for the same total input

• maintain total output by lowering total input

I’m looking for a word that means the second option.

Example:

Bob improved our combustion engine performance by X miles per gallon. We are then able to reduce fuel tank size by Y gallons, giving drivers the same range as before. Our vehicle is now more _____.

"Efficient" doesn’t seem quite right. I would describe the engine as efficient. To me efficiency is about ratio. I’m looking for a word that specifically means changes to total input, contrasting changes to total output:

Bob improved our combustion engine performance by X miles per gallon. Our existing fuel tank gives drivers an increased range of Y miles. Our vehicle is now more _____.

"Effective" is my best option so far.