this is what underdrive pulleys are for. work great!
Did an interesting fuel experiment in the past weeks as I needed to replace a part that required me to remove my serpentine belt until the part arrived. So without an alternator, power steering pump or A/C being driven for a little more than half the tank I gained almost 1L per 100kms. I was quite surpised that the load on these caused such a drain on fuel and of course the inconvenience of driving without the belt is far greater than the fuel savings.