Throughout the first half of the year, Kelley Blue Book reiterated that it has reported remarkable strength among values of fuel-efficient subcompact, compact and hybrid vehicles. From January through May, analysts determined fuel-efficient vehicle values increased 20 to 25 percent, far surpassing the overall used-market average increase of 5 percent during the same time period. With fuel costs dropping, however, prices for fuel-efficient models have been softening.