one thing I noticed, the smaller engines lost more mpg, for instance, the cast iron 325 V8 in my Silverado still gets 20mpg, but not quite 21 in the summer, My 4.0 V6 Tacoma drops from 19 to 17.5, I remember an old 1.9L I4 Isuzu I had went from near 30 to the teens all rough figures