That's not always the case especially when the engine is a 70's smog era engine. The cams they used in that period were sometimes installed retarded to help emissions and they were also real lazy profiles. In the late 80's I bought a real nice looking 74 Chevy Monte Carlo with a 350 2 barrel. Even driving it real easy it was an absolute pig on gas. Best I ever got with it was 11 mpg. I installed a performer intake and 600 holley thinking it might do better than the 2 barrel but it didn't help. I then installed a XE262 cam and was amazed at the power improvement and it got nearly 5 mpg better on gas