How much will cam effect gas milage?

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I just got the gears though, so I expect it'll take a hit on the highway.
I have the 233/239 cam too. I used every gear and didnt ever skip a gear. Most of the time left it in sixth. 75 up there and about 68-70mph back.
I thought that was good with a good size cam. I us GC oil too.


, which comes to....12-13 mpg. How are you guys getting 17
