So im building my 68 SS clone up to be a wicked pro touring car which i plan to take all over america. since there isnt a whole lot of information out there about overdrive transmissions and the effect on fuel economy i figured id create a post and see what solutions folks have come up with. i know theres some stubborn folk who will post on here about how little they care about fuel economy but this car is purpose built for mostly highway driving with some street performance and some strip every now and then (i dont care about winning really just like going fast) and believe me the back seat gets used lots. so in the past couple years ive done a few things to this car. and in the next few months i plan to do a few more things. currently it has an old 454 out of a dually pickup truck its low compression stock cam, a tremec t56 .050 od 6th and a 2.97 first with 3.73 rear gears, holley 750 holley street dominator single plane intake, 1 7/8 headers with 2.5" exhaust (which is in rough shape) the fan is direct drive. this combination got me 18mpg on the highway keeping my foot out of it. i believe that after i build and install my 496 around 600hp with 2" headers 3" exhaust, quadrajet 800 with edelbrock 2.0 manifold, dual electric fans, vintage air, rebuild the diff, freshen up the suspension with some tubular control arms and get the alignment nice and dialed in, throw some fat rubbers on the ass end of er and take it for a burn i can break 20mpg. my hypothesis is that i cruise at about 80mph at 2500rpm (if my tach is right) if i go much slower then i begin to lug a little bit up hills. i chassis dynoed this car before the trans swap, it had a th350 and it got around 270hp with about that in torque. i believe that my 496 will get at least double that allowing me to drop my revs to 2000 or even lower and safely cruise the speed limit.i have no idea what cam to put in it yet. what do you all think? i hope to get the swap done and the rest of the work completed before august. i hope to take my grandpa to bonnyville for speed week, there is so much for me to learn about how engines behave and ideal operation conditions. i will upload some pictures eventually most of them are on my phone.