the 300 runs a taller gear in the rear, so it turns slower at the same speed on the highway. the 240 runs a 369 the 300 a 346 so that is maybe 6% difference fuguring in my head. the increase in displacement is 20%. otherwise, the engines are the same. so if both are sticks or both are automatics i would expect the 300 to use 10 to 15% more fuel on a given speed. so if you are getting say 27 with the 240 the 300 would get maybe 23 to 25 under the same conditions. these are just round figures. it all depends on condition of car, driving conditions and driving habits. but the 300 will generally use more fuel. now if you put a 307 gear in the 300 na it will return about the same mileage or perhaps a bit better than the 240.

the turbo cars produce more power and again are fitted with a taller yet gear in the rear. 307 or for the 85 models a 288. and the fuel economy isnt much different than the 300na.

generally if the engine turns slower at a given speed you will get better fuel economy and sacrifice quick takeoff.

tom w

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.