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This thread seems to be losing a bit of focus. It also fails to mention the goals and what car it's intended for.

All else equal, 318 will surpass a 400 in mileage.

Also, I'm not buying the deeper gear might get better mileage. May for the 5 to 10 seconds it takes to accelerate from a light but once you are at a constant speed. Lower rpm equals better mileage.



You have a point, but the truth is that it certainly IS possible to get as good or better mileage AROUND TOWN with a lower gearset. The increased torque multiplication for acceleration from a stop can overshadow the steady-state rpm based fuel consumption. A lower rpm equals better mileage on the highway.....but around town that doesn't necessarily hold true.

Dave



true. a tale of two trucks one a 94 dakota V6 3.55's one a 95 sport dak w/ 5.2 and 3.92's. Both 5 speeds bought 2 weeks apart. My 5.2 got better gas milage than my buddies V6 and CRUSHED it an every catagory. Why the small engine has to work harder at low rpm to move the vehicle. A 318 w/ 2.76's in a heavy b-body would require the engine to work harder than the 400. Depending on cam, converter selection 2.76's might not even work as well as a stock cam. If you ahve any performance upgrades you need to look at converter and gear selection. For your 2.76's to pay off you will have to be doing a ton of highway driving at a maintained speed.



I dare someone to explain what it means from a physics point of view what happens in a engine that is "working harder" and getting worse MPG. The fact is the smaller engine is going to have less friction, less pumping losses due to smaller displacement and since you have to run a higher throttle angle to achieve the same HP output you will have less vacuum fighting the pistons downward movement. It will also have the air and fuel molecules packed more closely together so they burn better, just because mopar failed with the execution of the 3.9 does no mean it should get worse MPG than a 5.2 my current 3.9 dakota v6 is the first V6 truck I have owned after owning many 318 and 360 trucks and with the same driving habits it kills ( or CRUSHES) the other dodge trucks I have owned in the MPG department. I built a 318 just to get better MPG in my cuda and got 29 mpg, I got significantly worse MPG with a 2.94 gear than I did with the 2.76 INCLUDEING in town. It is all about execution of the entire package. And if you tune it to get proper air fuel mix and ign timing at low RPM than it will get better MPG when geared higher. Low speed/high load tuning is very hard with a carb and distributor. I also dare someone to explain how they built a 512 cube big block with 4.88 gears that got 29 MPG because it hardly had to work at all to move a cuda. How come a neon with only a 122 cube engine and even heavier than my cuda can get 33 mpg? That little engine is working its butt off to move those things, especially the newer ones that are bigger and have an OD gearset.

I am not causing global warming, I am just trying to hold off a impending Ice Age!