366 fuel mileage??

[lukieg]

 I recently swapped a 305 for a 366 in my half ton c10. It has a 700r4 with lockup and a holley 4160. I drive it easy and it has new plugs wires and such. I bet im only getting 8-10 mpg regardless of highway or town. This seems a bit poor. Fuel filters are new.. no exhaust leaks. Any ideas? Thanks Guys

[bigchevyc30]

ehh thats about what i get you should be getting better cause my 1ton dually with low gears makes 9mpg every time i fill up so i know some people will be able to help

[Captkaos]

That is what I would be expecting from one...

[lukieg]

 So in your opinion, theres no real way to net any better mileage then this?? Thats too much for a daily driver.. i was hoping that the smaller displacement may help with fuel mileage along with a mild carb (which i found to be a 600). Shame ,the torque is nice.

[78 Chevyrado]

those engines are just heavier in almost every way, and have more drag.  they have extra piston rings, heavier pistons, rods, cranks,  and blocks.

They'll move mountains, but you'll pay to do it.  its the only thing that keeps a big block from being the perfect engine.

[TexasRed]

You're using the 305 and you removed the 366??

My mileage went up when I replaced the ignition module and went with external coil.

[bigchevyc30]

no he switched from a 305 to a 366

[TexasRed]

that's what I thought but then saw the smaller displacement comment.

What gears are you running? That 700r4 may have the rpms too low.

HEI distributor? Like I said, I was running a coil that did the resistance test fine but would cause the module to be bad, it would still run and get 9mpg but tested at the parts store would fail. Of course, the only module that wouldn't fail off the parts store shelf was the accel, so go figure. But swapping that out and going with an external coil setup, I'm up to about 13 or so. 383 with 264 isky cam. I think I can get some more by going with a 3.40 rear end.

[lukieg]

700r4?? rpms might be too low??? for fuel mileage? Im sorry but Im confused? The engine is designed for nothing but low rpms? I thought small cams and low end torque help mileage.

[TexasRed]

There is such a thing as too low. Like if you have 2.56 gears, big tires, 700r4 and a 366 with more than stock cam. If low end torque helps mileage, get yourself a 502, LOL.

Check all the easy stuff. Are you spark plugs fouled? Have the parts store cycle your module a few times. If it's not good, check the replacement too.