305 vs 350 both carb which gives better mpg?

im thinking of swapping my 305 to a 350 but i want to know how much of a mpg fuel diffrence there will be both being stock engines


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With some simple mods, you can make the mileage better than stock.
It comes down to a few things also such as gear ratio, the state of tune, and how you drive it. The smaller mouse of course will get better mileage, but it reflects back on what I mentioned as well. Vehicle weight plays a role in this also.

All things being equal, probably neither one. You need X hp to sustain highway cruise, and your driving style won't change. You'll still accelerate at the same seat-of-the-pants rate.

gasoline has X btus per gallon, so you'll burn as much gas to suit your driving. Your right foot will determine your MPG.

That's assuming apples to apples. You can't always make that comparison; like if you were comparing Vortec heads to non vortec, or a chevy 307 to an Olds 307... they all have different design aspects that might squeeze different levels of BTUs out of the gas you give it, but when comparing old school chevy to old school chevy, it won't really make a difference.

I have a 85 chevy van, with a 305. horrible gas mileagepulling my race car, but when its just the van about 15 mpg. I think it would get better if it were a 350,

I'd be willing to bet that a 305 (due to the bore size and the attendant valve shrouding effect) would have a higher BSFC than a 350. Chevrolet crutched this up with the 305's smaller intake valve size.

Mileage in the real world? Probably not too different between the two, IMHO.

I think all else being equal the 305 will get better mileage in theory and practice for two or maybe 3 reasons...well for every reason really. Smaller displacement equals more thermal and mechanical (friction) efficiency especially with a more undersquare bore to stroke ratio. Also a smaller engine would have less pumping loss at the same load due to a wider throttle opening. How much more the 305 would get in the real world might be small but I bet it is noticable, maybe up to 10% or more I'm guessing. Of course there's more off-the shelf cam and head choices for the 350 but most of them aren't designd as being mileage adders in mind. Also it's hard to drive a 350 as mundanely as a 305, or at least it is for me as I would be stepping into the throttle more.

my friends 350tpi gets 17 mpg city, my 305 gets 17mpg city, my 3800 camaro gets 17 mpg city...all camaros

85 k10 305/700rv10-14 3.08 gears

93 k-1500 350/nv3500 16-18.8 3.42 gears

Shane