307 vs. 350 mpg ?

[FLDBRED]

With everything being as equal possible,what's the difference in gas mileage with the 307 and 350? I guess what I want to know is it worth the sacrifice in power to stay with a 307?

[ETsC10]

Seems if you had less power with the 307, you might push harder on the go-pedal
and maybe use as much or more fuel. Just a guess though.







.

[68gmsee]

I don't have an answer but here's my opinion.

On our trucks there's no set figure on my mpg vs engine size and I don't think anyone can give you a definite answer on this one. Personally, I'd go with the 350 if you have a choice 'cause I don't think you will see that much difference. I have a 250 six on my 68 GMC and my mpg's are almost the same as I used to get on my 69 with a 350.

There's a fine line that you have between horse power to weight ratio. Sometimes you get better mpg with a larger engine that has more power to move a vehicle with less effort. A 307 may be smaller in cubic inches but needs to burn more energy to move the same mass.

[Indyuke]

In my opinion, probably not. It takes more effort for the 307 to get the truck moving and keep it moving than it does the 350.

Imagine the 350 at it's best makes 290hp, and it takes 40 or so hp to keep the truck moving at optimal cruise setting. The 350 would be using 13% of it's potential. On the other hand a 307 at it's best makes about 200 hp. At cruise it would be using 20% of it's optimal power and working harder than the 350 for the same end result. The biggest difference between the two would be acceleration where torque is the most useful. The 307 would need to rev much higher to achieve the same torque and hp rate for acceleration than the 350 would. As a general rule, the 307 makes it's optimal torque and hp numbers (which are still lower than the 350's numbers) at higher rpms than the 350 does.

All that translates into the 307 working harder than the 350 to achieve the same thing, thus offsetting the fuel savings you would otherwise get with the smaller engine.

All things being equal, I believe a 350 will give much better power with a sacrifice of possibly 2mpg over the 307.

But if you already have the 307 in the truck, I'd imagine it's better to go with that 307. Unless it need to be replaced.

[trevrolet65]

My dad and I put a 307 out of a malibu in 1980. It got about 15 on regular gas, 2 jet carb. We changed it out with 350 out of an impaula in 83 and got 14 on same 2 jet and regular I think. He got more torque for sure and would run quicker. It was a 64 long bed. The thing was the 307 was needing rebuilt, and the 350 was rebuilt. Hope this helps.

[special-K]

From my own experience and what people running 307s have said in previous threads,the 307s are better on gas. They can definitely be built to get better mileage.

[Longhorn Man]

the reports on here do tend to place the 307 as MPG king in these trucks.
The theory up there of 350 verses 307 leaves out way too many variables.

[sanford66]

If you want MPG....i would look into buying a prius. All jokes aside, a well tuned 350 with a q-jet will get as good or better mpg than a 307 2bbl, and more power. Joseph

[Warrens69GMC]

If you want to keep the 307, some performance can be had by using a edelbrock 2101 cam kit (or summit eqv.), and installing 350 vortec heads or 87-95 TBI heads.

[special-K]

Originally Posted by sanford66

If you want MPG....i would look into buying a prius. All jokes aside, a well tuned 350 with a q-jet will get as good or better mpg than a 307 2bbl, and more power. Joseph

That's the key. Make it apples to apples by lapping a factory 4bbl manifold on that 307 with a small spread-bore,like a 450-500 CFM,and you will see a difference. A 4bbl 350 will get better mileage than a 2bbl 350.

I built a 307 for power and economy in the early-80s:
*RV cam * 1.94/64cc heads * 4bbl/Holley 450 Performance carb (can't recall name,not Economaster) *headers.*HEI with weights & spring kit *Hotter coil and Taylor wires * Dbl roller timing set *double stacked open element filter

My theory was a low-end torque,smaller displacement,slight bump in compression,better fuel/air flow,hotter/better timed spark,and just enough carb. Get the power without much throttle. It moved along on the primaries just fine and when I wanted more there was plenty there. I ran my plan past Dave Coleman,a world class engine builder,and he confirmed it sounded like a good plan. He even talked me out of a performance intake saying the flow would be fine at desired RPMs with a stock one.
The truck it was in was a '71 K/20 work truck. It had 4.57s,14.50/37 mud tires,utility body full of tools,ladder racks loaded with scoffold and ladders,and a bed full of materials. It got 13mpg and I ran it down the interstate,through the mud,city driving,everything,and it got mid teens. I always wanted to see what it did in a 2wd 1/2t with taller go big gears and no big loads