307 vs 350

[Awann99]

I’m rebuilding my gmc 1500. It has a crate 350 with a quad jet in it. I’m picking up another frame to start the rebuild and it has a stock 307 with a 2 jet on it. Which engine setup would give me better gas milage. I’m making the gmc into a 2500 so I can haul heavy loads easier. My shocks are beyond worn out atm. I’ve heard rumors of the fabled extremely efficient 307 but I’m not sure if they are true. And the 307 is all original.
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[CastIron]

I replaced my 307 with a crate 350 a few years ago. I didn't notice any significant change in fuel economy, but there was quite a difference in power. I think the 350 has a much easier time moving the truck, which means less throttle is necessary.

[CUDA8U]

had a 307 in a 73 ,it was gutless

[Mts67C10K20]

Of the 2 motors 350 for sure especially for towing
As previously mentioned the less it has to work the better you are
Built it for torque not horsepower
You won't regret it

Have fun

[Jcfcamaro]

Bigger cubic inches is always better.

[palallin]

What do you want to do with the truck?

The 307 in my wife's '68 Malibu was a very good motor--stock 2bbl would bark the tires upshifting with the Powerglide. And it did get good mileage.

The 350 will probably do a better job if you are pulling decent sized trailers or putting heavy loads in the bed. Or if you want to lay rubber, transform tires into smoke, and that sort of thing.

But, if you are past those sorts of showy acts and using the truck just as a cruiser/DD or with only light loads or trailers, you may be very happy with the 307.

FIRST, evaluate your goal. THEN choose the tool to accomplish it.

[54blackhornet]

Your building a 3/4 ton and worrying about mileage

[Steeveedee]

Originally Posted by 54blackhornet

Your building a 3/4 ton and worrying about mileage

IK,R?

OP, tell us what trans (stick, auto?) and differential ratio you have. I'd personally recommend the 350 if you are doing any towing. There is no replacement for displacement. I have a 402 in my truck and am looking at putting in a 454.

[jokers10]

Also, the efficiency of a first gen 350 is rather awful. For instance, I had one shoe-horned into an s10 blazer. Even with a properly tuned fitech fuel injection, I was, at most, getting 18 mpg. I was also running a 700r4 and 3:43 gears.

If you want a bit more mpg, run some highway gears, and get a more modern motor like the 5.3 or 6.0 LS based motors.

[Grumpy old man]

If your worried about mileage buy a Kia , You can't expect a lot from a 50 year old truck with no overdrive ,And by the time you add up all the costs to change anything your better off just dealing with the mileage your already getting .

[Redneck Rydes]

350 is the way to go,use the 307 for a boat anchor.

[Steeveedee]

Originally Posted by Grumpy old man

If your worried about mileage buy a Kia , You can't expect a lot from a 50 year old truck with no overdrive ,And by the time you add up all the costs to change anything your better off just dealing with the mileage your already getting .

That's right- life isn't all about mileage. I have a Kia for knocking around town, but when I need to haul something or tow my travel trailer, that little econobox stays in the driveway.

[GASoline71]

I think this is the first time I've read of someone making a C25 out of a C15. Especially when C25's are plentiful and cheap.

Stick with a 350...

Gary

[HO455]

No mention of transmission. Auto or manual? What percentage of empty versus loaded driving? Are the loads heavy but small in size (like gravel) or is it large and have a big aerodynamic penalty? Are you driving in a region with lots of hills? If your in a mostly flat area and and hauling small heavy loads occasionally. Then the 307 would work well with a manual tranny and maybe some sort of gear splitter. Be aware you won't be bombing down the freeway at 75 much with the 307 loaded. The more drag on the truck ( gravity and aerodynamic resistance) the larger the grade you have to negotiate the more you will want the larger motor. Most of the fuel milage gains made in the last 15 years or so have been because of transmission technology that comes in new trucks. Are you willing step up to a late model transmission and a controller. A lot of this decision has to how you want to drive the truck and if you like shifting the gears yourself.
My dad loved his 71 F250 4x4 with a 300 six 4 speed. It towed his fifth wheel trailer and got 19 miles to the gallon (not towing the trailer of course).
When the 305 in my Burban went south I replaced it with a 350. The biggest difference at this point is when I have to pull out on a busy road with a 45+ mph from a dead stop. The new 350 makes this much easier.
Enjoy the search for your perfect combination!

[mike16]

I've owned both, each serves its intended purpose fully, the 307 did every thing the 350 could do, the gas mileage did not matter. if you want better gas mileage then a better rear end gear ratio is what does that.

[67 chevelle]

My advise , get an l31 350 , very well designed engine , with towing in mind , very efficient and powerful

[draino76]

If you want to be a little different check what size the journals are on the 307. If it is a large journal put the 307 crank in the 350 block, order a 327 rebuild kit and a good set of heads. 327 would give you a happy medium and if set up right they can give fair milage and run like a scalded dog.

[truckster]

Originally Posted by draino76

If you want to be a little different check what size the journals are on the 307. If it is a large journal put the 307 crank in the 350 block, order a 327 rebuild kit and a good set of heads. 327 would give you a happy medium and if set up right they can give fair milage and run like a scalded dog.

Except you'd still be sacrificing HP/torque as compared to a comparably built 350.

[garyd1961]

Nothing wrong with a good running 307 but once it's worn out it's not worth rebuilding when you can get a L31-R 350 for around $2000. The L31-R is way better than the 307 or even the older 350.

[va72longhorn]

Back in the early ‘80s, I ran a 307 in a 2wd drive, 72 blazer. A mechanic buddy of mine gave me a set of double hump 202 heads that gave me all the power I needed. 18-20 mpg, depending on how I drove. Hanging around the shop one weekend with a case of beer, pulls the blazer in and inserts a piece of stainless steel door screen between the four barrel and the intake. Increased the mileage to about 25 and give me a little power. He commenced to tell me that the screen heats up and changes the fuel to a vapor when it hits it. Of course the vapor burns hotter and requires less fuel. Says it was developed back in the late 60’s, but the oil companies bought the patent and locked it away. Of course, I’m just a naive 17 year old kid at the time so I take it all in. All I know is I could burn ‘em at the lights and passed a few more gas stations.

[mike16]

I have hualed some crazy loads with my 72c10/307 w/ 3.73 gears. It was a good engine. gearing makes all the difference