Need some advice.

[1978chevy]

Hello all, I have a 78 k10 lwb, and I am trying to do some work to it to try to get around 15 mpg, it currently has a 305ho with a .458 lift cam and a 600 carter carb, 33" tires, 350th, np231, stock gears, not sure what they are but they seem to feel like a high gear. Any advice would be very appreciated thanks.
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[Firebirdjones]

What you want to achieve is doable. Probably will require alot of little things that add up. I've been successfull with gas mileage tricks, some simple, others take work.

Providing you got simple things out of the way I'll go over some things I do that help me.

33" tires aren't bad so long as they aren't wide. I run 10.5's on one truck which are slightly less rolling resistance. Getting into the wider tires starts to cost mileage, and they are pretty darn heavy too.
Go to an aluminum wheel if you haven't already. I went from a steel wheel and 33's to aluminum with 33's and saved 11 lbs. per wheel, that's not just 44 lbs. of weight, but 44 lbs. of rotational mass that takes less power to turn it.

Speaking of weight, that's a key factor as well. Look at ways to lighten the truck, especially rotational mass.

Getting into the tuning aspect of things. I generally tune my cars to run with about 16 degrees initial timing to start with, and about 20 degrees of advance built in. This takes trial and error as to what works best for your combo, but more initial with less centrifical helps throttle response and low speed torque to get you moving easier without so much throttle angle.
I then like to run my vacuum advance from manifold vacuum with an adjustable unit, (not ported or switched vacuum from above the throttle blades) and dial in about another 10-12 degrees of vacuum advance. These two tricks combined will give you outstanding throttle response that is very crisp. This alone is worth 2 mpg on most everything I've tuned.

From there, the best way to dial in a carburator is with a wideband. A carter carb is much like the Edelbrock carbs of today. You should still be able to source metering rods and jets to optimize that carb if you are really set on using it. A vacuum gauge would be needed to dial in the power piston spring for the power valve.
AFR I shoot for varies depending on the setup. 14.7 is optimum, sometimes I'll go a tad leaner (15.0 doesn't hurt anything) but in heavier vehicles, low numerical gears, smaller engines, or something I do alot of towing with etc...it might be best to error on the side of caution to prevent detonation, especially if you live in a hot climate. Anything in the 14's is pretty safe for part throttle, I'll ramp that up to the 13's for moderate loads, and that's where your power piston spring comes into play as vacuum drops.
You can look into long tube headers, intake manifolds, free flowing exhaust systems, without getting too carried away, all to free up a little bit of extra power.

Once all that is done, assuming the engine is running as it should, it then becomes a game of driving habits.

My 79 1 ton long bed pickup has a crate 502 with a 400 turbo and 3.73 gears and 29" tires. It's a tank at 5300 lbs. With tuning tricks I get 12 mpg around town and 14 mpg highway. Not bad for a brick with a thirsty engine.

My 72 blazer 4x4 with 33" tires (10.5's) with the original 355/350 turbo combo and 3.07 gears, intake, headers, 750 holley, would get 13 mpg around town and 16 mpg highway. Now with the 6.0 LS engine and 4L60E and 3.73 gears, I'm getting 17 mpg around town and 22 mpg highway.

It's doable. It'll take some money and time though.

[1978chevy]

I noticed that both of your trucks have larger engines than mine, but you are still getting a lot better gas mileage than I am, would it also help me if I was to put a 350 back in it because I do have one it just needs a top end. I am also wrong on the size tire I have,its 32x11.5s, sorry about that, they are the procomp ats, it also has aluminum wheels on it, I haven't checked the timming on it or tuned the carb, but I will try to do that ASAP, thanks for the advice, and anymore is welcomed.
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[1978chevy]

Oh ya and by the way I'm only getting 8 mpg with a mixture of city and highway. Also the engine only has maybe 15k on it.
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[Firebirdjones]

8 mpg,,,ouch! There is definately some improvement to be made.

Your tires aren't that big so not too much factor there, I'm not familiar with pro comps. Something I didn't mention is tread pattern, although sounds trivial, can affect gas mileage. I love knobby aggressive looking tires on a 4x4 but they kill gas mileage. A mild tire with tight compact tread is more friendly in that respect, just not as forgiving off road. It's a trade off.
I run BFG all terrains, which is a mild tire, not much for any serious off road, but gets me around the mountains for recreational purposes, since I spend 90% of my time on the pavement.

The 305 HO you have should be plenty capable, I wouldn't give up on that yet. Smaller engines generally tend to be a little more fuel efficient but there is a trade off when going too small or using them in a big heavy vehicle. If it has to work too hard to move your truck around it sort of defeats the purpose. More throttle angle means more gas. But I'd definately tinker with what you have and see what happens.

[1978chevy]

Ya the carb work and the timing is what Ill try next Ive got headers and 2 1/2 exhaust with flowmasters, it does take a lot of throttle to get it going but I think that is from the 1800 stall I have in the trans for the old motor which was a 383. The procomp tire that is the best tire I have ever had I'll never run another tire.
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[Firebirdjones]

I'm sure the pro-comp tire is a great tire, I just threw it out there that tread patterns will affect mileage, and if I'm not mistaken the pro-comp is aggressive? Stick with what you like, there are compromises that some people aren't willing to change, nothing wrong with that

The stall is going to hurt you a bit, especially if it's a non lockup. I assume it's an older 350 turbo trans? Playing with your part throttle shift points and optimizing them to your combo would also help mileage.

Stretching the shift points out a little bit to help the 305 get up to speed easier without so much throttle will actually help mileage. If it gets into high gear too soon it will then take more throttle to accelerate forcing the engine to work harder, and it will be driving "into" the looser converter more also....

Like I said, it's all a bunch of little things that add up.

[1978chevy]

Thanks for all of the advice. I'll get started on that asap and I'll let ya know what I come up with. One more question would you change anything in the gearing. Once again thanks.
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[Firebirdjones]

What gearing do you have?

[1978chevy]

Not very sure, but it takes forever to get up to speed, thats another reason why I have to keep my foot in it.
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