68 GMC gas mileage - 305 or 350?

[thaubol]

Hey -
This is my first build and pI'm struggling with the decision on what to put into my truck to achieve the best fuel economy.
It currently has what I expect is its original transmission with a 350 and some heavy rear end gears - I'm guessin 4.11, with 215/75-15 tires on the front and 275/75-15 tires on the back. I recently bought a t5 out of a camaro that I plan on using in the truck (in the interest of fuel economy). I also have a 88 monte carlo ss with a 305.
One dilemma I'm facing is which motor to use. Obviously, the 350 is able to have more power, and being in an older, heavier truck, that might be the better bet. But in order to attain the best gas mileage without running the motor lean I was thinking that the 305 might be the way to go, as long as I can find a way to up the torque so the truck isn't struggling while accelerating.
Another question I have is what gears I should use in the rear end. Most everyone is saying that a 3.08 is going to make the trans hunt for od on the highway, so I was thinking of going with a 3.43 or a 3.72? I'm pretty sure I'll be using 255/75-15 tires.
This is my first post on this forum, about my first build, so maybe i should shed some light on myself and my truck. I'm currently 21, I bought my 1968 GMC C15 last summer and have fallen in love with it. Any suggestions on any part of this build would be much appreciated as, like i said, this is my first build and am relatively clueless.
Thanks, TC

[69GWC]

Myself I would use the 350 with a 3:73 gear, but the 3:42s would be better on fuel. As for the 3:08s I would not stick them in there to me they are just to high for a heavy truck .
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[thaubol]

so in the interest of fuel economy, you think the 350 with 3.42s would be best? also, after i figure out the engine and gear ratio, im going to have to decide what cam shaft and carb to use. any suggestions?

[slipkorn34]

if your using the t5 the trans wont hunt for overdrive as its a manual and your the one controlling it... ive had 3:08 gears in a truck before and it was behind a 4.3 in a full size so it can work, but i would go 3:42 with the 350. as for carb and cam i would say quadrajet carb with an "rv" cam. there are much more knowledgable people on here that can give you better information with numbers to back it up.

[thaubol]

ah yeah sorry i was thinking of the automatic thats behind the 305 now for some reason ha late night....
thanks for the input. what im getting here is to not go through the trouble of trying the 305?

[69GWC]

Sure the 305 would work just fine but IMO I. Would want the 350 for more power just my 2 cents.
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[lap15]

If the truck already has the 350 in it and it runs good, I say let it go and just drive it sensibly. Swapping it out for a 305 doesn't seem worth it to pick up an extra mile or two a gallon. I have a 305, sm465, 3.08 in my '86 Chevy and I am getting 15 mpg empty, I would imagine a 350 the same way would get 13-14mpg....so going to all the trouble for 1mpg more is not worth it to me.

[Billett]

Definitely that^ the slight increase in mileage you'd get out of it wouldn't be worth it, and it would be easier to just try to save weight or drive more consciously. If you don't want to deal with a q-jet the edelbrock 1406 is designed for economy. You could probably find one on CL or ebay fairly cheap and just rebuild it.

[cubber]

The gutlessness of the 305 wil not be worht an extra mpg or two. Also as others have said, the 305 my actually lose mpgs due to overworking itself. If you really want to get good mileage, find an old 250 6 cylinder that someone has pulled out to put in a 350, freshen it up, put a 4bbl carb on it, and put that T-5 behind it with either the 3.42 or 3.73 gear and you will have one thats fun to drive and gets near 20 mpg.

[Wrenchbender Ret]

I would use the 350. The 305 is sluggish. I dought if it will increase the gas mileage any. I would use the 3.08 rear end . That gives you a choice. You can use 4th gear for heavy loads or hard pulling & still not be reving too high. The Q-jet is the best carb for MPG if you can o-haul it your self or drink beer with someone that can.

[Hubscrub]

I'm not gonna diss the 305, can i ask whats wrong with the monte carlo ss (i assume you would pull its engine) if it still has the 305 H.0. they were rated in the later years at aroung 190hp with the gears in the car it a fun driver ,it's a good respectable engine. I have a 86 monte ss that i've had since i was 17 and that 305 is still running great don't use a drop of oil heck the whole car has been great (i'm 30 now). If the 350 is in there an runs i wouldn't pull it i agree that a good Q-jet is by far the best carb choice and how you drive will affect gas mileage greatly too. The gearing is up to you and what trans etc. you want to suit your need in the end you still pushing a brick through the wind. Good luck

[GASoline71]

Either way you slice it...a 305 will not make the TQ that a 350 will. TQ is what gets the vehicle moving. The 305 will take more right foot to get moving and to climb grades... eating more fuel.

350 all the way.

Gary

[GuyO]

A well tuned 350 with TLC driving habits will go a long way towards MPG. 3.73 gears are normal, 4.10 are harder on gas but more fun. Type 3 carriers include 3.07's and 2.76's that give the best milage. I recommend a 200-4r overdrive tranny if you plan on having the truck a long time. If you want a immediate improvement increase tire diameter. The 275's are about 29" tall, 32's would increase milage by about 10% and not rub!

[thaubol]

I appreciate the input guys.

After reading through your responses, ive gathered that the general consensus is that the 305 wouldnt have enough balls to make it worth the trouble, and that i should just leave well enough alone and stick with the running 350 thats in the truck.
the only problem with this is that ha who can leave **** alone? Ive got 2 nice small blocks that will eventually go into one project or another, so i might as well build them now that theyre fresh and accessible. also, the 4 spd thats currently behind the 350 is a real stump puller and wont do for daily driving - anything over 45 mph and you feel like the motors gunna blow from high revs. so, either way im gunna have to pull that to swap a new transmission.

On another note, now that its out of the car and on an engine stand, I think im going to build the 305 to the best of my ability, geared towards fuel economy, not power, and keep the 350 in the truck for now. so far ive got that the qjet is probably my best bet for a carb (although im gunna have to find someone to help me build it) and that an rv cam might be best. any other ideas? im lookin for fuel economy first, seeing as how this is most likely going to be my daily driver, and then for a way to increase the torque in order to help get the truck moving and for help in when im hauling bikes.

thanks again guys.