305 v8 or 250 I6 for better mpg

[6977chev]

my 69 right now has a 355 with some kind of milder cam in it, i dunno the exact specs thats the way i bought it. it has 882 heads and a edelbrock 600 carb. the carb has the same rods and jets it came with when i bought it. it has a 350 tranny with a mild shift kit and i beleive the rear en is 3:73 gears. right now i get about 6-8 mpg and thats about a good as i can get it no matter how i drive it it stays about the same. so im thinkin of maybe swapping in either a stock 305 v8 or 250 striaght 6 which i can get both for free. i would rebuild which ever one i bought. which one would be the better choice to stick in for beter economy. i would love swap in the sm465 i have laying around but i lack all the parts to do so. so for now it will keep the 350 tranny. which motor would be the best? thanks guys

[69TowRig]

If it is a street rig I would consider swapping the rear axle ratio to something like a 3.07:1, since you have a decent engine and an automatic to push it. The high revs are killing your fuel mileage. I don't think an SM465 swap would save you anything because the engine will have to spin just as much to maintain the same rate of speed.

[Cyclone44]

My 69 is on it's second 250 and it has a saginaw 3 spd. and 3:73 Gears and I have never got any better then 12 to 15 mpg. Maybe my driving style but that all I get, I can't tell you about the 305 maybe someone else can, Maybe someone else gets more out of their 250, but that's best mpg. I have got with mine. George

[mvfd70]

I used to own a 68 with a 250 and a SM465 with 3:73 rear. That sucker only got about 10 mpg. I think your best bed for increased fuel mileage is to go with a 3:08 rear end gear. It won't be as good from the line but it might help at cruising speeds. Better yet would be to get an overdrive transmission.

[BCOWANWHEELS]

same milage with my 250 & 3 on the tree11-15 mpg best . i think a sm465 and 3.08,s would be worth tryin. you,ll need 1 st gear to pull out in for sure especially if your loaded my truck dont like it a speeds of 65 mph or more with the 3.73,s . i think a nv4500 and 3.73,s would be nice too .

[CCCC]

For normal driving with a near-stock engine setup that is LOW MPG no matter which engine is in there. That particular 600 CFM carb may be an issue, especially if takeoffs are racy and if one has an unsteady foot at highway speeds. To answer your question, experience here has been that a smaller cu. in. small block will yield as good or better MPG than a 250 I6. If you want some performance ability but mainly are interested in better MPG, my suggestion would be the small block with HEI (including correct wires/plugs) and a good 2 barrel carb. Have not used one, but have been told that the Holley Economaster/mizer is good. Hope this is helpful.

[LEEVON]

Sounds like a carb tuning issue. That thing has to be running rich to get 6-8mpg. I second the 2bbl and 3:08 gears and HEI, which should be good for at least 15mpg if not better. It won't be a powerhouse, but at least you'll still have the sound of the good ole' 350

[Brad]

Since you're going to rebuild either one, it really comes down to personal preference. I also wanted to say that it'll probably be cheaper overall to build the 305. I would make sure that either have a minimum of 9.0 compression. The reason all these old tired L6's get bad mileage is due to the poor compression ratios. Bump it up to about 9.5 and you'll get awesome mileage. That's what I'll be doing when the day comes. The trouble is, I'll be old and grey before my 6 needs a rebuild.
Btw, I'm getting 16.3 around town with an occasional 65mph highway trip and my engine has about 130K on the clock.

[68gmsee]

I agree with Brad. I, too, would go with the 305. It's more versatile in the carb selections and parts for upgrading are readily available.

With the engine conditon being at it's maximum efficiency, size does not determine mpg, unless, of course, you are in the big block category. A six cylinder has to work harder to get the truck going than a v8 so it may not buy you much.

A lot of other factors are involved here. As already mentioned, rear end ratios, driving conditions (city or rural), carb efficiency, tires, weight of vehicle, etc...

And as mentioned also, a single and 2 barrel carb are not the most economical. A well tuned quadrajet with the small primaries can give you as good or maybe even better mpg's than a 2 barrel. Plus you have the option of the additonal power of the secondaries when needed.

[Omnivec]

Step it up with a 305 throttle body injection with 3.08s

[streetstar]

Something is wrong with your setup if you are getting 6 to 8 mpg. 3.73 is not that wild a gear ratio.

I assume you have checked the obvious things already, like fouled plugs, old wires, exhaust leaks, etc. and if so, it has to point at the carb

My 69 GMC with a 307, 2bbl and Turbo400 only got 11 mpg on its best day-- i swapped a new Holley 600 and intake, and it still got 11.

However, my 75 with a 350, th350, Edelbrock Performer 650 carb and more modern HEI ignition gets12-14 .
Bigger motor, better mileage - because of a lighter duty tranny and more modern ignition ---- yours already has a T350 too, so it should be in the same ballpark --- swap the carb, you'll need one if you do another engine swap later anyway

[67ChevyRedneck]

There must have been something seriously wrong with my truck. I averaged about 10 with my 67 back when it was a 250/3spd/3.73 with a "highway cruise" speed of 55 I could pull off 12 mpg.

Even with my 350/700R4/3.73 I'm getting 12.5 city avg and about 16 highway.

Personally, I'd rather have the 250 over the 305. I had one in a Z28 and hated the gutless wonder. At least the 250 has lots of peppy torque. They would both probably get very similar fuel mileage #'s.

[Scott68-93-97]

Keep the 350 and put a q jet on it maybe an overdrive tranny, fuel injection, new gears. If you go with a 305 youll just have your foot in it farther to get it down the road, I had a 305 in my old malibu and swapped in a 450hp 350 and my milage didn't change much until I swapped out the 2.73 gears for 3.42 thats when my milage suffered. Even the bigger jets in the carb didn't kill it if you kept your foot out of it

[6977chev]

it has hei and im running 45TS plugs. if the engine is warm and i shut it off and try to start it after about 20 minutes or so the engine will crank over many times before it finally takes off, but if i give it any gas when its cranking its starts but it runs really rough and smokes black out the tail pipe i know it runs pretty rich because i can smell it out the exhaust. do you think i should change jets and rods. if so what size should i go with. it has the ones it came with in it now.

[68gmsee]

Sounds like you may be leaking raw gas into the intake while it's sitting. Could be float adjustment or something else. Let it sit for 20 minutes and remove the breather and look in the throat to see if you can see gas leaking or smell it.

Check you choke adjustment. It shouldn't close too tight while sitting for 20 minutes. If it's off, adjust it so that it barely closes when the engine is cold. Here in Texas, I back it off ever so slightly after I do this adjustment.

[screamin_c10]

You need to rebuild that 1406. I had the exact same symptoms you mentioned with my 1406. These old fuel tanks have years of crap flaking off into the fuel and it WILL eventually clog up your carb. replace the filter, clean out/ rebuild/ replace the carb, save your money for a 700R4 transplant and you should see 12-15MPG easy.

I'd personally stay away from the 305 unless you want a new boat anchor. If you really want a good low cube small block that is cheap to build and run, look at the original 307s.

[mrein3]

The first thing I would do is verify your data. If you don't own one, beg, buy, or steal a GPS. On a highway, check your speedo against the GPS. In relative terms, the speed on your GPS is more accurate than MOST speedometers. Check it at 55, 60, and 70 (where legal). Then in town check your speed at 30 and 45.

If all positions check out, and you REALLY are getting 6-8 mpg, you have problems. Get that carb working right. Tune up the engine. New air filter.

Do not immediately jump to throwing new and expensive parts at it like an overdrive tranny or higher (lower numerical) rear end gears.

The OD in a 700r4 is .7:1 where the final drive in your th350 is 1:1. The BEST you can expect switching from a th350 to a 700r4 is 30% or 2.4 mpg - assuming you really get 8 mpg now and you ONLY drive in OD - which doesn't happen.

Swapping gears will have similar results. Going between each factory gear set you gain or loose 10% of what you get now. So if you're running 2800 rpms at highway speed with your 3:73s, and switch to 3:55s, you'll take off approximately 10% so subtract 280 r's - you'll be running 2520 rpms and should see approximately .8 more miles per gallon. Drop to 3:08s, take off 560 r's you SHOULD see around 2240 r's and APPROXIMATELY 1.6 more mpgs.

So AFTER verifying your data (check your speedo), and tuning up your engine, you need to figure out how much you drive, how much you'll save with 10%, 20%, ... better mileage, THEN decide what YOU should to to improve the mileage on YOUR truck.

[panhandler62]

I'll just say "me too" to a couple of posts here. It's absolutely a carb tuning issue *AND* -- a q-jet that is in good shape and tuned right will beat just about anything for *overall* performance (which includes economy.)

Q-jets do *everything* pretty good. Most specialty carbs do a *couple* things better than a q-jet but give up performance in other respects to do it.

For instance --nothing makes top-end power like a big bore Holley type carb, but the damn things thing gas is still 38 cents a gallon.