305 v8 or 250 I6 for better mpg

[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

[BurnoutNova]

stay away from a 2bbl (the 2bbl is actully bigger than the primarys on a q-jet)... get a rebiult q-jet and put it on your 355, and then put some 3.07 gears in there and you'll prolly get 12-15mpg

my 71 k10 with 3.07 gears, q-jet and a fully loaded car trailor, and a fully loaded bed got 16mpg last swap meet i went to... it was mostly hwy driving, but i was still impressed

[FLEET72]

I had a '60 long step Apache 10. It had a 350/465/3:08 Quadrajet on an Edelbrock Performer. I averaged 16 - 18 mpg in that heavy beast. My '72 SWB has a 350/350/? Edelbrock 1506, best I can get is ~14. Both trucks had/have headers and dual exhausts.

[mnunn454]

Originally Posted by 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

I agree.

Before changing ring and pinions, or engines, or trannys, I'd start with the carb and HEI.

Regarding carb's:

A) Visit a chassis dyno and have them jet it for economy when cruising at highway speeds (NOT W.O.T. HP). Or,
B) Try carb tuning yourself by changing carb jets or needles and watching the plug colors (An O2 sensor in the exhaust pipe is a big plus in this regard). Or,
C) Replace the carb with a rebuilt Q-jet or Edelbrock #1405 or similar.

I'd think 12-13MPG isn't an unreasonable expectation for a V8/373 geared truck (once the air/fuel ratio is adjusted properly). The 50% reduction in fuel consumption will repay any hardware investment costs in short order.

To get 16MPG or more, you're likely going to have to drop to a 3.08 R&P or install an overdrive tranny (manual or automatic, you choose).

Upgrading to EFI and an overdrive tranny should net you over 18, but it's a lot of work and maybe cost to convert.

of course, all this assumes you don't have a gas guzzling cam, really low compression or the emergency brakes frozen up ('Been there. Done that. Got the T-shirt).

[6977chev]

sounds like the first thing i need to do is get my carb tuned right. i just bought it brand new a few months ago so i surly shouldnt need rebuilt. the engine already has HEI and i wish i knew the specs on the cam but i dont. it had that cam in it when i bought the truck. how do i know what size rods and jets i have now and how do i determine what size i need to put in. if thats even the problem.

[57sailplane]

i am pretty sure there is an instruction sheet at edelbrock on how to tune your carb. this sounds like your problem as these carbs are set rich so they will run a variety of combo's. if they were to set these carbs up lean from the factory they would probably be liable for detonation problems.

http://www.edelbrock.com/automotive_...ess_main.shtml

[mnunn454]

Originally Posted by mnunn454

I agree.

Before changing ring and pinions, or engines, or trannys, I'd start with the carb and HEI.

Regarding carb's:

A) Visit a chassis dyno and have them jet it for economy when cruising at highway speeds (NOT W.O.T. HP). Or,
B) Try carb tuning yourself by changing carb jets or needles and watching the plug colors (An O2 sensor in the exhaust pipe is a big plus in this regard). Or,
C) Replace the carb with a rebuilt Q-jet or Edelbrock #1405 or similar.

I'd think 12-13MPG isn't an unreasonable expectation for a V8/373 geared truck (once the air/fuel ratio is adjusted properly). The 50% reduction in fuel consumption will repay any hardware investment costs in short order.

To get 16MPG or more, you're likely going to have to drop to a 3.08 R&P or install an overdrive tranny (manual or automatic, you choose).

Upgrading to EFI and an overdrive tranny should net you over 18, but it's a lot of work and maybe cost to convert.

of course, all this assumes you don't have a gas guzzling cam, really low compression or the emergency brakes frozen up ('Been there. Done that. Got the T-shirt).

I just posted the response below on another thread. Pasted it here as well because I think it addresses this problem too.

Here's the reasoning behind my belief that greatly improved mileage can be had with a good carb tune (not just a simpe rebuild).

It's a Chevy High Performance article from a year or so ago called "Old school meets new school". It's on-line at
http://www.chevyhiperformance.com/te...age/index.html

In short, using some tech tools, this guy has a 500+ HP SBC and adjusted his Mighty Demon carb's idle air speed and high speed air bleeds to increase the operating range (RPM and load) of the idle circuit and delay the onset of the primary's main metering circuit. Result was smoother operation and 21 MPG on the trip home following a drag racing outing (granted he has an AOD tranny).

It's a technical article, so grab a cup of coffee before you sit down to read it. I think once you read it you'll agree that a lot can be done to improve mileage as long as you can see what your air/fuel ratio's doing.