mileage question

[spanky55amg]

Specifically if you are running an old 305, but any small block, what are you guys getting mileage wise?

I just put on a rebuilt q-jet. I think I have the idle tuned. They have the mixture screws set with tamper proof stuff. The idle is down to about 700-800 area. (its hard to tell with a 2 5/8 bosch tach). I dont get in the secondaries b/c it bogs down. [I think I need to adjust the secondaries as I think its too rich]. But on my first test, I was getting about 6 mpg which is.... well too low. My aim is about 12-15 mpg. I know these things are cinder blocks down the high way and Im doing about 65-73 mph. But I have read where some people have gotten 21 mpg out of a q-jet... once again, super wishful thinking on a square body.

 

[yevgenievich]

9mpg in city, 12mpg on the highway. 4x4 3/4 suburban

 

[Honky Kong jr]

If you have a 305 you gotta work that lil sucker to get that truck up to speed and keep it there which inturn sucks up the fuel.

 

[spanky55amg]

If you have a 305 you gotta work that lil sucker to get that truck up to speed and keep it there which inturn sucks up the fuel.

Yeah, but I cant even do that b/c I would be in my secondaries that bog down the engine. I'll adjust those next. Still. I think this 700r4 is trash. I mean, it hits all the gears but doesnt lock up. at 65 Im doing 1800 ish rpms.
Well... 285/45r22 is a 32.1 inch tall tire, at 65 mph, and assuming 2.73 gears, I should be running at 1907 rpms... which is about right. (I used an RPM calculator).

I guess Im just going to have to fiddle with the carb. I figured I'd get more than a 4x4 suburban though Im not asking for a unicorn... just north of 12 mpg. Even 12.1 would feel better.

 

[vkh]

Have you checked timing? I've picked up a couple mpg by advancing the timing to about 10 degrees initial (factory spec was only 4 degrees for my engine). That's where I'd start, then a basic tune up, check spark plugs, wires, cap, vacuum leaks, air filter, ect.

 

[spanky55amg]

Have you checked timing? I've picked up a couple mpg by advancing the timing to about 10 degrees initial (factory spec was only 4 degrees for my engine). That's where I'd start, then a basic tune up, check spark plugs, wires, cap, vacuum leaks, air filter, ect.

Yep yep. Just put on a new dist and its set at 10 degrees. New wires, new plugs, new air filter.

This is a rebuilt carb and reeks like its rich. But like I said, its got tamp stuff on the idle screws and something on the top too. So I figure it just needs to be tuned. Ive done the idle speeds. I need to work on the secondaries... I might need a new fuel pump too.

 

[spanky55amg]

And it is the correct q-jet for the year. I went off of model number and it is non-ccc and has an electric choke.

 

[Honky Kong jr]

Have you checked timing? I've picked up a couple mpg by advancing the timing to about 10 degrees initial (factory spec was only 4 degrees for my engine). That's where I'd start, then a basic tune up, check spark plugs, wires, cap, vacuum leaks, air filter, ect.

BBCs love them some timing if you put the right fuel in it.

 

[vkh]

BBCs love them some timing if you put the right fuel in it.

I was running 36 or 38 total timing before pulling it down to the frame this summer, I run ethanol free 91 and I have closed chamber heads. Considering my elevation I might even try 40 when it's back together.

 

[vkh]

If you can justify the cost I'd suggest a a/f gauge. Helped me dial in my qjet and it'll make it much easier to adjust for future things if you plan on modifying your engine as you go.

 

[Honky Kong jr]

If you can justify the cost I'd suggest a a/f gauge. Helped me dial in my qjet and it'll make it much easier to adjust for future things if you plan on modifying your engine as you go.

Very useful tool if you know what your looking at.

 

[1987 GMC Jimmy]

I get highway mileage in the twenties on my Caprice, which has a 305, a Quadrajet, and a 700R4. I've calculated it at around 21-23 when everything is working right and depending if I'm doing 70-75 or 80-85. Of course, I do a lot of city driving during the school year so my mean fuel economy plummets overall. I think the tire size and gearing have a lot to do with it. My rear is a 2.56, which is pretty rotten except for economy, and my tires are P215/75R15, which are just under 28'' in diameter. The car does weigh right under five thousand pounds, which I'd guess is a bit heavier than yours but not a bunch. And it does have a CCC carburetor, and the lockup sorta works these days, which helps noticeably. A lot of factors here are different, but I think you should be shooting for that 15 or even 17 on the highway. If you don't make that goal, at least you tried. If your lockup isn't working, it's definitely worth fixing whatever in the circuit is messed up. I also feel like you said you were doing something different with the wheels and tires, but I could be mistaken. Either way, there would be improvement in going to stock tire dimensions. If that 2.73 gearing is what you have, that's very conservative, which is good for economy. Some of the fault could be in the carb, too. I don't know if someone else rebuilt or if you did, but either way, it maybe worth spending the money (relatively little) to have it professionally calibrated if you're sure the rebuild's fine. That's what I would do. A CCC C10 is probably gonna get the best economy as far as carbureted squares go, but yours should be a very close second. It depends on a lot of stuff and how far you're willing to go on it.

 

[jake wells]

my last truck with a 305 was a 79 with 2 barrel and a 3spd auto got 14 highway running 50-55 mph because i don't do interstate.
i had a 87 with a 305 it had tbi 700r4 and a tall geared axle 2:73?? i got close to 20 with it running interstate at 75-80 mph.

 

[yevgenievich]

My suburban did 14-15mpg on a stock size tire when driving at 50-60mph. With a bit wider tires and speed limit of 75-80 have not seen much better than 13 . You should see lock up around 55-60mph unless gearing is off

 

[jake wells]

in the early days on the forum i had a 75 c30 of course larry remembers the truck
it had a 350 4bbl 4spd manual with 4:11 gears it only got 9-10 mpg.