The Stovebolt Site Forums The Shop Area Engine Shop some one must know the truth

hello Stovebolters, I know that this bulletin board is for pre- seventy three pickups but I'm gonna try to sneak one in anyhow, I have a 1975 Chevy 3/4 ton p.u.with a 350 and turbo 400 w/411 rear end gears. This is a smog motor and I live in California so I must maintain the smog equipment. This truck only gets about 8 miles per gallon. I have met guys over the years who claim to have 3/4 and larger trucks that pull down 16 to 18 mpg easily.I am ready to install new running gear. motor, 700r4, 373 rear end gear. My tire size is 33" tall. What I would like to know is if anyone has acheived this kind of efficiency and if so what cam, intake, exhaust, and any other tips you might have.

Thanks
Tony

OK, I have a 59, but my problem was similar. Hopped up 350, 350 turbo, 3.9 ratio, 31 inch tires -- lousy mpg and way too much power for my suspension/steering/brakes (stock susp/brakes).

Long story short- I decided to sell truck and keep almost new engine and trans. I installed a used (very) 305/350 turbo from my sis-in-laws car. This is after I had changed rear to a camaro diff with 3.05 gears. The combination of 2 barrel, less cubes and gear ratio took my mpg from about 8 to 14 in town. I don't run it on the hiway (small town, no need)but I run the A/C most of the time. Truck was so enjoyable with the new combo that I have kept it.

700r4 is a good choice. Low first gear, overdrive. Combine that with the 373 you will get better mpg and still have decent power/accel.
Don't know if you can get the mileage you are looking for though.

Hey Tony, how attached are you to that 75? I would get rid of it and buy a pre 73 and be free of the smog rather than spend a bunch of money on this thing. No more biannual tests etc. (Which are up to $85 now)

I like Tony M's idea. My '57 hardtop has a 350 with a 700-R4 and it easily gets 15+ mpg (20 highway). A 350 should be getting a lot better than 8 mpg.

Only way I've seen 350's get over 15 mpg consistently is with EFI. Went on a 700 mile day trip in a burb with EFI, most of the driving was around 80 mph, and got 17 mpg. One low-dollar improvment is to change your ignition advance curve. You need a timing tape for the balancer, spring & weight kit for the distributor, and an advance timing light. Ideally you will be running 32-36* advance at highway speeds, firing well before the intake valves close. Should get you another 10-20% mpg on the highway. Went from 11 to 14 in my 69, 3.07 gears and th400.

Everything you mentioned will help. I have a 650 Holley 4 bbl on mine, but I got better hiway miles with the Q-jet. Problem was that the q-jet was worn out, and I couldn't afford a new one at the time. The mechanical lockout of the secondary does help a lot on the highway. Edelbrock is currently making new q-jets, but they're over $400.

get a older truck
what carb does it have?
do you ahev any idea what RPM you are running at 60-65 mph?

I'd be very skeptical of anyone claiming a truck from the seventies, 3/4 ton or larger, 350 with 15+ mpg. I don't think it can happen.

In my experience, you can expect maybe 10 mpg with this rig. Probably won't make any difference between fast, slow, loaded and empty.

3/4 tons are heavier, have heavier rotating components, usually lower rear end gears, and sit higher, having all the aerodynamic properties of a shoebox.

My old '72 C-10 had a 350 4 bbl, three-speed manual trans and 3.08 rear. It could pull down maybe 17 mpg on the highway but only 10-12 around town. A friend bought a new '73 with a 250 six and manual transmission, and he couldn't quite match my mileage.

But I've NEVER seen a 3/4 ton come anywhere close to that. And the '73 and up trucks, as memory serves, didn't seem to do mileage well, according to the owners that I knew.

Maybe some of the later EFI engines with slicker bodies might be able to do a bit better, but not with carbs.

My 3/4 ton '91 GMC Suburban with TBI 350, overdrive automatic and 3.73 rear gears gets around 12-13 mpg. Another friend has a similar '92 with a 454 and he gets about the same mileage.... he just has a bit more reserve power.

Hi everyone, Thanks for all the replies. To answer some of the questions. Yes I have thought of an older truck (72 cheyene super 3/4 ton W/ 4 wheel drive)and all that takes is money. But this truck is actually pretty nice I am the second owner it only has 90k orig, miles and the stock paint and never dented. so its worth hanging on to. I hope to be turning 1800 rpm's@65 and EFI is an option only if this truck falls off the smog laws this coming year and I heard that was going to happen ( anybody know for sure? ) Thanks for the info on distibutor timing curve because I believe that this is a contributor to the problem in 1975 we were at the height of the gas wars and all the manufacturers were derating everything and this truck is one of the casualties of the war. Underpowered on the bottom, and at 55 you would think it was in the Flyin mile at Bonneville.Hey another question to add to the mix is. Do you know if when I install the 700r4 will I still be able to get into the lowest gear with the colunm shifter or will I only be able to go to second gear?

Thanks
Tony

I think with a good lockup OD tranny (like the 700r4) and a 3.08 gearset you should at least get your milage into the teens. Another thing to do would be to go to the narrowest treadwidth tires you can find to lower the rolling resistance. That should make a big differance. Also, lower the tailgate or take it out when you drive on the freeway, that helps quite a bit. The tailgate acts just like a spoiler on an airplane, and adds a lot of drag.

4onthefloor
I think you mean "flaps on a airplane"

Tony, I know you can run the 700 from a column shift, I just don't know exactly how to do it. Go ask at one of these boards, they know the newer trucks far better than I do.
My Old Truck
47-88 Chevy Board

as of jan 1 2004 1975 gas power is exezempt from smog check , but I dont know anything I only smog checked 40 cars today.
I own a smog check shop
any ? call 805-644-smog (7664)

Nope, the flaps increase lift. The spoilers are on the top part of the wing and their only function is to slow the aircraft down once it is on the ground.

I think you should get at least 12 to 15 mpg with the 350; my 77 w/454 and TH400 got 10 mpg, unless I was towing a trailer, then it got 7.5.

On the Tail gate issue... I get better 1 MPG with it up (based off 1000 mile round trip). [i'll be doing more research on this later].

Here is why I THINK (emphasis on THINK, as i'm just a dumb Marine), that a pickup would get better gas mileage with the tailgate up:

1. With the tail gate UP, the air flows over the front of the truck and cab then drops into the bed. When the truck is up to speed (stable speed), the air starts rolling inside the bed between the truck and the overhead airstream, this acts like a bed cover. This also forces the turbulance up off the back of the truck, which reduces the drag.

2. With the tail gate DOWN, the air stream flows over the truck and out the back, where it causes turbulance, so the truck is constantly fighting the drag.

Hmm, this is an interesting idea about the tailgate, you would think with the tailgate up, it would require the truck to work harder to keep going as the wind would slam right into the tailgate. Well, good luck on the research, I would like to know myself if your idea's are true.

- ChevyToughRebel
- Shane
[email protected]

But the wind only slams into the tailgate until it starts rolling in back of the bed, then the turbulence goes right over the top!!!

My '64 1/2 ton gets 16 MPG w/350/700R4 Q-jet 3.42 gears. I had a '90 EFI C2500 had 350/TH400 all it would get was 13MPG. I feel Overdrive will help mileage more than anything. My 1st boss said there were 3 thing a man would lie about, 1-how much money he made,2-his sex life,3-his gas mileage. Brian

Once had a 1977 shortbed stepside Chevy with 350/350. It only got about 9 mpg no matter how I drove it.

My 78 El Camino gets outstanding mileage.

All smog crap is pulled. 350/350 with 2.9 (something like this anyway) rear end gears. Edel 600 on performer intake. In town I get about 12mpg. On the highway last summer to Salem, OR, I made over 20mpg all the way down and back. Gotta love those tall gears.

- Ryan