Terrible Gas Mileage

[just_some_guy]

Well, the carb is running a little rich, but I didn't think it would make MPG that bad. Gotta adjust it. My TH350 has what I think is a factory transmission cooler. I guess it came w/the tow package?

 

[Goldie Driver]

Well, the carb is running a little rich, but I didn't think it would make MPG that bad. Gotta adjust it. My TH350 has what I think is a factory transmission cooler. I guess it came w/the tow package?

Separate cooler, or one built in the radiator?

I am not sure about the separate transmission cooler, but believe the factory towing package offered probably came with the factory engine oil cooler which has a separate heat exchanger.

If you know how to adjust the carb - I'll admit it, I don't- then that , plugs, cap, a rotor and maybe wires , and checking the timing would all be good 1st steps.

Or, just pull a few plugs and see how they look.

I would not expect you are gonna get 15 on the highway, but would think 12 is not out of sight.

( I say 15 cuz a buddy of mine could get that in his C10 , but he could hear the sweet spot on his engine and knew how to hit it on the highway. )

I also remember getting 9 - 10 highway in my full time 4wd rigs and when I bought a 79 Trans Am with the 403, auto, and 2.73 gears hitting 15mpg and thinking how bad ass the mileage was.



Different era for sure.

 

[Rusty Nail]

Pix?

Trans Am Pix?

 

[Goldie Driver]

Pix?

Trans Am Pix?

I'd have to look for the dam things. It got stolen out of a Carters Country (gun store ) in the late 80s or early 90s.

Solid black - was a factory black and gold special edition bought off the original owner but he had it painted.

Had I known it was being stolen I would have said give me a gun and ammo and I will be right back after I test fire it on some scum ...



Never recovered, BTW.

 

[Rusty Nail]

Sorry I axed.

AXED!

 

[Goldie Driver]

Sorry I axed.

AXED!

Well, on the plus side, the heater core had given out, and I never had it fixed.

And, had theft coverage, so I got something back.

I also learned I like 403's.

 

[jake wells]

major tune up and a carb rebuild is when i would start and keep your foot out of it. Keep it at 50-55.
I rebuilt my carb and did a major tune up and i am getting a little over 16 at the moment but i got a light foot.

 

[just_some_guy]

I have put new Autolite plugs, new wires, new MSD cap, rotor, and coil. I'm going to work on the timing, idle, and adjust the carb today - or at least try. I usually try to drive around 55 - 60 MPH on the 2 lane roads but when and if I ever get on the highway I usually drive around 70.

 

[Goldie Driver]

I have put new Autolite plugs, new wires, new MSD cap, rotor, and coil. I'm going to work on the timing, idle, and adjust the carb today - or at least try. I usually try to drive around 55 - 60 MPH on the 2 lane roads but when and if I ever get on the highway I usually drive around 70.

What size tires ? If larger than factory you need to adjust your numbers when calculating mileage.

Same applies to your speedo in case of different tires. For me, 70 real equals around 60 indicated.

 

[just_some_guy]

What size tires ? If larger than factory you need to adjust your numbers when calculating mileage.

Same applies to your speedo in case of different tires. For me, 70 real equals around 60 indicated.

They are larger than the factory tires but not by much. Cooper Discoverer AT3 XLT 32x11.5-50R-15's. Just had em' put on so they have good tread and pressure.

The local cops put one of those "YOUR SPEED IS" contraptions about a quarter mile down the road from my house, so I drive by it occasionally and check its reading against my speedometer and it appears to be accurate.

 

[Goldie Driver]

They are larger than the factory tires but not by much. Cooper Discoverer AT3 XLT 32x11.5-50R-15's. Just had em' put on so they have good tread and pressure.

The local cops put one of those "YOUR SPEED IS" contraptions about a quarter mile down the road from my house, so I drive by it occasionally and check its reading against my speedometer and it appears to be accurate.

Yeah, Eagle Drive on the way to my Moms house has one of those. For me, 35mph indicated = 40 mph on radar.

Always good to know.


What was your factory tire size ?

There are a bunch of these online, but I have this one saved as a favorite.

https://www.tacomaworld.com/tirecalc?tires=235-75r15-33-9.5r15

Yeah, those dinky things are what the OE tires were...

 

[MikeB]

I'm getting terrible gas mileage. Like really bad. Maybe 6 or 7 MPG.

Motor: 350
Trans: Turbo 350
Gear Ratio: not sure just know it's a 12 bolt rear from a '81 Suburban

Any advice on what could be happening here?

Did it get better mileage in the past? Does the engine run as well as it always has? Does it by any chance have an old cat convertor? How about a closed heat riser valve? Does your 1985 model have an electronically controlled Q-Jet?

Depending on axle ratio and tire height, 12-14 mpg should be attainable on the highway with a well-tuned engine (ignition and carb), along with low-restriction exhaust system. I've done just that in a 69 C10 with a mild 350, TH350, and 3.08 axle.

 

[just_some_guy]

Did it get better mileage in the past?

Don't know I've only had it like 3 months.

Does the engine run as well as it always has?

See above.

Does it by any chance have an old cat convertor?

No. When the motor was changed from a 305 to a 350 all the old smog stuff was removed. Just got dual exhaust coming off the manifolds with an exhaust leak I'm gonna get fixed as soon as my Flowmaster 70 series mufflers get here. The company I ordered them from said they were "in stock", then sent me an email saying it was gonna be like a month and a half until I got my damn mufflers. They are basically drawing interest off my money for 2 months. Last time I deal with those guys.

How about a closed heat riser valve?

Not really sure.

Does your 1985 model have an electronically controlled Q-Jet?

No it's got an Edelbrock carb on it. Not sure which particular model, though. The electric choke on it appears to be unplugged so I might need to get that looked at. Not sure which of these loose wires plugs into it, though.

Depending on axle ratio and tire height, 12-14 mpg should be attainable on the highway with a well-tuned engine (ignition and carb), along with low-restriction exhaust system. I've done just that in a 69 C10 with a mild 350, TH350, and 3.08 axle.

Just adjusted the carb and timing yesterday. I think it might have had a leak in the fuel line on the back side of the filter too, so I replaced those old hoses. Just filled her up with 31 gallons of 87 and reset my trip meter so I'll keep you guys posted. Thanks again for all the advice. Now I'm off to the post office to get a shipping quote for ol' Goldie here.

 

[wingman50]

1981 C10, 700R4. Not sure of the rear axle ratio however was a diesel. Now have a remanufactured 350 from Mabbco Motors. Quadrajet 4bbl, turning 2K rpm at 73 mph. Maybe getting 10 mpg. Replaced a 350 burning a quart of oil every 200 miles, that was getting 8 - 9 mpg. Mileage doesn’t change whether it was highway or city driving.

 

[Goldie Driver]

1981 C10, 700R4. Not sure of the rear axle ratio however was a diesel. Now have a remanufactured 350 from Mabbco Motors. Rochester 4bbl, turning 2K rpm at 73 mph. Maybe getting 10 mpg. Replaced a 350 burning a quart of oil every 200 miles, that was getting 8 - 9 mpg. Mileage doesn’t change whether it was highway or city driving.

That is disappointing to hear. I figured a fresh, more powerful engine would not work as hard an return better mileage. Well, better than a 1 MPG improvement.

2K at 73 MPH is just barely turning - wonder if it is out of it's desired powerband.

Which could be the case w/ Goldie trying to sling 33's with 3:42's and a weak ass 175 HP motor - when brand new, so 112K ago. LOL