Is 12 mpg attainable

[cam.man67]

So here's the dilemma I'm faced with: my '79 is my winter driver, and in preparation, I've been driving the truck around for the past few days regularly. Good Lord, is this going to be an expensive winter...

See, I've never gotten better than 8.5 mpg... last fill up was an appalling 4.5 (I think I have a fuel leak somewhere, but that's not the point of this topic). Anyways, my financial situation this winter is not conducive to sub-10mpg economy (I'm getting married in April)...I need to do something different.

And so the question: is 12mpg feasible, given the following combination.

-Olds 350, Quadrajet
-TH400
-mud terrains out back
-12-bolt, 3.73 open

I like this truck a lot, and I really need to get some better fuel mileage out of it. Part of this is that I don't want to dump a bunch of parts at it in hopes of better fuel eco. I want to focus primarily on tuning, timing, etc. Is this possible?

[dfrank]

Originally Posted by cam.man67

So here's the dilemma I'm faced with: my '79 is my winter driver, and in preparation, I've been driving the truck around for the past few days regularly. Good Lord, is this going to be an expensive winter...

See, I've never gotten better than 8.5 mpg... last fill up was an appalling 4.5 (I think I have a fuel leak somewhere, but that's not the point of this topic). Anyways, my financial situation this winter is not conducive to sub-10mpg economy (I'm getting married in April)...I need to do something different.

And so the question: is 12mpg feasible, given the following combination.

-Olds 350, Quadrajet
-TH400
-mud terrains out back
-12-bolt, 3.73 open

I like this truck a lot, and I really need to get some better fuel mileage out of it. Part of this is that I don't want to dump a bunch of parts at it in hopes of better fuel eco. I want to focus primarily on tuning, timing, etc. Is this possible?

I think it is possible. Just start with a tuneup. New plugs and wires, check timing and seriously look into a rebuild on the carb. Are you using vacuum advance? That usually helps with fuel mileage. Also most importantly keep your foot out of it. Lol be gentle on the throttle.

[68post]

I was at 12 mpg on the highway with a warmed-up 350 (headers, cam, AL intake, Qjet) in a K30 with 4.56 gears/305 tires and SM465, and that was with a good load too. You should have no problem.

As part of your tune-up set the carb's idle mixture screws with a vacuum gauge @ the highest attainable reading.

[05duramaxhd]

i have a 86 all original with 63K miles 305/700r4/2.73 factory gear 2wd. I just did a 220 mile trip and she got 19.311 MPG's I was stoked. had cruise set at 58 MPH

[D13]

Do some tire and gear math to get the truck to run 2000 RPM at 70mph. Or, if you like, my trucks run a little over 1700RPM at 60mph. You can adjust this with gears or tires or overdrive trans etc.

#1 cause of fuel economy is driver. I can change 10% or more by how I drive. My dad got 13 mpg in the 81, I got 15+. Shift early and drive soft.

Assuning the Olds doesn't have any amout of cam (performance cams hurt low end torque which is what you are after), a good tune up, then it's gears or a 200-4R, both of which are bucks.

FWIW, 87 R10 3.08 P235/75R15's 700R4 and 6.2 diesel = 23mpg on mixed driving, close to 26mpg at steady 55mph. 70mph is more like 24. 52-58 is maxengine torque range wic is where fuel economy is waiting.

[Dead Parrot]

What speed are you measuring at? My 75 gets 14 @ 60 and 12ish @70 and worse @ faster. If you are measuring around town mileage, all bets are off.

For the tune up, start with the ignition stuff. Check the plugs, wires, cap, you may need new ones. If you are non-emission or the check is post winter, move your vacuum advance to manifold vacuum. Verify that both your vacuum advance and centrifugal(mechanical) advance are working. It is fairly common for the vacuum pot to have failed at least once by now. Not uncommon for the mechanical advance to have stuck in one spot. Failure of one or both cause poor mileage.

If you still have stock exhaust, make sure the riser valve isn't stuck shut.

If still equipped with the OEM air cleaner, make sure the THERMAC system is working. Again, fairly common for the vacuum pot to fail which causes longer warmup times and worse mileage.

Is the choke working? A stuck or slow to open choke can really hurt mileage.

Have you done the Q-jet fuel well epoxy fix? Q-jets often leak there and hurt mileage. Have also had issues with leaks on Chevy Q-jets at the fuel filter casting plug. If you have a Olds Q-Jet, you may not have that plug.

[tucsonjwt]

You should get 10-12 mpg city only.

A few suggestions:

Install manual choke - about $12. Then you will know when the choke is engaged and it should never be "stuck." Also, you can adjust the amount of choke you want - perhaps open it all the way shortly after a cold start if you are on icy pavement to avoid spinning wheels. I have found the most common reason for poor fuel economy is running rich due to a stuck choke, and this is a problem you can fix yourself for a little time and money.

Find an old mechanic who still has a SUN tune up machine. You can likely spend $40 for a half hour of diagnostics and get a good idea of the performance of your engine. You can also get an evaluation of the carburetor condition and assess whether or not you need a new one. I would avoid parts store rebuilds - if you need a carburetor get one from the professional Qjet rebuilders mentioned elsewhere on this forum. Modern mechanics don't want to be bothered with non-computer vehicles.

Drive like grandma on her way to church. Accelerate very slowly from a stop and take your foot off the gas long before you get to a red light or stop sign so you coast to a stop. If you are not irritating other drivers then you are not doing this right. If this improves your fuel economy then you know that driving style is a factor, then you can go back to driving any way you want if you think it is worth the gas $$$.

For reference, I have had 3 C20s (1973 454/TH400, 1976 350/TH350, and current 1983 454/SM465 granny) Of these, the 1973 got 11 city/13 hwy, the 1976 got 10 city/10 hwy, the 1983 gets 11 city/13 hwy (but it is currently only about 8 city due to excessive idling and being driven only 2-3 times per month.)

[rich weyand]

Go back through Dead Parrot's list.

I would put new plugs and wires on it as a matter of principle, because they're cheap.

Check the rotor and cap carefully.

Put the vacuum advance (79 got vacuum advance? 78 does...) on manifold vacuum.

Set the base timing to 12-16* BTDC.

If you are coming up on new tires, get them now. Good all-season tires will get better mileage. You don't say what size tires those are.

I think the Qjet well plugs are leaking. You can fix those a couple different ways.

[71swb4x4]

In your quest for mpg's be careful, it's easy to spend $2,000 to save $500 in fuel economy. That being said 8.5 mpg's is not good, you either have a major tuning issue or a lead foot. You should be able to increase that to 12+ mpg's fairly inexpensively.

[donut]

All good points. I'm assuming everything is mechanically sound? No brakes dragging, front end all wonky etc.? Mud Terrains out back... How accurate is your speedometer and odometer? Tire pressures set?
All these can have an effect on fuel economy.
Check this here, not expensive, but time consuming (and worth it) http://67-72chevytrucks.com/vboard/s...d.php?t=290498

FWIW, My 2 sig vehicles, '86 averages 11mpg, the K-20 would average 13. Both had/have 4.10's and I kept tire size pretty reasonable.
When the '73 went on the highway, didn't go over 65mph. More than that, you could watch the gas gauge drop.

[cam.man67]

Wow, lots of great advice here, guys. Thank you!

I've made a little progress in the tuning dept...regapped the ANCIENT (and soon to be replaced ) plugs, added a couple degrees of timing and adjusted the mix screws a bit...netted 10.42 in a13 mile trip of country and highway driving. So, we're getting there! idle vacuum has stayed around 19-20, so I'm pretty confident in no sort of leaking.

The qjet is damp on top of the fuel bowl, so I'm seriously considering rebuilding it if/when I get a chance. Plugs and wires are waving as soon as I have money.

Oh, and just for future reference, my odo doesn't work. been using a gps-based iPhone speedo for all my calculations.

[rich weyand]

For spark plugs, don't get anything fancy. Waste of money, and some won't work well in our trucks. ACDelco R45TS or Autolite 24.

For wires, You can use stock wires, but best to use IMO are Taylor.
http://www.summitracing.com/parts/TAY-74206

[dfrank]

Originally Posted by rich weyand

For spark plugs, don't get anything fancy. Waste of money, and some won't work well in our trucks. ACDelco R45TS or Autolite 24.

For wires, You can use stock wires, but best to use IMO are Taylor.
http://www.summitracing.com/parts/TAY-74206

+1 on acdelco plugs and +1 for Taylor wires.