MPG question

[69long bed]

I searched the site a little bit and found some pretty good info, but I still want to ask. What MPG should I exspect to be getting. And what can I do to improve.
Truck is 86 SWB stock 305, 350, 3.08 gear, 255/60/15 rear tiers and lowered. Has stock manifolds and cat into a flowmaster dual outlet muffler.
Carb is Q jet, I drive 30 miles a day comb of city and Hwy, I believe I.m getting 9 miles to the gallon.

Thanks

[85burb]

I would loose the cat and get a good exhaust ran from the headers/maniflods and loose the q-jet if you dont know how to work on them. Not sure what you should be getting but i got 7 with my 6.0 with a cam and a carb intake with a 670cfm carb and that is in a 85 burb 4x4 with 33s. Seems low to me.

[Hotrodrobert]

If you have a non electronic Q-Jet, you cannot get a better mileage carb period.
Get the Rochester book by Cliff Ruggles available from Summit, etc.
Make sure you have a non computer ignition and that the vacuum advance works properly, all your ignition system is good.
Headers, esp. try-y design do help mileage.
Having the converter lockup working on the freeway helps, too.
Use a stock type air filter with the heat tube properly hooked and working.
Be sure the mechanical advance in your distributor is working and all in by 2800-3000rpm, total about 36-38*.
GET A VACUUM GAGE AND USE IT!!!!
I have had many people over the years that I worked on their vehicle to get more mileage only to have to teach them how to drive for mileage.






1

[hatzie]

Wider tires will cut down your fuel mileage.
My 1985 C10, 305, NP465, 2.71:1 rear, and 235 75 R15 tires got 14mpg hwy 12mpg in town with a stock electronic HEI and electronic Rochester Quadrajet.
My 1975 C20 beater farm truck with 350, NP465, 3.73 gears, 245 85 16 tires, Rochester Quad and regular HEI got 17 hwy and 13 in town.

[donut]

Stock, even when they were new, I used about 15mpg for a 2wd, 10-12 for a 4x4. (smallblock powered)
YMMV, this was a general rule I used. Covers most trucks of this era. (that I found)

Big blocks and/or lower gears, all bets are off.

[andrewmp6]

9mpg does sound a bit low whens the last time it had a full tune up and carb adjusted?That and driving habits will make or break your fuel.donut its some newer trucks too my brothers 01 f150 4x4 only thing done to it is a set of 31s and it gets 14mpg on the highway on a lucky day.

[tucsonjwt]

I have owned 3 squares - all 3/4 ton, one was 350/TH350, one was 454/TH400, current is 454/4spd granny. All got 10-11 city, 13 highway in best stock operating condition. For reference, my last new truck was a 2010 GMC ext cab 1/2 ton with 4.8L and 3.42 rear axle - got 15.5 city/18.5 highway but driven only 2800 miles so not broken in. New Silverados get 21 highway. I had a 2000 V6 auto Silverado that got 20 city with very careful driving,
10 mpg city is normal for a full size carbureted truck. You can gain or lose 1-2 mpg with driving conditions (A/C on, stop and go traffic, heavy foot, etc.). Considering the high cost of gas, it makes no sense to drive these old trucks unless you must. I drive mine only 3,000 miles per year or less - only when I need to haul or as a second vehicle. Fuel injected 1987 1/2 ton is a different story - similar to new truck gas mileage in top operating condition.

[379redneck]

Originally Posted by tucsonjwt

I have owned 3 squares - all 3/4 ton, one was 350/TH350, one was 454/TH400, current is 454/4spd granny. All got 10-11 city, 13 highway in best stock operating condition. For reference, my last new truck was a 2010 GMC ext cab 1/2 ton with 4.8L and 3.42 rear axle - got 15.5 city/18.5 highway but driven only 2800 miles so not broken in. New Silverados get 21 highway. I had a 2000 V6 auto Silverado that got 20 city with very careful driving,
10 mpg city is normal for a full size carbureted truck. You can gain or lose 1-2 mpg with driving conditions (A/C on, stop and go traffic, heavy foot, etc.). Considering the high cost of gas, it makes no sense to drive these old trucks unless you must. I drive mine only 3,000 miles per year or less - only when I need to haul or as a second vehicle. Fuel injected 1987 1/2 ton is a different story - similar to new truck gas mileage in top operating condition.

Do you know what a stock fuel injected 350, th350 3/4 ton 4x4 got? I have an 87 and did some work, just trying to get a better idea on the milage other than "sucks" Not tryin to hijack

[Your41Plague12]

Not much better. A 4x4 truck is a 4x4 truck, they're not going to be sipping gas. If a typical carbureted truck gets 10 mpg, I would say a TBI truck gets 11 or 12. But they obviously had other advantages and disadvantages. My truck has 9" of lift and 38x15.50 tires, and I got 11 on the highway the other day (I was ecstatic, by the way). My brother has a 1994 F150 with the 300 c.i. straight six, and he gets 12 or 13 on the highway. Old trucks are old trucks, and fuel economy will reflect that. Something with the aerodynamics of a brick is hard to make efficient.

[379redneck]

I know its not going to be anything good, thats why I have a car to. I was just wondering.

[tucsonjwt]

With that 3.08 rear and it being a 1/2 ton, I don't see why you can't get at least 12-13 mpg if you are 50/50 city/highway. I would take your truck to a shop with a good engine analyzer and have it checked out. You might have a simple problem that will show up quickly in the hands of a pro. I took my old BB to a shop that does restorations on old vehicles and they had 2 guys spend about 40 minutes tweaking things while on their engine analyzer and it now runs better than ever - well worth the $70 I spent there. It did not improve gas mileage much, but now I know it runs as good as it can.
.

[C-10 simplex]

It could be something simple---like the air filter is dirty and needs to be replaced. Also, consider that most gas is E10 or even E15 and that is probably not helping MPG. If you can find a station that sells 100% gas, this might help.


The first step is to make sure you are calculating MPG properly and accurately. How are you doing this? Is your speedo and odometer accurate---tested against mile markers?

If you have time, read my very anal, in-depth, on-going scientific thread regarding mpg:

http://forum.73-87chevytrucks.com/sm...ic,8474.0.html

2) i think aerodynamics can really help out a pickup truck. The main areas are tonneau cover, grill---and let's face it---your 305 does not get THAT hot and it does not need THAT big of a grill opening unless you're towing, but towing with a 305 isn't a good idea to begin with but that's another topic, and the underside---lots of turbulence there apparently.

i think the ecomodders site is good if you want to get into aero.

[Cue-Ball]

I agree with Tusconjwt. Something is wrong, and it's probably something simple. My truck is similar to yours: '80 SWB, 2WD, 305 V8, TH350 trans, 2.73 gears, 235/75R15 tires. I get consistently 12 mpg in town. I can get ~15 mpg with steady highway/freeway driving.

Go over all the basics. New air cleaner, fresh plugs, check your plug wires, check your cap/rotor, make sure your timing is set correctly, etc. Check non-engine stuff too. Make sure your wheels spin freely and that your brakes aren't dragging. Make sure your trans fluid is clean and full. It's easy to check your catalytic converter too. Have your exhaust tapped for an O2 sensor before the cat. Attach a vacuum/pressure gauge (you can get one for like $15 at harbor freight) to the sensor hole and make sure you aren't building pressure when the engine is revved. If all of that stuff checks out, then your carb probably needs adjusted. Best to have someone who knows Qudarajets handle that part, because it's easy to do more harm than good. This is where the O2 sensor bung you installed in your exhaust will really come in handy.

[BigRed76]

My only bit of advice is keep the quadrajet. They will do great when tuned properly. When my '76 4wd was stock with the quadrajet i got 14mpg city/highway. I put on an edelbrock 600 cfm that is properly tuned and lost 2 mpg simply because the primaries are bigger...

[sethv]

Originally Posted by BigRed76

My only bit of advice is keep the quadrajet. They will do great when tuned properly. When my '76 4wd was stock with the quadrajet i got 14mpg city/highway. I put on an edelbrock 600 cfm that is properly tuned and lost 2 mpg simply because the primaries are bigger...

Agreed! Quadrajets rule when they're tuned properly. I was getting between 15-18 highway in the K10 back before I got an Edelbrock 1407. Now I'm lucky to see 12

[Hotrodrobert]

Keep the Q Jet!!
The Q has the best fuel atomization of any commonly available carb. If other carbs had the same atomization, they could get the same fuel efficiency.
Good info on Q Jets at Quadrajet Info by Lars on the Digital Corvette forums board-long, but very informative. Also, the Rochester Carburetors book by Doug Roe and the Quadrajets book by Cliff Rugggles. Roe's book has a lot more theory, Ruggles more performance mods.

[gofastbobby]

so this game of mpg is like golf right, the lower score the better? cause i have you all beat. i calculated the mileage on my 76 just before I put it away last week. 5.7, thats mpg not the engine size.

[glos55]

Originally Posted by gofastbobby

so this game of mpg is like golf right, the lower score the better? cause i have you all beat. i calculated the mileage on my 76 just before I put it away last week. 5.7, thats mpg not the engine size.

That's cause your big right foot is always IN IT!!!!

[gofastbobby]

Originally Posted by glos55

that's cause your big right foot is always in it!!!!

whatdya sayin im fat?

[bigredchevy]

i have a c20 3+3 383 base proformance engine, edelbrock carb, moddifide 400, 4.10 rear, im about 15 mpg. its all back country roads ruff 55 mph, truck wheight is 6660 last scrap run i did and this is my daily driver

[silverbullet01]

i have a 1984 suburban, 3/4 ton, 4x4, 305 4 barrel, th400 tranny, 411 gears and 33's, and i get around 18mpg. i also have a 1984 2 wheel drive half ton, i also have a 305 4 barrel, with a th350 tranny and 273 rear gear, and 235/70/15 tires, and i havent calculated it yet, but i get between 20-25 MPG, i have dual exhaust on both trucks.

[69long bed]

I added a converter lockup toggle switch over the weekend. Filled up the tank this morning and did the round trip. I could hear a big difference in the RPMs. And the fuel gauge hardly moved. All good signs of better MPG. I will report at the end of the week.
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[Your41Plague12]

I'm running 4.10 rears in my truck and 38" tires (not sure what the re-adjusted final gear ratio is), and STRICTLY highway, I still only get 11 MPG. I know it takes a lot to get the big tires rolling, but once I'm there and cruising, I should get better than that. I have a 350 (350 HP Crate) from Jegs with about 4K miles on it. Runs just like it should. No cats, true dual setup with 2.25" pipe and Thrush Welded mufflers. Carter AFB 650 CFM carb. It runs great, but on the highway I should get higher than 11 if you guys get 15 with 3.73's and normal sized tires. The time it takes for me to get to highway speeds shouldn't affect the mileage that much on a 100 mile trip. I'm also running a TH400, not a 700R4 like a lot of board members.

[donut]

Your41Plague12-rolling resistance can play a roll, as well as 65mph vs. 70 on the highway. The lift doesn't help the areo of a barn door either.

My K-20 used to avg 13mpg. Never clocked just the highway, but it was a noticeable difference between 65 and 70. This is with 4.10's and 32" tires (245/75/16), no lift, tbh350.

Your engine may be working to hard just to keep it rolling.

@69long bed, crossing my fingers for ya.