Gas Mileage is poor

[BruthaMan]

Originally Posted by BruthaMan

My '72 gets about the same, on the highway.

4.11 gears with turbo 350 trans. At 50 MPG, I'm already at 2500 RPM.

It's a blast in-town, but on the HWY, she's a screamer! Hoping to add a 2004R down the road and/or find someone locally that wants to trade out axles for 3.73.

I really think keeping the RPM low is the key to good MPG on the HWY. I've been driving my truck from San Angelo to Lubbock for the past six months. It's about 200 miles each way. At 65-70, I typically get 10-11 MPG on the HWY.

Motors is tuned very well. I've spent a great deal of time on timing and tuning. Carb (Edelbrock 1406) is tuned well, recently rebuilt. One thing I did notice was, the plugs that were in the motor were .35 gap AC Delco. I swapped those out for Bosch Split V's gapped at .45 and it ran much better. Not necessarily MPG, but just idled better and was a snap to tune to the motor.

Coming back from Lubbock yesterday afternoon, I decided to just take it easy and keep the truck at 55 MPH. At 55 MPH, I'm at around 2750 RPM. I normally drive at 65. Gas mileage was considerably better. I normally have to stop in a town about 45 miles from home to refill with 1/8th tank left. This time, I was able to make it all the way back on one tank. 13.8 MPG.

Really need to get an overdrive to drop the RPM down and be able to drive at reasonable speeds. The only good thing about driving that slow is, I never have to pass anyone!

[truckster]

Originally Posted by BruthaMan

I really think keeping the RPM low is the key to good MPG on the HWY. I've been driving my truck from San Angelo to Lubbock for the past six months. It's about 200 miles each way. At 65-70, I typically get 10-11 MPG on the HWY.

Coming back from Lubbock yesterday afternoon, I decided to just take it easy and keep the truck at 55 MPH. At 55 MPH, I'm at around 2750 RPM. I normally drive at 65. Gas mileage was considerably better. I normally have to stop in a town about 45 miles from home to refill with 1/8th tank left. This time, I was able to make it all the way back on one tank. 13.8 MPG.

Really need to get an overdrive to drop the RPM down and be able to drive at reasonable speeds. The only good thing about driving that slow is, I never have to pass anyone!

RPM at highway speed isn't the only factor affecting fuel economy. How much air you have to push is also significant. The Department of Energy estimates that an average family car will have a 17% fuel mileage penalty at 70 MPH vs mileage at 55 MPH, but that's for an average family car. A 67-72 Chevy/GMC truck is far less aerodynamic.

[Average Joe]

Yet another mpg thread lol. 12mpg with a 250 is about what I used to average with a freshly rebuilt 250 in a 69c10 I used to own. It had 3.73's and 3ott. Best I ever got was 16.5 cruising at 60mph on flat ground with no wind. Im currently running a tired smog era 454 in my Burb and getting close to 12mpg on the highway with 8 in town. My take on it, like others have said, is a 250 is really underpowered in these trucks. A 350 will get as good or better mileage and be much funner to drive.
If you like to tinker you may be able to improve your mileage with a dialed in HEI. Add a electric or clutch style fan and build a cold air intake. Lots of idears on here for maximizing mpg. Just remember at the end of the day its still a heavy brick with aerodynamics akin to a Westinghouse and old technology. Mileage will only get so good.

[67ChevyRedneck]

My 67 used to be a 250 straight six, 3spd, 3.73 rear. Back then it got 11mpg no matter what, city/highway/loaded/unloaded... didn't seem to matter.

It now has a 350/700R/3.73. Still gets 11-12mpg "city" but I can squeeze 15-16mpg out of it on the highway.

[Eddie H.]

You think 10 mpg is bad, I once owned a 3/4 ton Ford with a 390 engine that was such a gas hog that I couldn't even get the tank full if I left the engine running while trying to fill it up.

[ho70]

Originally Posted by Eddie H.

You think 10 mpg is bad, I once owned a 3/4 ton Ford with a 390 engine that was such a gas hog that I couldn't even get the tank full if I left the engine running while trying to fill it up.

My dad had a 460 Ford-same thing-he only owned it for two months

[48richard]

I have a 71 longbed, 350/350 with 3.08 rear end and 31.5" tires which are about 10% taller than stock. I kept track of the mileage over 3 years and got 11.5 mpg using it as a driver, about 2500 miles per year.
Then I pulled the engine because it leaked oil. I re-sealed all the covers and intake manifold, painted it and put it back. No more leaks, but over the last 15 months it has averaged only 10 mpg. The only thing that I disturbed that might have some impact is the timing since I removed the distributor, but it is timed properly as far as I can tell, and certainly runs as well as ever.
Any ideas, other than global warming?

[Joel69]

Mine is a 72 long bed, 350 and 700r4 trans with 3.08 gears it gets me 15 mpg everyday with the occasional 16 mpg every now and then; but i have the TPI fuel injection system from a 1989 corvette (that was the donor for the engine and trans),

i guess the fuel injection system makes a difference.......

[1968c15]

just to chime in, I'm gettin about 12 city and 17-19 hwy if my calculations are correct with a 350 and .308 in the rear withh a 700r4 on my 68 gmc stepside but after reading what everyone else is getting I'm not so sure anymore haha

[DreamRyder1963]

Originally Posted by 1968c15

just to chime in, I'm gettin about 12 city and 17-19 hwy if my calculations are correct with a 350 and .308 in the rear withh a 700r4 on my 68 gmc stepside but after reading what everyone else is getting I'm not so sure anymore haha

It does sound high but then again my brother gets about that with his 305 v6. Granted he just drives it to school and back.

[profe56]

If gasoline prices were at least$ 2.50 a gallon, I'm sure that nobody will worry, by modifications we do is almost impossible to make our trucks have more performance. I already I convinced that my truck is like a toy for my. I have a 305, Rochester4 bbl, headers, 700r4 transmission, 3.42 rear end and yields only 11-12 per gallon.Last what I intend to do is: change to 350 engine, and 3.73 rear end ratio, and only because I want to convert it to rear disc brakes, hope to win another 2 miles.

[1968c15]

Originally Posted by profe56

If gasoline prices were at least$ 2.50 a gallon, I'm sure that nobody will worry, by modifications we do is almost impossible to make our trucks have more performance. I already I convinced that my truck is like a toy for my. I have a 305, Rochester4 bbl, headers, 700r4 transmission, 3.42 rear end and yields only 11-12 per gallon.Last what I intend to do is: change to 350 engine, and 3.73 rear end ratio, and only because I want to convert it to rear disc brakes, hope to win another 2 miles.

wouldnt 3.73s yield worse milage than 3.42?

[profe56]

Good question for those who have 3.73 ratio.

[hamjet]

With the OD tranny I don't think the gear change will be a factor. going to more cubic inches probably would hurt more.

[motorcritter]

Swapping out the 350 SBC w/ a Holley (poorly tuned), 465 trans and 4.10 Dana that's getting about 5-8 MPG for a 305 TPI, a 2.95/.63 T5 in the '69 GMC C2500. There's a good chance that mileage will be better. If it's not good enough, then I'll swap out the rear end gears for something more economical. This truck will get run around town and in the Foothills a bit. The '72 C10 Cheyenne (402/QJet/TH400) will get the LSx and OD automatic. That should improve the solid 10 MPG it's getting.

[my67c20]

Before i started to do any mods, i would first try some different driving styles and routes, assuming the vehicle had a good tune-up. Get a good baseline, then change one thing at a time, starting with lowest cost items. Ultimately you are looking for your vehicles sweet spot. The point where fuel consumption and torque play nicely together. Remember you are moving a brick, so fuel consumption is going to be max at takeoff, and should drop at the sweet spot. And remember, like any good engineering project, it requires many data points.

[68Gold/white]

I had only heard of economaster 4 bbl carbs, just wondering.

[data89]

Originally Posted by 68Gold/white

I had only heard of economaster 4 bbl carbs, just wondering.

You know what, I bought that Holley for my 69 Dodge 318. Now that you mention it, I bet it won't fit on the Chevy with the monojet. I don't know much about carb interchange (obviously), but, you think it can't be used?

[68Gold/white]

Originally Posted by data89

You know what, I bought that Holley for my 69 Dodge 318. Now that you mention it, I bet it won't fit on the Chevy with the monojet. I don't know much about carb interchange (obviously), but, you think it can't be used?

Not sure if it can or can't be used. I'm sure your carb mounts w/ 2 studs, you'd have to measure both to make sure. You need to make sure the throttle plate on Holley doesn't get hung up on inside of intake. Fuel line needs to hook up, as well as throttle linkage, and air cleaner. Choke type on both?
Try it!

[71K20chevy]

I got a 71 K20, that gets 13-14 highway. It's lifted 4", has 4.10 gears, 33" tires, TH350, a MILD 350 with a Qjet and HEI. Not too bad if you ask me.

[72MARIO]

I love the honesty in this thread ! I will also join the 10-12 mpg club in my 67 C20. Now my 68 suburban has a 327 holly economaster 450cfm3:73 s I added a gear vendors but it didn't change much other than driver comfort. . It does get a solid 18mpg to a us gallon. I have no idea why the engine was built in 1982. Just not going to touch it.

[dsy1]

My '71 C20 longbed started with a 350/350 with 411 gears, and got a consistent 10-11 MPG. Now it is a 350/2004R with the same gears and still gets a consistent 10-11 MPG. I only drive it about 2000-3000 miles per year, so I don't fill it up very often.

[Z10]

When I purchased my 72 the PO had replaced the 6 with a small block 400 and a 350 trans. 10mpg pretty much was all it would get. Changed the engine for a ZZ4 350 and 700r4 with 3.73 rear. It's now getting about 16 on the highway with and engine that's not broken in and hasn't yet been filled with Mobil 1. Hoping I'll end up with around 17 when it get's a little looser.

[DGulliver]

Anyone who bought one of these old trucks with any gas mileage concerns is barking up the wrong tree. And any comments about Hondas and Prius or whatever gas mileage they get are irrelevant. I would walk before I'd drive either of those anywhere. Fuel up and ride!

[mrein3]

Am I the only one that keeps track of these things?

Those 1.8 mpg to 4.5 mpg tanks were winter tanks of gas where I plow my driveway and drive out to the fish house. That type of use involves lots of backing up, lots of idling, lots of engine running time with the odometer not advancing.

The graph is from excel. If I add a trendline it looks like I started out a touch over 10 mpg and now average a touch under 8 mpg.

If I take a trip in the summer, not pulling my boat, I get 13. My worst tank ever was 1.49 last winter. The winter from he!!. I didn't make it through the winter on my seat tank plus one saddle tank. I had to pour in FIVE 6 gallon jerry cans (I don't drive the truck on the salted roads in the winter).