trouble with GAS MILEAGE!!!

[71 3/4 ton]

i have a 71 chevy 3/4 ton 4 speed with a recently rebuilt 350 motor and the only things that were not replaced was th distributor and the carbarator.
i drive it very easy and cant get it to do better than 10 mpg. the other thing i noticed was that it blows out carbon/soot when i start it. does something sound like it is adjusted wrong or does the carbarator need rebuilt so i can get better gas mileage.

[Longhorn Man]

Sounds like it is running a bit rich.
Don't expect a whole lot more than 10 - 12 MPG

[Mint68]

I wish I could get 10. Seems like every time I check it is just over 7. Maybe tire size is killing it, or just throwing off the actual count of miles driven.

[lowrollin70gmc]

The distributor and the carb are the two items most affecting fuel mileage. Are you still running stock points? If so, HEI is a cheap swap for better burn. Also, most carbs can be overhauled with a $10 kit and some time. Or just tuned with a good ear.

A K20 is a lot of cool pickup to move.

[tsetsaf]

I have a Holley that is running a little rich and I am pulling consistent 12 mpg with a turbo 350 on the same truck. The above feedback is dead on.

Consider a tranny swap for even better improvements. In my case it will pay for itself in 9 months assuming I pick up 2-3 mpg (I expect to do a little better though)

Mark

[WorkinLonghorn]

I'd check that the choke is opening up all the way (I disabled mine,but I'm in a warmer area).Then I'd suck on the vacuum line going to the distrib/ advance and make sure the diaphram isn't shot.(Mine just went out) Then I'd check the mechanical advance with a timing light.If all that checks out I'd maybe look at the float level in the carb. Etc. All the standard stuff. My 5200 lb. 3/4 ton gets close to 12 no matter how I drive.

[Premium72]

um...no sounds like your driving an cool old truck!!.. enjoy it!

[mrein3]

I've been keeping track of mileage since I bought my truck in 1994. Since it is NOT a daily driver I don't worry too much about fuel economy.

My average in 27,592.9 miles is 9.3 miles per gallon.

The low outliers in the graph are ice fising miles per gallon. Idling around on the lake, pulling the fish house around, and letting it warm up makes for crummy mileage. I see 2-6 mpg but only burn 20-30 gallons between December and March.

The higher outliers in the graph are the times I took the truck on the highway without towing something. There I see 12-14 mpg.

Since there are VERY few trips for me on the highway not towing, the payoff for an overdrive tranny wouldn't be that great. However I will put in a 700r4 some day because the torque converter lock-up in drive while towing will save me eventually.

When I took the 3-speed manual out and put in a th350 I saw a drop of .5 mpg. This seems reasonable because the final drive ratio of those two transmissions is the same however the th350 has some slip while going down the highway where the straight stick does not.

Long story short: 10 mpg is about right for a 4-speed stick. If you look to the left of the graph, you'll see I ramped up from just under 8mpg to 12. That was a HEI, new plugs, and new wires. This at a minimum will help.

[SunTzu]

Originally Posted by mrein3

I've been keeping track of mileage since I bought my truck in 1994.....

WOW, that is impressive attention to detail, do you do taxes?

Good information, thanks for posting this.

Rob

[68gmsee]

Originally Posted by Premium72

um...no sounds like your driving an cool old truck!!.. enjoy it!

How true... I don't know how many young teenage nephews have said they want my truck (350/350 two wheel drive)... I tell them they can't afford the gas to drive it to school and back.

Reality sets if you start to drive one of these old trucks daily. My brother in law drove it for when he wrecked his truck. He said all his profits were going to gas (160 miles round trip twice a week)... Mine was getting 12-13 on the highway but he was used to 25 to 30.

[greasy68]

It is the price you pay to play. I would follow the advice above and do the best you can. It is tough for me to get out of my 07 charger and drive my truck every day but i love it.

[Wreckless_71]

haha...yeah I make enough money at work to drive home then back to work the next day. Now if I break down thats another story, cause there is no alternate transportation, and I have to go to school.

[LockDoc]

Originally Posted by mrein3

(snip)

Since there are VERY few trips for me on the highway not towing, the payoff for an overdrive tranny wouldn't be that great. However I will put in a 700r4 some day because the torque converter lock-up in drive while towing will save me eventually.
(snip)

Not to hijack the thread but I don't think that will work. Everything I've read on the 700r4 says not to tow in drive. If I remember right you are even supposed to shift to a lower gear when you are bucking a strong head wind. The trans will keep shifting in and out of lockup and eventually burn it up.....

LockDoc

[Shyguy]

LockDoc, you are right. Towing in overdrive is not recommended. In the RV world the saying is if the transmission is shifting in and out of overdrive, take it out of over-drive or overheating and damage will occur.

I kind of got a round that a little by having my transmission built heavy duty and running synthetic fluid and a cooler.

I had a '72 GMC 3/4 ton with a 350, with a 4 speed and 4.10 gears and the normal mpg was 10 mpg.

Now I have a '71 C-10 with a 350 with a 4 speed and 3.73 and 31x10.5x15 tires and I can get between 13-15 mpg cruising the main roads.

Danny - Southeast Missouri

[greasy68]

700r4 is not so good for towing. I have one and it is great but for towing it is a pain. If you plan on towing a lot go a different route than 700r4. I have heard 4l60 and 4l80 are much better if you want to stay with OD.

[mrein3]

Re-read what I said. Towing in DRIVE - not OVERDRIVE is no different than towing a th350 in drive. Except that the torque converter locks which except for the slip until it locks I'll get back to similar economy my 3-speed manual had.

Now maybe I used the wrong words. When 700's came out they had L1, L2, D, OD to select with the shifter. Since then I've called the 3rd gear in a 700, the 1-to-1 gear which is identical to the 1-to-1 in a th350 or th400, "drive", and the next one, "OD", overdrive.

Because I do so few miles on the highway without a boat attached, a th350 with a lock-up converter would be the ticket. However those times I travel without a boat, I'm usually on a 70mph road. Dropping about 1000 rpms those few times I'd use it would be worth it to me.

I hope that clears that up.

[LockDoc]

Originally Posted by mrein3

Re-read what I said. Towing in DRIVE - not OVERDRIVE is no different than towing a th350 in drive. Except that the torque converter locks which except for the slip until it locks I'll get back to similar economy my 3-speed manual had.

Now maybe I used the wrong words. When 700's came out they had L1, L2, D, OD to select with the shifter. Since then I've called the 3rd gear in a 700, the 1-to-1 gear which is identical to the 1-to-1 in a th350 or th400, "drive", and the next one, "OD", overdrive.

Because I do so few miles on the highway without a boat attached, a th350 with a lock-up converter would be the ticket. However those times I travel without a boat, I'm usually on a 70mph road. Dropping about 1000 rpms those few times I'd use it would be worth it to me.

I hope that clears that up.

I guess, but every vehicle I have had with a 700r4 the shift pattern was P R D 3 2 1. "D" was "drive" until you reached a certain speed, then it shifted into "overdrive" so anytime you are in "drive" you are technically in "overdrive" because it will shift into overdrive once you reach highway speeds. If you didn't want to use overdrive you put the selector in third. 3 is 3rd, 2 is 2nd, & 1 is 1st.... That's what I was taught anyway.

LockDoc

[jay6891]

I parked my truck 2 years ago when gas got so high. I was going 44 miles round trip to work and burning two tanks a week of super unleaded. Now I just start it every month and use a one gallon can as my fuel tank. Unhooked the fuel pump and run to the gas can. I get approx 30 minutes of run time..idle and a few feel good blaps of the throttle and that one gallon is gone.

[mnunn454]

Originally Posted by SunTzu

WOW, that is impressive attention to detail, do you do taxes?

Good information, thanks for posting this.

Rob

I agree with you, Rob.

Mrein3, Your father isn't Ross Perot by any chance, is it?

(Oops. Just dated myself............)

[Premium72]

ehh. i drive my truck daily(when im home), i drove it everyday when i was going to highschool and another 3 years ontop of that until i tore it down, an i was broke!!..if your complaing about the gas mileage maybe u should get a honda...the only thing i dont like about goin to the gas staition in my truck is haven to talk to every goober who wants to talk to me about my truck. other than that it aint nuthn but a thang....