1969 gmc 4x4 1500 deluxe

[dgardhawk]

i have a 350 eng. 350 trans. and get terrible fuel mileage. is there anything that i can do to get better mpg without changing engine and trans.
thanks,

[jocko]

options:
a. lower numerical (i.e. higher) diff gears, front and back ($$$)
b. that's about it considering your restraints....

If you're really concerned, the o/d trans is about the only other option - and it requires moving the eng fwd about 2", a kit to mate the o/d trans to the t-case, etc etc.

I don't have any other ideas than that.

Remember to include the tire diameter in your gearing calculations, big deal on a 4x4 - and may be part of your current problem, depending on your current tire size/gearing, a tire change might be all that is required.

If you know your differential gear ratios and tire diameter, we can give you an idea what rpm you're running (and how it affects gas mileage).

WELCOME to the board!

[dgardhawk]

not sure of diff. gear ratio but i am running around 3500 rpm at 55 mph and i can watch the gas gauge go down.

[special-K]

2bbl or 4bbl? You can do best with a 4bbl. When are you running 3500 rpm? Kinda vague info so far

[67_C-30]

The tune of the engine and condition of the carb has a lot to do with it also.

[dgardhawk]

Originally Posted by special-K

2bbl or 4bbl? You can do best with a 4bbl. When are you running 3500 rpm? Kinda vague info so far

it is a 4bbl. 3500 is highway speed 55 mph or higher

[blue 72 gmc 4x4]

Originally Posted by 67_C-30

The tune of the engine and condition of the carb has a lot to do with it also.

Sure does! Start there!

[Shyguy]

Are you really going 55 or is that what the speedometer reads?

[jocko]

Yes. If you don't know if you have the original gears, check rpm vs speed with a GPS unit. What you are reading on speedo is not relevant. 3500 at 55 doesn't provide enough info to know where to start. Tuning the motor, yes, could provide 1 or 2 mpg improvement, but not much more. Btw, what IS your current fuel mileage, specifically?
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[396ci]

is your truck running in 4wd all the time? you could remove your front drive line when your not off road. might get you a few mpg's.

[swamp rat]

Originally Posted by dgardhawk

it is a 4bbl. 3500 is highway speed 55 mph or higher

Thats high rpm's, I have 4:10's and 33" tires i'm at about 2700 at 60 mph, i have a turbo 350 tranny.

You probably have 4.56 gears on your differentials

[dgardhawk]

Originally Posted by jocko

Yes. If you don't know if you have the original gears, check rpm vs speed with a GPS unit. What you are reading on speedo is not relevant. 3500 at 55 doesn't provide enough info to know where to start. Tuning the motor, yes, could provide 1 or 2 mpg improvement, but not much more. Btw, what IS your current fuel mileage, specifically?
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i get 5 miles per gallon whether i get on it or not

[dgardhawk]

Originally Posted by 396ci

is your truck running in 4wd all the time? you could remove your front drive line when your not off road. might get you a few mpg's.

no it is not

[So.Cal.Super]

what size tires are you running?

[MARKDTN]

Fuel injection swap. TBI or TPI is easy to add to a stock bottom end. With only a TPI swap in my '83 K20 (same transmission, same rearend) I went from 14.5mpg city to 17+ and 17 hwy to 22.

[BrianG]

Agreed, that's a lot of rpms for 55 mph. I'd check the gear ratio. You might not want to go with too high of a gear depending on what your tire size is, and how you normally drive it (city/highway). you want to get into the right rpm range for your engine and your style of driving. 5 mpg is pretty bad, but a lot of factors can contribute to that. It does sound like your gearing is pretty low though. Could the tranny be slipping at all? Do you notice that it shifts into second and then third right away when you take off? Is it actually going into all three gears. Is it stuck in low lock? Just a few things to think about.

[White70C20]

There's some good info here. Makes me wonder if my truck has 4.56 gears in my rear diff. At 65 MPH my truck is screamin'!

[White70C20]

Originally Posted by BrianG

Agreed, that's a lot of rpms for 55 mph. I'd check the gear ratio. You might not want to go with too high of a gear depending on what your tire size is, and how you normally drive it (city/highway). you want to get into the right rpm range for your engine and your style of driving. 5 mpg is pretty bad, but a lot of factors can contribute to that. It does sound like your gearing is pretty low though. Could the tranny be slipping at all? Do you notice that it shifts into second and then third right away when you take off? Is it actually going into all three gears. Is it stuck in low lock? Just a few things to think about.

I don't want to hijack this thread, as I think others could benefit from this too. My truck shifts into second and then into third right away unless I baby it. I can't ever hear it shift into third unless I putt around like a grandma. Is this normal for a TH350?

[swamp rat]

Originally Posted by White70C20

I don't want to hijack this thread, as I think others could benefit from this too. My truck shifts into second and then into third right away unless I baby it. I can't ever hear it shift into third unless I putt around like a grandma. Is this normal for a TH350?

Shift points can be adjusted internally, plus there is a modulator at the rear of the TH350 that is adjustable as well, you take the vacuum hose off the modulator and get a small screwdriver in there, i forget which way is which but it makes it shift firmer or softer, may even change shift points a little too?

[mmiddle]

HEI, Q-jet, Good tune up. 3500 @55?????You should be able to climb a tree!
put 3.55:1 gears in it. headers & free flow exhaust. It seems a though gearing is a major issue here.

OD tranny, EFI take the lift kit off?

There are many things that can be done, but $$$$$$$$$.

[dgardhawk]

i have 33x12.50x15 tires
also how do you tell what kind of gears you are running.

[dgardhawk]

had a dead battery. changed battery and when i hooked up the positive back up i smoked a wire that goes to the firewall. i know i probably have a short to ground so i changed all of the wires to the battery and put a fuse in line on the wire that got smoked and checked the wires to see if i could see anything that was obvious. hooked up positive and popped the fuse immediately. i dont see anything that is obvious, any thing that anyone can suggest would be appreciated. is there a way to bypass the wire that is smoking to see if that one is the problem. and how do you pull the wire out of the plug on the firewall.

[swamp rat]

First thing that jumps out to me here is that by your statement you hooked up the negative cable first then when you hooked up the positive cable the wire got smoked.

Always, Always hook up the positive cable first, tighten it, then hook up the negative last. If the negative cable sparks when you try and install it then it tells you that there is a short in the system that needs to be tracked down, also when you install the negative cable last if you accidentally touch your wrench to the body it won't spark as your tightening the ground, sparks around the battery are not good..

If you try to hook up the neg cable last, touch it to the battery and it does spark, go to the fuse panel and remove one fuse then go back and try reinstalling the neg cable, if it sparks then go to the fuse panel and replace that fuse and try another. do this until you don't have a spark and you have isolated the circuit

What you don't mention is what wire? what color? where does the wire originate and lead too?

In the Electrical FAQ section there is a full color schematic available you could find this and figure out what wire your talking about.

[dgardhawk]

chased wires most of the day, it is the red wire, it looks like it is the hot going to the ignition switch.

[dgardhawk]

so i pulled every fuse and i still have a short issue. could it be the ignition. it is the red wire going from the battery that melts when i put power to it. so i did a dumb thing and pulled the plug off the back of the ignition under the dash and of course some springs fell out. any idea how to get the ignition off the dash so i can fix it. cant seem to get it out.