C20 milage improvements

[swamp rat]

After reading the thread for C10 milage I started to think about my truck and how bad the milage is and how much better the milage is in the newer trucks.

I am left to wonder what other 3/4T owners have done to improve milage?

Different gearing, Tranny, running gear or engine mods? Fuel injection or a newer engine?

Has anybody here ever yanked a body off of a late model truck and mounted the classic body style into it?

[ridin98ci]

Im there with u. I have a 69 3/4 ton and am getting about 9mpg. I would be happy with 12-14mpg.

[custom72c20]

Originally Posted by ridin98ci

Im there with u. I have a 69 3/4 ton and am getting about 9mpg. I would be happy with 12-14mpg.

same here i drive a 72 c20. its always thirsty. i really feel it when i go to fill up.

[brianthelion02]

My k-20 always got 8 mpg,went to a 700r4 and went to a solid 13mpg!!!!

[WorkinLonghorn]

I've got a heavy (5200 lb.) longhorn 3/4 and I get 12 MPG no matter what I haul or pretty much,how I drive. But it's a stick and it is running about as lean as it gets,plus an adjustable vac-advance on the dizzy,dialed in.

[ridin98ci]

Originally Posted by brianthelion02

My k-20 always got 8 mpg,went to a 700r4 and went to a solid 13mpg!!!!

Im new to the chevy game.....mopar convert.....how would I go about swaping in a 700R4 for the grannie low 4 speed in her now?
James

[WorkinLonghorn]

ridin98ci,I don't think it's practical.You'd need a bunch of parts (trans+torque converter,collumn shift or floor shifter,cross-member,cooling lines kick-down linkage,radiator with cooler,modulator line etc) just to get a little better mileage,and I don't think his gains are at all typical.If you need to lower your cruise RPM you can get taller gears in rear end. I get 12 average with 3.73 rear and a granny 4 spd.

[swamp rat]

Originally Posted by brianthelion02

My k-20 always got 8 mpg,went to a 700r4 and went to a solid 13mpg!!!!

Now were talking! Do you happen to know what gearing you have?

I seem to average 8 last time I checked, I could live with 13 too!

[BCOWANWHEELS]

change the rear r&p ratio and install a O.D. trans. = 15-20 mpg

[swamp rat]

Originally Posted by BCOWANWHEELS

change the rear r&p ratio and install a O.D. trans. = 15-20 mpg

What do you drive? 3/4 4X4?

[davidh]

Most 3/4 tons with the 6.0 engine are only getting 13mpg on average. I think that can be matched.

[mrein3]

First thing I would do is figure out the gear ratios in your axels. The chances are they are 373s. If they are 308s, going to an overdrive transmission won't buy you much. In fact some here have posted that 308s with OD HURTS their fuel mileage.

If you have 373s or 411s, a 700r4 upgrade may help. You need to locate a core 700r4, buy the Advanced Adapters 700r4 in place of th350 output shaft, and take them to a rebuilder you know and trust.

Pull your engine. Remove the th350 from the transfer case and adapter. Move the engine cradle up. Bolt in the 700r4. Bolt the engine in its new location - approximately 2 inches closer to the radiator. Deal with the fan shroud. Buy a TV made EZ 700r4 kick down cable kit from Bowtie Overdrives. Set it up correctly.

Those mods, about $1500 - $2000 worth, will net you APPROXIMATELY 30% increase in fuel mileage.

But before you do all that, figure out what you spend on fuel per day/month/year. Then calculate what 30% (2.7 miles per gallon if you are at 9 mpg now) will save you. You may find that the payback is 10 years down the road.

Let me put it this way. I get 9.5 mpg in my 1/2 ton, 383, th350 4x4. The payback for a possible 30% increase in fuel mileage would be WAY down the road. So far down the road that so far I've left the th350 in my truck.

I had to drive the truck to work today because my daily driver is on jack stands waiting for parts. I go 38 miles one way, most of it in a 70mph zone.
I'd love to reduce my highway rpms by 30%. But for the amount of money I'd have to sink in a 700r4, needing to replace one daily driver, wife, kid, another kid on the way, justifying a $1500 expense is pretty tough in my case.

Do the math for yourself. Then decide if that potential 30% increase is worth it.

[ccd3]

Smart math- you make some really good points.

[swamp rat]

Originally Posted by mrein3

If you have 373s or 411s, a 700r4 upgrade may help. You need to locate a core 700r4, buy the Advanced Adapters 700r4 in place of th350 output shaft, and take them to a rebuilder you know and trust.

Pull your engine. Remove the th350 from the transfer case and adapter. Move the engine cradle up. Bolt in the 700r4. Bolt the engine in its new location - approximately 2 inches closer to the radiator. Deal with the fan shroud. Buy a TV made EZ 700r4 kick down cable kit from Bowtie Overdrives. Set it up correctly.

Those mods, about $1500 - $2000 worth, will net you APPROXIMATELY 30% increase in fuel mileage.


Do the math for yourself. Then decide if that potential 30% increase is worth it.

Is it worth it right now? no but mabey when I decide to do the frame off resto it will be. I do have 3.73:1 gearing. If you have knowledge of any reading for this mod on line I would like to read it. Thanks!!

[mrein3]

Originally Posted by ezglide

Is it worth it right now? no but mabey when I decide to do the frame off resto it will be. I do have 3.73:1 gearing. If you have knowledge of any reading for this mod on line I would like to read it. Thanks!!

Just do a search here or in the 4wd forum on this site. Many here have done it. I have a 700 core but still can't justify the money I'd need to invest in actually putting it all together.

373s with OD is a GREAT candidate for a 700r4 upgrade. PLUS you get a lower first gear (in the 700r4) which is awesome for pulling stuff at low speed.

Another nice thing about this mod in a 4x4 is that the speedo cable comes out of the tranfer case. If your odometer is reading correct now, it will be the same after the mod.

In a post you made after the one I'm quoting you state that you are running 33 inch tires. If you didn't correct for that in your mpg calculations I'm guessing your speedo is off by 20% and so are your mpg calculations. A factory th350/NP205 with 373 gears had NO correction for the speedo. If you got 307 gears they put a little gear reducer on the output of the transfer case and your speedo goes into that to correct for the different gears.


The difference between factory rear end gears is always 10%. 4.56 to 4.11 is 10% different. 4.11 to 3.73 is 10%. 3.73 to 3.07 is 20%.

Factory tires are 28 inches tall. 10% of 28 is 2.8. 2.8*2 = 5.6. 28 + 5.6 = 33.6.

Doing the math, if you have 3.73 gears and 33 inch tall tires you are in effect making your front and rear end gear ratios similar to factory 3.08 gears.

You MIGHT want to go back to 28" tall tires if you do a 700r4 as many people here who put a 700r4 in a 3.08 geared truck found a DECREASE in fuel mileage.

That is enough math for one night. I hope everybody reading this paid attention to your math teach in high school. That stuff comes in real hand for stuff like this.

[swamp rat]

Originally Posted by mrein3

Just do a search here or in the 4wd forum on this site. Many here have done it. I have a 700 core but still can't justify the money I'd need to invest in actually putting it all together.

373s with OD is a GREAT candidate for a 700r4 upgrade. PLUS you get a lower first gear (in the 700r4) which is awesome for pulling stuff at low speed.

Another nice thing about this mod in a 4x4 is that the speedo cable comes out of the tranfer case. If your odometer is reading correct now, it will be the same after the mod.

In a post you made after the one I'm quoting you state that you are running 33 inch tires. If you didn't correct for that in your mpg calculations I'm guessing your speedo is off by 20% and so are your mpg calculations. A factory th350/NP205 with 373 gears had NO correction for the speedo. If you got 307 gears they put a little gear reducer on the output of the transfer case and your speedo goes into that to correct for the different gears.

Doing the math, if you have 3.73 gears and 33 inch tall tires you are in effect making your front and rear end gear ratios similar to factory 3.08 gears.

Thanks again I will do some searching and do what I can to learn about the mod.

What I did was jack up the rear of the truck, marked the driveshaft and tire and spun it and counted the revolutions of the shaft, this was well over 10 years ago, it came out to roughly 3 and 3/4 turns per 1 turn of the tire, I concluded that I have 3.73's. I have not been able to find any markings on the rearend but its a GM unit from what I have gathered.

Now that said heres the conflict, I do have that little adapter sticking out of the transfer case too, kinda makes me wonder if the 1st owner did some mods to the truck? I bought it from my step dad and am the 3rd owner.

I would do the math if I had the formula! LOL! <- Means laughing out loud.

[ridin98ci]

Yep the math makes sense.....looks like Im gonna be in the slow lane and hittin all the gas stations for a long time!!

[Lemield]

My C20 with a 454 gets around 9 MPG....I've tried everything I can think of....no real improvement...I did go from a 750 CFM edelbrock carb to a 650 CFM...mileage actually went down a little...my newest solution....I leave it in the driveway unless I'm hauling something....helps a little.

As my buddy says...'It's up to us to burn up the fossil fuels so that they will be forced to find something better' I feel that I'm doing my part....

[rlm316]

Taller tires will cut down the engine RPMs. Fewer turns to make the same distance. Just need to do the math to convert the posted speed to the actual speed. Do a good tune-up. Make sure you are getting as much power out of the motor as there is available. My $.02

[Old Chevy Mech]

My 70 K-5 gets 15+ on the highway. The engine was built for the truck in 1993 and has most of the curve set in the cam, has oversize dia tires, not over width. It has a carb mod called a "carburator enhancer" installed and I run a Slick 50'd engine & drive train wit Amsoil 10W-40 engine oil. The speedometer is 8-9mph off/low side. Truck has a vacuum gage and I use it to drive by as much as my tach & speedometer. Normal cruise RPM is @ 2500. Adjustable advance set "just shy of ping on a long hill" LOL, K&N air filter, MSD 50K volt coil. Points ignition.

That 15MPG has been verified by 3 people to make sure I wasnt screwed in my calculations.

Frank

[CrashBob]

My '67 has a 327 TH400 and 4:56 gears and it doesn't see much speed above 65 as I can't afford a new engine. Mileage was 8-9 mpg depending on the wind direction. Put in a new billet HEI dizzy, 8.5 mm wires and hot coil. Now i'm averaging 13-14 mpg. When the money is available, I'm going with a 700R4 and 3:90 gears.
Try the HEI, worth the bucks, I have about $175 in mine. Good luck!

CrashBob

[swamp rat]

Originally Posted by rlm316

Taller tires will cut down the engine RPMs. Fewer turns to make the same distance. Just need to do the math to convert the posted speed to the actual speed. Do a good tune-up. Make sure you are getting as much power out of the motor as there is available. My $.02

I run 33's, at 40 I am around 23-2400, at 60 I am around 2700 (all by memory) and without wheel size corrections to the odometer.

[TotalChevy1]

I've owned alot of 3/4 ton and 1 tons and none of them have every done better that 10 mpg...pulling or not. I even had a 98 dually(7.4L) that under heavy load(14-16k) only averaged 3.9..... the exception is my duramax..it's a beast and pulling the same 14-16k I get 9-12 depending on how many hills I've had to pull and if I've got the 35's on.

[Brad]

While mrein3's math is basically correct, what's missing in his statement is that the price of gas will only increase. That means that the payback won't be as far off in the future as one might think!

[heyjoe]

My '67 C20 250ci, 3 on the tree, 4.57 rear gets 10 mpg city.