C20 milage improvements

[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.

[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.

[BCOWANWHEELS]

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

[Green Machine]

Highest gear in most of the 3/4 was 4.10 with an eaton and many have 4.56, no better ratios available for these. If you can locate A Dana 60 from a big block truck, they were 3.54 ratio. Got a dana 60 3.54 posi sitting in the garage waiting to go into the 1971 when we get time, that's our plan.

[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.

[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?

[68K20 x Drill]

Originally Posted by 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.

3/4 4x4 with 4:10's, 350 and 4 speed rolling on 35's, the only thing I think every time I drive my truck is ...'where is 5th gear?'

I was getting 11 MPG with a tired 350, but it cracked a valve.
New low miles with fancy parts 350 from PnP got 8.5 the other day
It was raining and I idled in traffic a bit but...
You can feel 'lumpy idle' to 1700 RPM and it seems to pull real hard at high speeds. I think someone put in one of those 'light car with high stall only' cams, and it sucks in a 6200 Lb truck with a stick

At 70, aerodynamics can be a pretty big factor. I know a few VW guys who can measure an economy and top speed difference by taping the drip rails(harder to measure with a big motor). A front lip and smooth undertray should make a noticeable difference as well. Mythbusters did some testing, and found a net or open tailgate also made a difference.

My big plan is to bump the compression up around 22:1(diesel)when I have too much time on my hands.

[mrein3]

Originally Posted by ezglide

stuff deleted...

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.

I'm guessing that with 20% taller tires, and the factory 3.07 speedo corrector unit (I don't know what they're called) between the speedo cable and the x-fer, you're probably reading the correct mileage. Meaning you're probably right on with your mpg calculations.

A good way to check to see if your speedo is on and therefore you mileage calculations are correct is to drive down the road with a GPS. In relative terms a GPS is way more accurate than just about anything out there. Set the GPS to read speed. On my old 5-channel Eagle unit it is "SOG" or Speed Over Ground. Place the unit where the antenna isn't shielded by the truck and where you can see the display without causing you to not pay attention to the road and drive around a bit. If your speed reads the same as your GPS, you know your calculations are correct.

If your speedo isn't correct, you may be getting better mileage than you calculated. Or worse. Without a correct speedo your odometer is off and you don't have good data.

On the RHS of your rear axle facing forward is a three letter code stamped in the axle tube. Write that down then get on the google and search for Chevy rear axle codes. That will tell you what your rear end ratio is.

If your data is good, and you think you may want to go with a 700r4, click over to the 4x4 and off road forum and ask if anybody over there runs 33" tires, 3.73 rear end gears, and a 700r4 before you start spending money. These forums are full of helpful people. In all likelihood somebody there has a K20 with a small block, 3.73 gears, and 33" tall tires. The chances are they'll be more than helpful.

Some links on the 4x4 forum:
http://67-72chevytrucks.com/vboard/s...ighlight=700r4

http://67-72chevytrucks.com/vboard/s...ighlight=700r4

One more thing. When I was researching putting a 700r4 in my 4x4, I found that the Advanced Adapters output shaft is cheaper from Summit than from Advanced Adapters. Go to the AA website, get the AA part number for the correct shaft, then search on Summit's website for that part number.