calling every1 who went 1/2 to 3/4 ton axles!

[dinotruck68]

3/4 ton axles and gas mileage
i know that upgrading to 3/4 axles is good for more abuse and bigger tires. Im not gonna go any bigger than 35's on a 6 inch lift and i want to know my axles wont fail me when i need them but i also want to know that i wont die when i go to the fuel pump. i know bigger tires will lower the mpg a bit. i here 3/4 axles are pretty bad on gas cuz of gearing is this true? if so then cant higher gears be set in in order to help? i know these trucks waist alot of a gas as it is.. i just want to save as much as i can.

[msgross]

if you are running 35's with any axles then don't worry about gas mileage... gears, tranny, engine, carb etc.. will all play a big part but don't ever expect over 13 MPG i would guess.

[mothertrucker424]

13mpg is a pipe dream in my world

[72cheyennesuper20]

Originally Posted by msgross

if you are running 35's with any axles then don't worry about gas mileage... gears, tranny, engine, carb etc.. will all play a big part but don't ever expect over 13 MPG i would guess.

13 MPG, I wish.

[dinotruck68]

my lil 305 isnt that bad on gas it does better than my lil bro's 68 with a 283. u can get higher gears in a 3/4 ton axle tho right?? that would make the biggest difference wouldnt it?

[shilo]

4.11 is probably the most common 3/4 ton gearing. Which is about perfect for 35's. 3.73 are available too factory as well aftermarket can probably get you higher. But you dont want to be lugging your motor all the time just to drop your highway rpms a little bit. That will get you worse over all economy.

[dinotruck68]

i just want to be able to cruis at 55-60 mph and not hav my motor screaming and still get double digit mpg....if its possible...
i hav a 4 speed trans with a granny gear... if it helps any?

[msgross]

maybe you should look for an overdrive tranny if that's what your goal is.

[dinotruck68]

i really like my tranny. my dad had the same trans on his old truck and he was able to get it goin all the way up to 130 .i want the capability of 3/4 axles and i want to make it down the street without wasting all my gas lol.

[estig70]

67-72 Chevy 4x4 trucks = no gas mileage, but this may help in your search...................http://www.gearvendors.com/cg2wdt.html

Eric

[johnnywagon]

36's, 4:11 gears, mildly built small block, Q-Jet, TH350 and I get 8 mpg on the highway......when it sees the highway.

[79oldj]

I agree with estig70 your best option is a gear vendor overdrive unit. I am planning on putting one on my truck for mileage and simply so it is not deafening at 65 mph inside the cab. Plus with a 4 speed it gives you a reason to buy a T handle with a button in it.

[vtblazer]

Two quick ways to jump up in fuel mileage.
#1 and bigger difference - overdrive
#2 would be fuel injection

Remember, your trying to push a brick down the road, mileage won't ever be stellar.

[dinotruck68]

yea a fuel injection setup is on my to do list for the ruck. i'l look into the overdrive thing a bit more to...
what about the 3/4 axles.. whats a good gearing setup for highway use?? since i will be driving the truck every day on the highway.

[vtblazer]

Originally Posted by dinotruck68

yea a fuel injection setup is on my to do list for the ruck. i'l look into the overdrive thing a bit more to...
what about the 3/4 axles.. whats a good gearing setup for highway use?? since i will be driving the truck every day on the highway.

Find an online gear calculator and spend some time messing around with the figures.
It'll let you type in and change all the parameters for each scenario.
Then you could look deeper and find what tranny's offer what for a 1st gear too.
A th-350 has a better 1st than a th-400 but neither is as good as the 1st gear in a 700r.


It's all about tranny/gears and overall tires size. (height)

I know that a th-350 w/4:10's and 33" tires will turn right around 3K on the highway.

[shelby987]

It seems like something is being lost in translation here.....the fact that you have 3/4t axles doesn't really mean anything for your gas mileage, its the gears in them that counts. That being said, it seems like you are trying to get back to what your previous gearing was with 1/2t axles, if your 305 was getting mpg that you were happy with there are two questions that need to be answered.
1. What was the gearing in your old axles?
2. What was the size of your old tires? (Keep in mind that taller tires act as a gear reduction....an engine turning 35" tires through 4.10 gears will only run 75rpm faster at 65mph than the same combo with 31" tires and 3.73 gears).

All this being considered, there are limitations to what can be done to improve the mpg of a rig like this.....being up in the air exposes the undercarriage to a TON of air....while our trucks are shaped like bricks up top, at least they are smooth, all the turbulence you create by trying to push/drag air over your axles, driveshafts, transfer case, etc is NOT condusive to fuel economy. Gearing will only help SO much, short of building an engine to put out peak torque down low (think inline diesel) you might just be out of luck.....even with a double over drive 0.50 final gear set, you are gonna need an engine a HECK of a lot stronger than a 305 to turn those tires at 65mph.
I would say the cost of new gears isn't worth it, especially when you factor in the loss of power down low that will result, an overdrive (aux or trans) will give you a MORE highway friendly RPM....if you change the gears you lose the ability to spin the tires in the lower gears......you can always start using your creeper as a first gear if you go to tall in the gears.....but the jump from 1st to 2nd will be a nightmare.....and you think your engine humms on the highway......wait until it sounds like that everytime you make a 1-2 shift!

[shilo]

Originally Posted by vtblazer

Find an online gear calculator and spend some time messing around with the figures.
It'll let you type in and change all the parameters for each scenario.
Then you could look deeper and find what tranny's offer what for a 1st gear too.
A th-350 has a better 1st than a th-400 but neither is as good as the 1st gear in a 700r.


It's all about tranny/gears and overall tires size. (height)

I know that a th-350 w/4:10's and 33" tires will turn right around 3K on the highway.

T-350 has 2.52 T-400 has 2.48 Hardly enough to notice. However the 700r4 gots a 3.06 first and .70 overdrive. The shift from 1st to 2nd is pretty spread out.

[vtblazer]

Originally Posted by shilo

T-350 has 2.52 T-400 has 2.48 Hardly enough to notice.

Non the less, there is a difference.

Here's a great tranny info page for you guys, a good one to bookmark.
http://www.race-track.com/news.asp?TOPIC_ID=532

[Yukon Jack]

While I do agree that the 3/4 ton & 1/2 ton doesn't factor into the mpg question too much, but do remember that a 3/4 ton rear axle such as a 14 bolt does weight a fair bit more than the stock 1/2 ton axle. Extra weight does drag fuel economy down.

[vtblazer]

Originally Posted by Yukon Jack

While I do agree that the 3/4 ton & 1/2 ton doesn't factor into the mpg question too much, but do remember that a 3/4 ton rear axle such as a 14 bolt does weight a fair bit more than the stock 1/2 ton axle. Extra weight does drag fuel economy down.

That and if you decide to upgrade to a th-400 for that extra beef while your at it, it takes 54 horse power to run the thing.

[dinotruck68]

yea i know ts not really the axles themselves its the gearing... i dont hav 3/4 axles yet but i want to get em. as for the motor . my dads friend has a good 350 waiten for me. i dont want to get rid of my 4speed.. i jsut dont want suuuper low gearing in my axles.

[Fry]

I believe running the extra bearings and the bigger gears will rob some HP and subsequently take some mpg. I noticed a bit from my semi floater 14b to FF.
My 6000lb 3/4ton 85 Diesel w/6.2/sm465/3.73/33s gets around that 15-17MPG range town or highway it doesn't seem to care. I hope to hit 20 on the highway when I get bigger tires. If I wanted to get better I could get a yaris...
My new 1/2 ton work chevy with a 5.3 only gets 15.

[68K20 x Drill]

My 68 with 8 lug 4.11's on 35"s and a stick got(at best) 11 MPG with my old 350. I put in a much fresher 350, and now it seems to get 8.

[shelby987]

I would agree....the 350 is going to suck down even more gas than the 305.

It is your truck, and you are more than welcome to do whatever you want to it, but putting a bigger V8 into it, lifting it 6", putting 35" tires on it, and keeping your 4spd. manual is not going to be condusive to making much better than 10mpg......something in the equation has got to give....

4" of lift and 33's will cost less and result in less drag, unsprung weight, and less inertia to get rolling, and you could probably get away with 3.73 on a healthy 305 with an RV cam and a 600cfm carb (this would be my recommendation).....still not going to net more than MAYBE 14mpg highway....but it will also be a LOT cheaper.

good luck!

[dinotruck68]

ive already got sum 33's and a small lift not really into it much i still love the 6" with 35's look. My 305 was rebuilt about 4 months ago and runs real strong, every time step on it it throws me into my seat. so the 350 was an iffy? idea. As long as im in the double digits in mpg im happy. i had just heard the lower gearing in 3/4 tons kill gas mileage bad but from what a couple of u guys said its not THAT bad.any one getn over 11mpg in their trucks??