3.07 or 3.73 gear decision.....

[lukecp]

As stated in my earilier post, it looks like i am going to be in need of a new rear axle for my truck pretty soon.

So....i'm debating on putting in another 3.07 geared axle or swappiing to 3.73's. Either axle will be a used salvage yard unit.

Right now with my mild 350 and 3.07 gears i pull ~3,000 rpm at 75 mph, and usually get around 13 mpg on the highway, going around 65 mph with the engine pulling 2,500 rpm and lots of donwshifting/passing. With the 3.73's i will be pulling ~3,000 rpm at 65 mph. My mild cammed engine makes peak tourqe in the 2500-3000 rpm range.

Any guesses on how much the 3.73's will negatively affect my gas mileage?

I want 3.73's in the truck for my future plans, which are either an OD tranny, or putting a much bigger cam in my 350 and slowly turning the truck into a non daily driver hotrod. So i don't want to swap in anohter 3.07 geared rear only to swap it out for 3.73's six months or a year down the road.
I would go with 3.42's, but they never came in 5-lug coil spring axles, and getting my rear rebuilt is far out of my price range.

Any imput is greatly appreciated

[Gordo]

The last time I drove my 71 on a decent trip, from CT to PA, 325 mi., traveling light, mild 350, 700R4, 373's, it gave me 20 mpg. The 373"s still made it sh!t and git. The 350 is 30 over, 9 to 1 flat tops, 194 heads, 600 cfm Edl and matching performer intake with a speedpro RV cam. Before the 700R4 above I would average 13-14 mpg.

[Palf70Step]

with a 3.07 rear with what you described, you can probably get around 17-18 on the highway and still be able to pull without alot of problems. Drag race pulling...nope...but regular truck stuff shouldn'e be a problem. If you opt to go with a 700R4 or another OD unit. You can probably see 20 on th highway. Mine with an OD 3:42 rear, I run around 2100 @ 70ish. I think if you are looking at keeping it as a daily driver, you would probably like what the 3.07 give ya.

[lukecp]

I still don't know how you guys get 15+ mpg out of these trucks
The best i have ever got out of mine has been 13.2 mpg....usually it is around 13.0 or so. This is with my 9.7:1 350, RV cam (204/214), headers, duals, re-curved HEI, 1406 edelbrock jetted 6% rich (was too lean for my engine), and stock TH350.
Of course i'm still new at tuning and i'm not all that confident in my tuning skills. I'm getting a G-tech meter pretty soon (they are going for like $40 new on Ebay) so i can tune it for performance...but MPG is a different thing.

I'm really not sure if the truck will stay my daily driver or not....if i could somehow get 20mpg out of it, heck yeah, i would keep driving it! Maybe with a 200R4, good tuning, and a JET quadrajet i could get 16-17...which would be cool...
I do love driving the truck daily, but latley it has been pretty expensive to keep on the road, with the high fuel costs, and all this little crap that is currently wrong with it and making it a pain to drive. I don't want to sell it...but it would be nice to take it off the road and fix all the minor problems if i were to get a cheap alternatre ride.

Sorry for the rambling, and thanks for the responses!

[gunnersm8]

4.11s and i dont see 65 for long periods of time...man i need an OD.

[lukecp]

ttt

[FRANKIEPOTATAS]

gas mileage......what is that. I do just fine with my 4.56's out back cruisin the highway at ohhhhh 50 to 55! as my daily driver she is always flirting with any gas station we pass!

[68CST327]

4.56's are the way to go! No need for a speedometer, I just look at the gas gauge to see how fast I'm going!

[FRANKIEPOTATAS]

GAS GAUGE with 4.56's????????? I had to take my gas gauge out........ the needle was moving so quick from full to empty it was distracting my driving!!!

[fast14]

Hey yellow72, what is a g-tech meter and what does it do?? thanks

[Rooster's 67]

only note to say is dont put a 2004r trans on anything but a trot-line for a weight, that was one of the worst trannys that gm ever built!!!!!!!! 700r4 is the only auto od or the 4L80 trans, or for those that wnat to stir a stick the six speed is really slick double od and all.

[cdowns]

i have a 2004R tranny that had a custom rebuild it's rated for more power than a similar rebuilt 700r4 have put 30k miles on it in 2years and hauled tons of stuff some crap some not and for the ease of install you can't beat thee 200 and at 18mpg at 80mph with 4:11's i can't complain about gas milage

[lukecp]

Originally posted by fast14
Hey yellow72, what is a g-tech meter and what does it do?? thanks

It is a tiny little electronic meter that you attach on to the windshield of any car, and it measures 1/4 mile times, 0-60 times, and HP. Supposeldy the 1/4 mile times are accurate within 2 tenths of a second, but the HP is all over the place. I have heard they are really accurate.....and they have been going for $40 brand new on Ebay (summit sells them for $140 new)

[jku72]

You are facing a dilema. You have two fundemental choices, whether you are going to spend or not. The best choice irrespective of money is to go with a 3.73 and a overdrive trans.
If you dont have that kind of coin, my money rides on the 3.07. Knowing how I am, which is to say poor, I also would some day like to have the 3.73 overdrive combo, but, I also know that raising money is a difficult proposition, and realized that I am driving this truck now. I went with a 3.07 in my burb, and doing that brought me from 13 mpg to 18, at 65 mph highway. A few other tuning mods got me a little more mileage. My truck is torquey enough as such.
My personal advice is to go with the 3.07, and save over time for the other stuff.

[lukecp]

Well guys, thanks for the responses. I called all the local yards and only found one dealing in these trucks (one said "hell, we crushed all that old stuff years ago" ). The guy said he had a good rear for $150, and he would pull if for me....gonna go check it out and see what ratio it is in a few minutes

[Baradium]

Originally posted by yellow72custom
Well guys, thanks for the responses. I called all the local yards and only found one dealing in these trucks (one said "hell, we crushed all that old stuff years ago" ). The guy said he had a good rear for $150, and he would pull if for me....gonna go check it out and see what ratio it is in a few minutes

A lot of guys do that

Went to a place once. Only 67-72 he had was a '67 panel truck (needs a TON of work and he wanted more than I was willing to give him). I think he'd crushed everything else before and only kept that one because the bondo has it looking solid and he seemed to have an affinity for panels.

He also told me of the IH's he'd gotten, one with a perfect body that didn't one. One in driving condition and another that had a lot of dents but also in drivable condition... all of which he promptly crushed because "who'd want those things?" It was really tempting to deck him...

-Ryan

[70c10]

Lucas, I would go with 3:73's. You know you will be going more performance oriented in the future!

[68c10owner]

i just bought a 3.73 posi out of a mid 70`s 4x4 for $100. i ordered new axle saddles from early classic and it will go right in. i would have prefered a 5 lug but my 68 is 6 lug anyway so it doesn`t matter. my point is, you can add any rearend you want. just get the saddles from early classic or anyone else selling them and find a rear with the ratio you want. i wanted a 3.73 cuz my truck was originaly a 6 banger and the gears are pretty deep and it`s howling too. i`ll probably get better mileage with trhe 3.73 gears but it`s my toy so i don`t care about mileage right now.

[lukecp]

Well i just got back from the yard....drove 20 minutes to look at a rear, and the guy tells me it is in a different place! Grr.....he said he would go look at it for me though, and call me back and tell me the ratio in the morning......i think if it is a 3.73 rear, i will proably buy it....but i will think about it if it is a 3.07 rear. I do want to upgrade to 3.73's one way or the other eventually though. Plus i need to make sure my pinion nut hasn't backed off (was reccomended by a member, and i want to check it before getting another 3.07 rear).

As far as the saddle things go, don't they have to be welded in? What about U-joints, brake lines, and shock mounts? Are they in the same place with the '73-up rears? There are 5x as many 73-87 trucks in the yards around here.....and some came with factory 3.40 geared 12-bolts too....but i don't have a welder and i really don't want to pay someone else to put it in for me......

[Duffy]

Having had two different trucks with 3.07 and 3.73, both 4 speeds, I would go with the 3.07 every time. You can't beat it for mileage and if you want more acceleration, you can always hop up the engine. Just my opinion, but then I drive the truck because its a truck, not a hot rod project.

Duffy

[68c10owner]

the saddles do need to be welded in place. the u joints should be the same and i`m going to use the brake line that came with the rearend. the shocks will bolt in like always because the bolt to a plate that gets bolted to the saddle it i remember right. i don`t have a welder either but my neighbor does. it doesn`t take much to weld them in. i just ordered the saddles today and i`ll be doing this swap as soon as the rain stops. i`ll keep you posted on how it goes. also, if you use a different rearend, make sure you measure it. the one i`m puting in is about the same length as the stock one. i think it might be a bit wider but it isn`t enough to mess things up with wheel fitment.