3.08 or 3.73 Rearend

[72peace]

I've got a 69 with a 250 6cyl, manual steering, manual brakes, three on the tree and the current rearend is a 3.73. I've got a cleaned up 3.08 rear I was thinking about replacing it with for better gas mileage. But my mechanic say's I don't want to do that because the 3.08 will hurt if I try to tow anything. What do you guys think? I may do some minimal towing but nothing major. Not sure what to go with, help me out here!

Thanks guys,
Nathan

[Boog]

The factory spec'd that 6 cylinder with the 3.73 for good reason. It's no powerhouse. But since you have a 3.08 rearend ready to go you could swap them out to see how well the 6 manages with it.

[weq92f]

I would stick with the 3.73. With the 3.08 and the 6 cylinder, the fun factor might be diminished too much. That few % mileage improvement probably isn't worth loosing the pep in the truck.

-klb

[YBNORML]

If you go with the 3:08's the truck will have a high top end but will take forever to get there. And with the truck being underpowered, you probably won't see much of an increase in mileage.

[68Gold/white]

Originally Posted by YBNORML

If you go with the 3:08's the truck will have a high top end but will take forever to get there. And with the truck being underpowered, you probably won't see much of an increase in mileage.

X2!
These old pickups are HEAVY!
A 250 6 cyl needs all it can get to help move the vehicle along, in ANY kind of driving.
You'd be lugging the engine a lot more with the 3.07 gear.
I don't even recommend using a 3.07 gear with a V8.
There's a darn good reason the factory gear is a 3.73, stay with it.

I've learned a lot driving these older heavy vehicles. If you think you will get any king of great MPG number, it is not likely, especially with an old school engine of any kind.

My point of view is to make it fun to drive, have at least "good" power, tune that combo to do it's best for effeciency, let the MPG number fall where it may. 12-15 MPG are good #'s for one of the old tanks, anything better is awesome!!!

[El Dorado Jim]

I think you would hate the 3:08...clutch will chatter when starting out and not much power with the straight 6

[Alex V.]

Heavy compared to what? My '67 Suburban, C10, V8/4speed, with lots of HD options, weighs approx. 4,600 lbs., so I doubt the OP's truck weighs much over 4,000 - not light, but not especially heavy compared to today's vehicles. Aerodynamics is far inferior, of course, which is probably a much bigger factor in efficiency loss than weight when compared to a newer vehicle.

[n33k0]

You might actually get worse MPG with the 3.08 since the I6 will be working harder to get moving from a stop. I don't have much experience with that, just what I was told when I was pondering a similar swap on my truck. I stuck with the 4.11 that my truck came with; MPG sucks, but the truck has plenty of pep.

[C10 - C90 Bill]

I don't even tow anything with my 3.73's with a 3-Speed that only has the 2.85 1st Gear.

Anything that would have any weight to it anyway.

The great answer to our situation would be an Overdrive Unit. But the Factory ones were rare and the aftermarket ones are too expensive.

I compromised with taller Rear Tires. I put on 265/75/15's and it made a noticeable difference.

[LongBox]

What C10 - C90 Bill said. I once had a 1/2 ton 250 six Chev pickup with the 4 speed manual. Because it had a 4 speed, the rear gear was higher (lower numerically) than the normal 3.73. That truck was frustrating to drive on the highway, because 4th always seemed too high, and if you put it in third, the engine was spinning it's little heart out. A buddy had virtually the same truck, but with the 3OTT and 3.73. It was much more comfortable to drive at highway speeds.

If you change to the 3.08 or whatever, you'll likely have the same problem.

[Daimler]

Just thought I would mention - back in the late 70's I saw a Hot Rod magazine with a picture on the cover of a Chevelle car doing a wheele down the track taken from the back. The nose was in the air. When I read the article it said the guy had to replace the rear end after that run. He was running a 292 six. I never forgot it. Maybe a 250 six is capable of some performance upgrades too?

[weim55]

Originally Posted by Daimler

Just thought I would mention - back in the late 70's I saw a Hot Rod magazine with a picture on the cover of a Chevelle car doing a wheele down the track taken from the back. The nose was in the air. When I read the article it said the guy had to replace the rear end after that run. He was running a 292 six. I never forgot it. Maybe a 250 six is capable of some performance upgrades too?

Posted via Mobile Device

[LongBox]

Originally Posted by Daimler

Just thought I would mention - back in the late 70's I saw a Hot Rod magazine with a picture on the cover of a Chevelle car doing a wheele down the track taken from the back. The nose was in the air. When I read the article it said the guy had to replace the rear end after that run. He was running a 292 six. I never forgot it. Maybe a 250 six is capable of some performance upgrades too?

I think Clifford Research offers lots of performance upgrades for inline 6 engines. Multi-carb intakes, headers, split exhaust, and so on. A 292 six hopped up produces lots of torque, and with headers and a split (dual) exhaust, a really great "bark" out the pipes. I bet there are other companies offering hop-up parts for the 6, and it's probably a good way to go.

[robs71k5]

Is that a 308 possi if so I might be interested in it if you decide can't use it

[weim55]

I did this swap myself ('71 C10 250 - column 3 speed). The truck was such a dog with 3.08s it struggled with with even slight grades. Mileage was no better. Went back to 3.73s....

Steve weim55 Colorado
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[trac209]

I have the exact opposite opinion than most here,I would go with the 3.08 rear. These trucks are no heavier than the 73-86 trucks that replaced them and they had inline 6 with even less gear like 2.56 ratios. My old square had a 250 and a 3speed with 2.56 and it was just fine and I even had a 16 foot travel trailer I towed with it. 3.73 gears on the highway are useless unless you like holding everyone up or like 3000 plus rpm. Im looking to swap out my 3.73 for those as well

[VWNate1]

As one who lives Way Out West and only has i6 powered old Chevies I can tell you that very few are properly tuned and yes, in the hills they'll be slow but you can fiddle the tires outer diameter to compensate for the 3.08 final drive .

It all depends on your usage .

I drive in the Desert Mountains a _lot_ , I don't tow much but I do haul old engines and Motocycles .

Dammit ~ i have to go now , I'm keen to hear what you do .

I'd jump for joy if I could find a 3.08 final drive .

[robs71k5]

I have 308 but not possi riding on 33's behind turbo 350 and a 383 strokes. I don't have a problem with power to keep forward motion but I often wonder about gas mileage . People are always telling me performance would be so much better with 373 but I like to keep the rubber on the tires and know that if I do a gear swap my tire life will be cut into a mere small percentage lol
I can't smoke the 33s from line but my carb and timing might have something to do with that as I was able to with previous carb which was a different quadraujet
308 not sure if they are built for towing or not . I don't see them being a deciding factor for a 6 cyl engine . It all depends what you use it for mainly. I use my truck for pulling a 5000 lb boat but only short distance and twice a year once in the spring and out in the fall. Then it's my daily driver but I think that's where my gas mileage might improve because 308 in the city might be to much and 373 might offer savings. This has been my experience over the last couple years driving with my personal habits . I haven't done a gas mileage calculation simply because that's not why I have this truck and that might give my wife ammunition to use :-)

[ohboy321]

I have 3.07 axles in my K10, and with my 327 it has lots of giddy up on the flats but I run out of steam pretty quickly on hills.

The truck is new to me and I am still trying to sort out the 327, which apparently was rebuilt by a PO, but could still use some adjustments. I will be rebuilding the older Holley 4 barrel carb in the spring.

Would a properly tuned/adjusted 327 handle the 3.07 axles, or should I look for 3.73s?

[tdangle]

Years ago I had a Dodge with a 318 V8, auto trans and 2.94 gears. Towing anything was almost scary. I switched to a 3.5x rear and not only did it tow much better, my mileage actually increased empty with a mix of city and hwy driving by a couple of mpg. I'd stay with 3.73

[LockDoc]

-
Do what I did with my '67. Put an S-10 5 speed overdrive trans in it. It made it a lot more fun to drive, and it will still tow if I need to. I did the swap in 2010 and no problems so far.

LockDoc

[Craftsmen]

I removed the six from my truck, also had the Saginaw 3 on the tree with 3:73 open rear end. I dropped on a 350 crate motor and a 4 speed Saginaw, went to the floor with a hurst shifter. I rebuilt the rear end, installed trutrac posi unit and installed a new gear set. I kept the 3:73 gear ratio, glad I did. I would never want to go any higher gear. This is a fun combination best of both worlds, cruising and playin.

[Mike C]

Originally Posted by LockDoc

-
Do what I did with my '67. Put an S-10 5 speed overdrive trans in it. It made it a lot more fun to drive, and it will still tow if I need to. I did the swap in 2010 and no problems so far.

LockDoc

This. This is THE answer to keeping a 6 in the truck. And an S-10 T5 from the 4.3 is easy to source. It's the V8 trans that is hard to get. Your truck will do EVERYTHING better with the T5. Tow, accelerate, cruise, MPG.

New cars make great power and they correct themselves for altitude and temperature, but what makes the biggest differences is the transmissions. You will absolute LOVE the T5.

[TwoFiftyShifter]

Originally Posted by ohboy321

I have 3.07 axles in my K10, and with my 327 it has lots of giddy up on the flats but I run out of steam pretty quickly on hills.

The truck is new to me and I am still trying to sort out the 327, which apparently was rebuilt by a PO, but could still use some adjustments. I will be rebuilding the older Holley 4 barrel carb in the spring.

Would a properly tuned/adjusted 327 handle the 3.07 axles, or should I look for 3.73s?

Automatic - 3.73s
4 speed - 3.08s

[Craftsmen]

There were 3 or 4 different ratio 4 speeds, with 4th gear all 1:1 ratio. All personal preference I guess on what your doing with the truck.