What is the best rear gear for??

[72 Cheyenne402]

What is the best rear end gear for a 402 w/ a 400th transmission? right now i am running a 373 rear end gear ratio but I I am thinking of a 355 or 342 posi any Ideas? what would you go with with this setup?

[Longhorn Man]

depends on what you want out of the truck, and if the engine is built up or not.

[68Stepbed]

[72 Cheyenne402]

The engine was built about 5 years ago Crane Fast bleed lifters - low end rv cam - 427 crank (because I couldn't find a 396 crank) - Crane chromollly pushrods - Crane steel stamped rockers - bored 40 thousands over (if I remember correctly) - High dome pistons - The heads were reworked with new valve job, valves, and valve seats for unleaded fuel. I thing reworking the heads costed almostr as much as rebuilding the engine. I almost bought some new BB aluminum heads.- Aluminum intake and AFB 750 carb- 3" 3 chamber flow masters 3" exaust all the way back to 4" tips.

Any Ideas?? I am still leaning to the 355 posi. This truck is a crusier anymore and the engine is not ratical just a good build

[BUDDY]

Originally Posted by 72 Cheyenne402

The engine was built about 5 years ago Crane Fast bleed lifters - low end rv cam - 427 crank (because I couldn't find a 396 crank) - Crane chromollly pushrods - Crane steel stamped rockers - bored 40 thousands over (if I remember correctly) - High dome pistons - The heads were reworked with new valve job, valves, and valve seats for unleaded fuel. I thing reworking the heads costed almostr as much as rebuilding the engine. I almost bought some new BB aluminum heads.- Aluminum intake and AFB 750 carb- 3" 3 chamber flow masters 3" exaust all the way back to 4" tips.

Any Ideas?? I am still leaning to the 355 posi. This truck is a crusier anymore and the engine is not ratical just a good build

That seems like a pretty good combination to me. If you're looking for a little better mileage out of it, you might consider swapping the TH400 out for a Th700R4. For that mild BBC the 700r4 is plenty strong enough, plus it has a lower first gear, (for a little more kick off the line), & the added benefit of OD for those jaunts on the highway.

Swapping the gears would definitely be cheaper, but you could always do that or the trans swap afterward.

For me, I'd swap in a t-56 6-speed from a camaro or corvette. I like to row my own gears.

Later,

Buddy

[68Stepbed]

I don't think you can get 3.55 gears if it's a truck 12-bolt. 3.42 is the only choice, which will work well also. Do you know what kind of rearend you have? The OD tranny is a good idea if you want to spend the money. If you want to get by on a budget, put a set of 3.42's in it.

[Longhorn Man]

that sucker should have plenty of grunt. Myself, i'd say screw the overdrive, and just go with a 3.08 gear. You'll ned up with a good cruise RPM, maybe a couple hundred revs more than wanted, but not much, and the low end torque should walk right over that gear. Plus, this would be WAY cheaper, and more reliable than any GM overdrive with the exception of the 4L80E, but the cost of that swap would be as much as your motor was.

[67ChevyRedneck]

Originally Posted by Longhorn Man

that sucker should have plenty of grunt. Myself, i'd say screw the overdrive, and just go with a 3.08 gear. You'll ned up with a good cruise RPM, maybe a couple hundred revs more than wanted, but not much, and the low end torque should walk right over that gear. Plus, this would be WAY cheaper, and more reliable than any GM overdrive with the exception of the 4L80E, but the cost of that swap would be as much as your motor was.

AND you're more likely to find a 3.08 than a 3.42. 3.42's were rare for these trucks right? If you can find a good 3.08 you can swap the whole axle directly (as long as its a leaf rear with a lear rear or a coil rear with a coil rear, otherwise you'll have to swap the mounting blocks.) If you do decide to go OD I'd leave the 3.73 in the truck.

[BrianJordan]

i like 411s,350/th350

[72 Cheyenne402]

I had a 3.08 posi in it stock the gear just wore out so I went with the 3.73 which cam from another 72 truck it works well and will nearly throw your head through the rear window. I just run hi rmp's at 70-75 MPH. I'm thinking a different gear may help out. I don't want to lose any power or off the line speed???

[crazy longhorn]

I like a mid 3's gear with a BB or big inch sb.......that said, I agree with Andy, that sucker will pull a 3.08 just fine . The swap from 3.73, to 3.42 is not worth the trouble(not enough change in rpms) on the hyway. If you change tire height, you can probably make up the diff in rpms tween 3.42 & 3.73....... L

[Longhorn Man]

3.42 was never found in these trucks, so there would be no simple axle swap.
I can't believe I said 3.08.... and no one jumped on me for it. (I always tell ppl it's not 3.08, ot's a 3.07)

[72 Cheyenne402]

Right now I am running 15x10 with 295-50-15 on the rear and a 15x8 265-60-15 in the front I have been looking at moving up to a 17x10 on the rear with 285-40-17 and on the front 17x8 with a 245-40-17. I'd like to see how this combination works out. Maybe going with a bigger wheel could make all the difference

[67ChevyRedneck]

Originally Posted by Longhorn Man

3.42 was never found in these trucks, so there would be no simple axle swap.
I can't believe I said 3.08.... and no one jumped on me for it. (I always tell ppl it's not 3.08, ot's a 3.07)

I can't ever remember which is the correct one (3.07 or 3.08), I'm lysdexic so it don't matter anyways! I was shown a book at a junkyard with rear axle codes for 67-72 chevy trucks and some of the listings I remember were 3.07, 3.42, 3.73, 4.11, and 4.56. Maybe the 3.42 was used in vans or something? Maybe I was looking at the wrong years, maybe it was for a car? I just remember thinking, "man! if I could get a 3.42 with my 700R4 that would be awesome!"

Yup, larger diameter tires (overall larger height) will effectively give you a higher gear ratio. There's lots of online calculators out there, just google it and find one that uses a 1:1 ratio or you can find one and play with the tire/gear/OD ratios (a 700R4 uses a .70)

[Longhorn Man]

3.07 is correct.
The 3.42 wasn't offered, not as a RPO anyways. I'm sure it was used in cars, and maybe vans, but I know for ertian it was not an RPO on the 67 - 72 10 - 30 trucks.

[Palf70Step]

I agree with Andy. With a BB get the 3.07 or 3.08. Unless this is being used as a serious work/pulling/racing truck, that will give you the best overall driving ratio.

[crazy longhorn]

The 3.42 gears are available aftermarket (Richmond, ect). The 75 12 bolt under my truck actually has e 3.40 gear..... L

[stope4]

Longhorn Man

When you do the calculation isn't it something like 3.07692307692. It's so close I think you can get away with either 3.07 or 3.08.

[jsbgump]

With that setup I would run 3.08's

[Longhorn Man]

There was a reason for the 3.07 and the 3.08 gears. The 12 bolt in our truck is not the same as the 12 bolt for a car. This way, back in the day, there was no confusion when a tach ordered gears. If he ordered 3.07...it was a truck, if he ordered 3.08, then it was a car.

[409biscayne]

I love my 3.73's in my 68 bb t400 c10 . I like being able to outrun the new trucks , at least stoplight to stoplight . Especially those v8 ricer wannabe's .

[Bill72BB]

I went with 3.54 for the Dana rear , it works out great for city and freeway driving .

[72 Cheyenne402]

I think I may just go with some bigger wheels maybe some 17" or 18" and see how that works out. I planed on going bigger on the wheels anyway. Can't beat that 3.73/400 trans/402 from one stoplight to the next. I used to take the old 5.0L Mustang's in Houston. You should have seen the anger on there faces. it was great!!

[68Stepbed]

The 275/40/17 tires won't help you. They are actually a smaller diameter tire. The 295/50/15's are 26.6" tall. The 275/40/17's are 25.7" tall. It would actually be like going to a lower gear and higher revs. If you go with a 275/50/17 or 275/55/17, that would help a little. The 3.42 gears would still work well, a good mix between good off-the-line acceleration and still turn lower rpms at highway speeds.

[mbgmike]

Originally Posted by 68Stepbed

I don't think you can get 3.55 gears if it's a truck 12-bolt. 3.42 is the only choice, which will work well also. Do you know what kind of rearend you have? The OD tranny is a good idea if you want to spend the money. If you want to get by on a budget, put a set of 3.42's in it.

yeah no 3.55 3.42 is closest gm #12-342 for truck 12 bolt. Also it helps to know your tire diameter