Swap the tranny or the rear diff??

OK, HAMB. I just bought(today) a 1948 Chevy 3/4 ton. It has a 250-6 with a THM 350. Original rear(4.57) and it is not good for highway speeds.

What are my best options?

Rear end change ?
****** change to 7ooR?

Help a Brutha out....

 

Even with overdrive, it'll still be spinning the motor pretty fast on the highway. You probably want to change the rearend....

 

A 700r4 will be 3.19 in overdrive a 2004r will be 3.06.

 

The flip side--- The 700r4 first gear effect vs. the 200r4 as compared to a T350

2.52 x 4.57 = 11.52

2.74 x 4.57 = 12.52 (would launch like a T350 with a 4.97 gear)

3.06 x 4.57 = 13.98 (would launch like a T350 with a 5.55 gear)

plus 200r4 is an easier swap to replace a T350 with a more even gear spread.

 

I think you have 6 bolts wheels if you don,t care,s10 rearend and the 4x4 rearend maybe wider.

 

I'd do both the ****** and the rear end...

 

8 bolt rear....

 

Until you change the rearend none of the trannies mentioned being put it in will give you any highway speed. With the 3/4 ton rearend your engine will be screaming.Change the rearend and see how it does with the 350turbo before blowing bucks on a 700r4 or the 200r4.

 

Changing either will get you halfway to where I suspect you may want to be. Do both while you're at it and get the full benefit of better highway cruising and fuel economy. If you're not wanting to haul heavy loads or shred the tyres at the traffic lights, a p***enger car diff ratio plus an overdrive box will bring you much closer to p***enger car comfort and performance.

Especially if you've got to start cutting driveshafts to change either diff or gearbox, you might as well do both at the same time to save the h***le of doing that job twice too.

Cheers, Glen.

 

how tall are the rear tires and what are your freeway speed?

 

I'd just do a OD trans..3.19 end result with probably 29" [at least?] tires = 2585 rpm; doubt if the engine would be any happier, don't think you have enough power to run slower..

 

It's 700R4 and 200-4R, not 200R4. Just nitpickin'

 

Consider my nit picked my vote is the 200-4R as it's a similar size to the 350, can be built stronger than the 700 (not that he needs to) and doesn't have that deep first (the 4.57 will take car of the oomph)

The low 3series effective cruise ratio in OD will be happy unless the tires are 30+inches...

rick

 

You wrote 200r4 not 2004r but who is nitpicking

 

Now that we have determined that the HAMB agrees on nothing(I keed!).

Which ****** will swap out the easiest? 700r4 or the 2004r? Again, 250 I-6 and a stock rearend.

Popcorn please, extra ****er......

 

Are you reading what is being said.
Get a S10 rearend and leave the 350 trans get it.

 

change rearend gear

 

I read a lot of different opinions....you suggested the s-10 rear, but it is a 3/4 ton truck, I would like to keep the eight bolt pattern. Can I do that with an s-10 rear?

 

how are you planing on actually using it? are you going to haul alot with it, or are you just using it like it's a Honda civic?......

The 250 is going to want a cruising rpm in the 2000-2500 range, again it depends on how you're going to use it as to where in that range you want it to be.

You want to stay with the 8 lugs? if so, look at what else (probably late 60's or newer 3/4-1 ton) that has the same bolt pattern. Figure out if they are the right width and what ratios were available.

Run the numbers through an online gear calculator, ***uming the right rpm on the engine, and see if the rearend gearing available in the later rearend is anywhere close to what the calculator says it should be.
If not, then a trans change is also probably needed.

The 700-R4 is stronger than the 200-4R, but if you plan on using it like a Honda Civic, it shouldn't matter....
Make sure you hook up the TV cable correctly, or you'll burn it up....

The TH350 is the simple way to go, but if you run too low of a rearend ratio, you will loose some of your heavy towing ability, and it would be a bit of a slug off the line. again, use it like a Honda and it probably won't matter to you....

 

how are you planing on actually using it? are you going to haul alot with it, or are you just using it like it's a Honda civic?......

The 250 is going to want a cruising rpm in the 2000-2500 range, again it depends on how you're going to use it as to where in that range you want it to be.

You want to stay with the 8 lugs? if so, look at what else (probably late 60's or newer) that has the same bolt pattern. Figure out if they are the right width and what ratios were available.

Run the numbers through an online gear calculator, ***uming the right rpm on the engine, and see if the rearend gearing available in the later rearend is anywhere close to what the calculator says it should be.
If not, then a trans change is also probably needed.

The 700-R4 is stronger than the 200-4R, but if you plan on using it like a Honda Civic, it shouldn't matter....
Make sure you hook up the TV cable correctly, or you'll burn it up....

The TH350 is the simple way to go, but if you run too low of a rearend ratio, you will loose some of your heavy towing ability, and it would be a bit of a slug off the line. again, use it like a Honda and it probably won't matter to you....

 

are you sure of rear gears? original rearend was torquetube. maybe swap gears not whole rearaxle.

 

Just track down a later 1-ton van with a 3.08 or 3.23 and try that. Will be 8 lug, you want the 14-bolt semi-floater, the full floater only goes down to a 3.73. May be a tad wide for the truck but should give you an idea what you want, or just let the tires stick out.

Chevy trucks go open drive with the AD design in '48.

 

Look at the 67 to 72 trucks as they have a similar rear end and the center section will swap,I think the 3/4 ton suburbans had 3.73 gears as a option.

 

Just change the trans, and keep the same rearend and bolt pattern. And no, a 700-R4 is not stronger than a 200-R4. I have a stock 200-4R in a Turbo Buick thats withstands 21 lbs of boost at the track. Just use a 700-4R. They are easy to find, and will be fine for what you are using it for.

 

Thanks for all the replies. I am just looking for a direction to head in. Unfortunately I am not a DIY guy on a project like this. I hate replacing a perfectly good transmission, but I need some road speed. It is OK for around town, but I want to get out to some car cruises across the state. I think either (or both) fixes will work, I just need to find which one works best!

 

No, they aren't. Just because they put them in those doesn't make them stronger. Plenty of guys blew them up and replaced them with TH350's.
There is an article on webrodder.com that goes into detail about the differences between them.

 

3/4 ton trucks were open, or more specifically they were a 2 piece driveshaft with the front half a torque tube that turned into an open drive for the rear half. You can swap the center section with newer years to get the ratio down, but you would need around a 3.08 without overdrive, and I don't know if that's available with a center section that would fit?

 

The 3/4 ton is not a torque tube rear, and they used the same rear untill the early 70's. There are rears outthere with gears as low as 3.73, look for automatic ice cream trucks and campers. The pumpkin swaps out like a 9 inch ford. If you swap the
entire rear axle you can get rid of the Huck style brakes at the same time, but I do not know of a later 8 lug as narrow as the 3600 series A-D trucks. The bendix brakes
from a later 3/4 ton AD about 52on is a direct swap if you get everything. I believe 1 tons used the same brakes and rears so you can check those also.

Chevy used a hybrid drive shaft in these trucks so they could use the same trans in
1/2 ton and 3/4 ton trucks- it uses a tourque tube to the center bearing, then it uses universal joints. Jim

 

The 700-R4 is longer and the mount is farther back.
The 200-4R is possibly the same length as the TH350, depending on what tailshaft housing is on the TH350.