Preparing for rear end change

[NC67Chev]

I want to start preparing to put a wider rear end, so I figured I would explore options of making it a little more highway friendly. My truck has a 250 with a powerglide, 3.73 and 215/75/15 tires. I know there are better transmission options, but I'm not planning on changing it.

From what I can tell, it's turning approx 2940 rpm's at 65 mph.

Here's some other combinations (all based on 65 mph) I found that I might consider:

235/75/15 with 3.73 - 2822 rpm's
215/75/15 with 3.42 - 2697 rpm's
235/75/15 with 3.42 - 2587 rpm's
215/75/15 with 3.08 - 2429 rpm's

So what I'm looking for is some expert opinions on this subject. I realize my calculations might be off, so please feel free to correct me.
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[BurnoutNova]

If it were me, I'd go with 3.42's. I don't really drive on the highway much though. If you do a lot of highway driving I would go with 3.07/3.08 gears.

[SS Tim]

235 tires, 3.08 gears and a th350.

http://www.summitracing.com/expertad...tio-calculator

[TBONE1964]

You need to consider how often the truck is actually traveling at 65mph. With everything you have now and just changing to a 3:08 you will drop you RPM's by about 500 give or take a few and after that a change to a 235 tire will get you about another 100 RPM which would get you close to 2325 RPM's at 65.

The draw back to that is you will lose you low end get up and go. It is going to be a little more sluggish moving from a stop. Having a powerglide does fix that a little also.

[SS Tim]

An automatic's torque converter does help with the taller gears and tires a little but the additional gear of a TH350 would help keep the engine in its power band. But then a 200R4 and 4.11 (or 3.73s) with the 235 would be a good alternative too.

[NC67Chev]

Thanks guys for taking the time to give some input. Finding a rear end with a 3.08 should be easy enough, and wouldn't cost much either. I would really prefer not to change transmissions. I would say the majority of driving I do with the truck is 50mph and up.
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[SS Tim]

If you are doing a lot of zooming around really consider a three or a four speed automatic. A TH350 would be cheap to build and give you a good usable gear between either and or that you have now. A 200R4 would cost more but then you can run the digger gears and still pull down good highway speeds at a reasonable rpm.

[SS Tim]

If you can try it somehow. A set of 35" tires would simulate 3.08s with 235 tires (29") with your present gears.

[NC67Chev]

I must say I'm starting to think a little more about considering a transmission change now. The charts on this page have helped me understand things a little more:

http://www.race-track.com/news.asp?TOPIC_ID=532
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[50mercfan]

if you change transmissions a 3 speed auto (350 turbo) won't help. a powerglide or 350 turbo have the same hi gear (1 to 1 ratio). you'll need to go with either a 200r4 or 700r4. the 200r4 would probably be a better choice in your truck because it has a higher first gear than a 700. the 700r4 has a pretty low first gear and your already running 3;73's. i run 700r4's in everything i have. a tall rear tire would help some, but, not like an overdrive transmission. if your running at 50mph and up most of the time, you'll really like the overdrive.

if you get an overdrive trans from an 85 or earlier, it will have a mechanical speed-o-meter. if you get a newer trans, a good transmission guy can change the tail housing over to mechanical for you.

[SS Tim]

Originally Posted by 50mercfan

if you change transmissions a 3 speed auto (350 turbo) won't help. a powerglide or 350 turbo have the same hi gear (1 to 1 ratio). you'll need to go with either a 200r4 or 700r4. the 200r4 would probably be a better choice in your truck because it has a higher first gear than a 700. the 700r4 has a pretty low first gear and your already running 3;73's. i run 700r4's in everything i have. a tall rear tire would help some, but, not like an overdrive transmission. if your running at 50mph and up most of the time, you'll really like the overdrive.

if you get an overdrive trans from an 85 or earlier, it will have a mechanical speed-o-meter. if you get a newer trans, a good transmission guy can change the tail housing over to mechanical for you.

Apparently you are taking things out of context. Overdrive is not the only reason for a trans swap. If he changes to a higher gear and or a taller tire, a swap to a three speed automatic will make the truck more drivable and start off better. Swapping in a four speed automatic will give the best of both worlds but at a cost. I'd suggested the TH350 as it is a very inexpensive upgrade for his truck even if he does nothing else to it. That long wait for everything to get up to speed and shift into direct will be gone and the too little or too much gear choices when loaded or on a grade are also buffered better with a three speed. Face it two-speed Powerglide transmissions were outdated when the truck was new and even more so at today's traffic speeds and densities.

[TBONE1964]

Here is a gear ratio chart for GM automatics as well as some manual transmissions.

RWD TRANSMISSION GEAR RATIOS

AUTOMATIC

Trans Type First Second Third Fourth Fifth
ALLISON 1000 3.10 1.81 1.41 1.00 0.71
POWERGLIDE 1.82 1.00 - - -
POWERGLIDE 1.76 1.00 - - -
Turbo 4L60 3.06 1.62 1.00 0.70 -
Turbo 4L60E 3.06 1.62 1.00 0.70 -
Turbo 4L80 2.48 1.48 1.00 0.75 -
Turbo 4L80E 2.48 1.48 1.00 0.75 -
Turbo 350 2.52 1.52 1.00 - -
Turbo 400 2.48 1.48 1.00 - -
Turbo 200-R4 2.74 1.57 1.00 0.67 -
Turbo 700-R4 3.06 1.62 1.00 0.70 -

MANUAL

Trans Type First Second Third Fourth Fifth Sixth
BORG WARNER T10 2.23 1.76 1.30 1.00 - -
BORG WARNER T10 2.43 1.64 1.24 1.00 - -
BORG WARNER T10 2.64 1.76 1.35 1.00 - -
BORG WARNER T10 2.64 1.64 1.24 1.00 - -
BORG WARNER T10 2.88 1.76 1.35 1.00 - -
BORG WARNER T10 2.88 1.91 1.35 1.00 - -
BORG WARNER T10 3.42 2.28 1.46 1.00 - -
BORG WARNER T10 3.44 2.28 1.46 1.00 - -
BORG WARNER T5 2.95 1.94 1.34 1.00 0.63(TBI) -
BORG WARNER T5 2.75 1.94 1.34 1.00 0.74(TPI) -
BORG WARNER T56 2.66 1.78 1.30 1.00 0.74 0.50
BORG WARNER T56 2.97 1.94 1.35 1.00 0.84 0.62
BORG WARNER T56 3.36 2.07 1.35 1.00 0.84 0.62
MUNCIE M20 2.56 1.91 1.48 1.00 - -
MUNCIE M20 2.52 1.88 1.46 1.00 - -
MUNCIE M21 2.20 1.64 1.28 1.00 - -
MUNCIE M22 2.20 1.64 1.28 1.00 - -
M-12 2.97 2.07 1.43 1.00 0.84 0.56
SAGINAW (4spd.) 3.11 2.20 1.47 1.00 - -
SAGINAW (4spd.) 2.85 2.02 1.35 1.00 - -
SAGINAW (4spd.) 2.54 1.80 1.32 1.00 - -
SAGINAW (4spd.) 2.54 1.80 1.44 1.00 - -
ZF6 3.07 1.80 1.43 1.00 0.71 0.59
ZF S6-650 5.79 3.31 2.10 1.31 1.00 0.76

[50mercfan]

Originally Posted by SS Tim

Apparently you are taking things out of context. Overdrive is not the only reason for a trans swap. If he changes to a higher gear and or a taller tire, a swap to a three speed automatic will make the truck more drivable and start off better. Swapping in a four speed automatic will give the best of both worlds but at a cost. I'd suggested the TH350 as it is a very inexpensive upgrade for his truck even if he does nothing else to it. That long wait for everything to get up to speed and shift into direct will be gone and the too little or too much gear choices when loaded or on a grade are also buffered better with a three speed. Face it two-speed Powerglide transmissions were outdated when the truck was new and even more so at today's traffic speeds and densities.

well, tim, you suggested the same thing i did, 3 times before i did.

i've got 25 years of hard core hot rod building experience. so the following is not theory, it's fact.

there is no way you can change 2/3 of the drive train cheaper or easier than a trans swap. a 3:08 gear and a taller tire would definatly help. but, did you ever try to crush a crush sleeve on a gm rear end while laying on the ground. it's almost impossible. plus the 3:08 gear with the low horse power of the six cylinder would not be nice to drive at all.

here's the swap in a nut shell

you'll have to get a flex plate from a later 250 6 cylinder. because the torque converter holes will be different than the powerglide.

move the trans cross member back 3/4 of an inch. so the mount will align with the cross member.

shorten the drive shaft or find a junk yard piece the right length

you can hook the shift lever onto the new trans, it won't read right on the selector, but, it will work fine.

the toughest part of the swap will be connecting the detent cable on the over drive to the throttle lever, if it where in my shop, i'd make a bracket that bolts to the base of the carb. and drill a hole for the ball mount that the cable connects to. the problem with that is, the detent cable has to be adjusted absolutley correctly or you will burn the transmission up. so you should find someone that is very familiar with 700's to help with this part of the swap.

here is the one i made for my 72. so you have an idea what i'm talking about.

[SS Tim]

Good, Its nice when people at least consider why we recommend a change. As you can see the drop off between gears is lessened with more gears. With a 200R4 the o/d in conjunction with deep rear gears give a nice performance/ability spread. Its nice to be able to get up to highway traffic speed quickly when you need to.

[NC67Chev]

My mind was made up to change transmissions, and I still very well may, but I have some new considerations. In any case, I recently found some factory 16 inch wheels, so I'd like to get some more thoughts on this. As mentioned above, I'm turning approx 2940 rpm's at 65 mph with my current 15 inch tires. The new tire sizes I'm now considering and corresponding rpm's at 65mph are:

235/85/16 - 2568 (The least tire selection)
245/75/16 - 2676 (A ton of tire selection)
235/75/16 - 2727 (Some tire selection, but mostly A/T...and maybe more $)

Again, I'm not necessarily changing my mind about upgrading to overdrive, but being able to upgrade to factory 16" wheels which work with disc brakes and helping my highway rpm's is appealing. Thanks for your input on tire selection!

[SS Tim]

Again taller tires will help top end RPM reduction. But at the same time they will reduce off the line power. Drivelines are a balance. Looking at a 400 RPM drop I don't think you will be happy with how the truck drives with a powerglide (two speed). Pretty much everything else I suggested remains the same as before. But if you do decide to run taller tires adding the th350 changeover would give a lower first than the powerglide thus giving back the takeoff RPM (plus some) and a well balanced second, middle gear. It will drive much better than the powerglide's all or nothing reduction.

As far as tires go its up to you. Again a set of used or borrowed tires might be the place to start in chosing the size that works best for you.

[NC67Chev]

Originally Posted by SS Tim

Again taller tires will help top end RPM reduction. But at the same time they will reduce off the line power. Drivelines are a balance. Looking at a 400 RPM drop I don't think you will be happy with how the truck drives with a powerglide (two speed). Pretty much everything else I suggested remains the same as before. But if you do decide to run taller tires adding the th350 changeover would give a lower first than the powerglide thus giving back the takeoff RPM (plus some) and a well balanced second, middle gear. It will drive much better than the powerglide's all or nothing reduction.

As far as tires go its up to you. Again a set of used or borrowed tires might be the place to start in chosing the size that works best for you.

Thanks Tim, I really appreciate you taking the time to reply! The 16's had 245/75's on them when I found them, but they were so rotten that I didn't even want to try to drive down the street on them. I'll have to see if I can find some road worthy cheap tires to try out.

[SS Tim]

Originally Posted by NC67Chev

Thanks Tim, I really appreciate you taking the time to reply! The 16's had 245/75's on them when I found them, but they were so rotten that I didn't even want to try to drive down the street on them. I'll have to see if I can find some road worthy cheap tires to try out.

Luckily you only need a pair for testing! :-)
Most tire shops wil help out for a good cause project.
Been lucky enough to often get good "hold ups" for free if I'd bring in a carcasses to keep the disposal count right.

Remember for anything more than seat of the pants testing use a calculator, like the link I posted way up top there.
Since the speedo will be off, a calculated speed adjustment needs to be applied to keep from fudging the results. A tach cross check too, even if it is only a temp tach/dwell meter installation will add acuracy to the testing.

Of course recorded baseline numbers are a must start point before you change anything. If you can't quantify a change then its effect is only a guess.

[Hatzman]

If you're looking at swapping out the rear end look into the 79 Cadillac Seville rear. The two ratio options were 2.56:1 (In California) and 2.24:1.

The rear is narrower and has disc brakes. Some complain about the calipers leaking but I have not had any issues. You will need to purchase a trailing arm for it though.

Good luck!