Need help solving 61 drivetrain / RPM problem

[SMR]

Ok guys I could use some real help on this problem. Go easy on me if it’s something stupid. Here is my problem. I have a 61 C10, 283 stock motor with 2 speed powerglide transmission, running 29 inch tires. I’m running about 2k rpms at 40 mph and wanted to step down the rpms by changing the rear end gear set to a 3.38. Based on conversations with Ring&Pinion.com, I was told I had at least a 3.90 set in the rear end and that going to 3.38 would lower the rpms to a more highway friendly 2600 rpms at 65. The killer is that when my mechanic opened up the rear end the gear set was already 3.38 (44/13). Something doesn’t make sense as I should be running at around 1600 rpms at 40 mph. I had the trucks digital dash, which displays rpm checked for accuracy, by having my mechanic connect up his own equipment to see if there was a problem with the rpm reading. Both were inline with one another. According to my mechanic the transmission is shifting fine and does not appear to be slipping. Any ideas?

[63burban]

Mechanic should check his math, the rear should be 3:73. Count the ring gear and count the pinion teeth. The numbers should be stamped in the ring gear. Let use know

[markeb01]

I haven’t taken the time to calculate the math, but a couple of points to consider – most vintage automatic transmissions will lose up to several hundred rpms in the torque converter. A Powerglide isn’t a stick shift, so there could be 400-500 rpms going to waste before the output shaft actually turns the driveshaft.

The other issue is how low to go in final drive rpms with an older, smaller engine. Dropping the rpms too far will leave you with a dog on acceleration. The engine needs to be operating in its torque range to move the truck most efficiently. New vehicles can propel a vehicle at highway speeds using very little engine rpm because they were designed that way. Our trucks were not.

And if the numbers provided were in fact counted by your mechanic (44/13), you do indeed have a 3.38.

[geezer#99]

Originally Posted by 63burban

Mechanic should check his math, the rear should be 3:73. Count the ring gear and count the pinion teeth. The numbers should be stamped in the ring gear. Let use know

Maybe you should check your math. 44divided by 13 equals 3.38.

[kieth]

Here is what my spreadsheet calculations show:

R/A 2000 rpm
MPH

3.08 -- 56.02

3.36 -- 51.35

3.50 -- 49.3

3.73 -- 46.26

3.90 -- 44.02

4.11-- 42.09

4.33-- 40.0-----GUESS

4.56-- 38.0

HOPE THIS HELPS ALL SHOWN ON 29" DIAMETER TIRES

KIETH

[SMR]

Originally Posted by 63burban

Mechanic should check his math, the rear should be 3:73. Count the ring gear and count the pinion teeth. The numbers should be stamped in the ring gear. Let use know

Pretty sure the counts are correct..but will have him check the ring gear stamp to confirm and let you know.

Thank you

[SMR]

Originally Posted by markeb01

I haven’t taken the time to calculate the math, but a couple of points to consider – most vintage automatic transmissions will lose up to several hundred rpms in the torque converter. A Powerglide isn’t a stick shift, so there could be 400-500 rpms going to waste before the output shaft actually turns the driveshaft.

The other issue is how low to go in final drive rpms with an older, smaller engine. Dropping the rpms too far will leave you with a dog on acceleration. The engine needs to be operating in its torque range to move the truck most efficiently. New vehicles can propel a vehicle at highway speeds using very little engine rpm because they were designed that way. Out trucks were not.

And if the numbers provided were in fact counted by your mechanic (44/13), you do indeed have a 3.38.

I was actually a little worried about the acceleration issue that I would experience when I was thinking I was dropping the gearing from a 3.90 to 3.38. It doesn't appear that many people are running powerglides on this forum and maybe this is why. I'm kind of old school and like to keep as much of the orginal powertrain as possible. Also I've always heard powerglides were bullet proof. Really hoping someone out here can school me on the options, if any, to make the truck a little more highway friendly without switching out the powerglide. Thanks for your input.

[SMR]

Originally Posted by kieth

Here is what my spreadsheet calculations show:

R/A 2000 rpm
MPH

3.08 -- 56.02

3.36 -- 51.35

3.50 -- 49.3

3.73 -- 46.26

3.90 -- 44.02

4.11-- 42.09

4.33-- 40.0-----GUESS

4.56-- 38.0

HOPE THIS HELPS ALL SHOWN ON 29" DIAMETER TIRES

KIETH

Hello Kieth,

I would have thought that I was running 4.11 originally using Ring&Pinion.com calcualtors, which basically is in line with your calculations above. However, the problem is that I'm actually running 3.38 gears and get rpms levels more commonly assoicated with 4.11 gears.

[SMR]

Originally Posted by SMR

Pretty sure the counts are correct..but will have him check the ring gear stamp to confirm and let you know.

Thank you

ring stamped 1344

[Captainfab]

I agree with what markeb01 said.

Most of those rpm calculators are based on a manual trans, which has virtually no slip. With an auto trans your rpm's will be a little higher due to losses thru the torque converter and trans.

You don't want your rpm's too low with a 283, as they don't produce very much low end torque. They also produce peak power higher in the rpm range.

[SMR]

What"s your thoughts on replacing rear end with perhaps an 87 12 bolt running 3.08 gears
Posted via Mobile Device

[Sharps40]

Why not can the powerglide and go with a 700r4? Its the same $1K to totally overhaul the rear axle and it sounds like its been gone thru before anyway.

[kieth]

Originally Posted by SMR

What"s your thoughts on replacing rear end with perhaps an 87 12 bolt running 3.08 gears
Posted via Mobile Device

If you will send me a private message here with your e mail I will send you the spreadsheet my info came from, it also shows usable torque to the ground with different transmissions and rear axle ratios. here is how your truck will perform speed wise but you can see what happens to the torque as you change the transmission with your engine. (CALCULATED WITH 29" TIRES)

RATIO----SPEED@------ TORQ TO-------TORQ TO----------SPEED@-----------------------------------------------------------------------------------------------
------------2000RPM------ GROUND--------GROUND----------2000-------------------------------------------------------------------------------------------------
----------------------------1:1 PGLIDE------700R4-.70-------- RPM--------------------------------------------------------------------------------------------------

3.08------56.2mph-------544---------------416----------------80.03-------------------------------------------------------------------------------------------------

3.36------51.35-----------594---------------415----------------73.36------------------------------------------------------------------------------------------------

3.55------49.3-------------618---------------432.78------------70.43------------------------------------------------------------------------------------------------

3.73------46.26------------659---------------461.22-------------66.09----------------------------------------------------------------------------------------------

3.92------44.02----------- 692--------------- 484.72-------------62.88----------------------------------------------------------------------------------------------

4.10------42.09------------724----------------506.98-------------60-------------------------------------------------------------------------------------------------


ONE HAS TO PUT THIS TORQUE NUMBER IN THE PROPER PERSPECTIVE, COMPARE THEM TO WHAT YOU HAVE NOW AND HOW THE TRUCK PERFORMS.
AS A EXAMPLE MY 66 HAS 31" RUBBER AND A 200HP 350 TBI 3:73 WITH A 700R4. MY TRUCK HAS 442 FT LBS OF TORQUE IN OVER DRIVE. WELL IT DOES GREAT ON THE HIGHWAY BUT IT IS NOT A BARN BURNER, IT WILL GO 85MPH IN OD BUT DOES NOT PULL A HILL WELL AT ALL. SO I USE 450 FT LBS OF TORQUE AS A JUDGE OF HOW CHANGES WOULD AFFECT THE PERFORMANC OF MY TRUCK. WITH THIS IN MIND WE ARE GOING TO INSTALL A 300HP LT1 SOON AND THE RESULT WILL BE 637 FT LBS OF TORQUE. SO WE FEEL THE EXTRA 100 HP WILL MAKE MY GMC MUCH MORE DRIVABLE WHILE STILL MAINTAINING LOW RPMS (2000@ 65MPH) .

[Captainfab]

An '87 pickup dufferential will be the 8.5" 10 bolt. The 3.08 gears will definitely help your highway cruise rpm's, but your accelleration will suffer.

Leaving your current 3.38 geared differential in place, and changing to a 700R4, will give you both lower highway rpm's, but also improve your accelleration over the Powerglide. Plus it will make your truck, more enjoyable to drive IMO.

Originally Posted by SMR

What"s your thoughts on replacing rear end with perhaps an 87 12 bolt running 3.08 gears
Posted via Mobile Device

[Clyde65]

ditto^^^^^^!!!!!

[kieth]

Originally Posted by kieth

If you will send me a private message here with your e mail I will send you the spreadsheet my info came from, it also shows usable torque to the ground with different transmissions and rear axle ratios. here is how your truck will perform speed wise but you can see what happens to the torque as you change the transmission with your engine. (CALCULATED WITH 29" TIRES)

RATIO----SPEED@------ TORQ TO-------TORQ TO----------SPEED@-----------------------------------------------------------------------------------------------
------------2000RPM------ GROUND--------GROUND----------2000-------------------------------------------------------------------------------------------------
----------------------------1:1 PGLIDE------700R4-.70-------- RPM--------------------------------------------------------------------------------------------------

3.08------56.2mph-------544---------------416----------------80.03-------------------------------------------------------------------------------------------------

3.36------51.35-----------594---------------415----------------73.36------------------------------------------------------------------------------------------------

3.55------49.3-------------618---------------432.78------------70.43------------------------------------------------------------------------------------------------

3.73------46.26------------659---------------461.22-------------66.09----------------------------------------------------------------------------------------------

3.92------44.02----------- 692--------------- 484.72-------------62.88----------------------------------------------------------------------------------------------

4.10------42.09------------724----------------506.98-------------60-------------------------------------------------------------------------------------------------


ONE HAS TO PUT THIS TORQUE NUMBER IN THE PROPER PERSPECTIVE, COMPARE THEM TO WHAT YOU HAVE NOW AND HOW THE TRUCK PERFORMS.
AS A EXAMPLE MY 66 HAS 31" RUBBER AND A 200HP 350 TBI 3:73 WITH A 700R4. MY TRUCK HAS 442 FT LBS OF TORQUE IN OVER DRIVE. WELL IT DOES GREAT ON THE HIGHWAY BUT IT IS NOT A BARN BURNER, IT WILL GO 85MPH IN OD BUT DOES NOT PULL A HILL WELL AT ALL. SO I USE 450 FT LBS OF TORQUE AS A JUDGE OF HOW CHANGES WOULD AFFECT THE PERFORMANC OF MY TRUCK. WITH THIS IN MIND WE ARE GOING TO INSTALL A 300HP LT1 SOON AND THE RESULT WILL BE 637 FT LBS OF TORQUE. SO WE FEEL THE EXTRA 100 HP WILL MAKE MY GMC MUCH MORE DRIVABLE WHILE STILL MAINTAINING LOW RPMS (2000@ 65MPH) .

Well as Captain Fab said changing to 3:08 will lower your cruising rpm, but coming out of the hole will not be as good. As you can see from the above chart you currently have 594 ft/lbs of torque available 2000 rpm should equal 51mph but you said it was only going 40mph.....
changing to 3:08 will only make you go 56mph @2000rpm so the work will not be worth it. Go to the 700r4 it will make the most dramatic difference, i.e. it will make your truck come out of the hole better due to a lower first gear, you should actually have increased speed in 3rd gear to where the charts show it should be (trans will not slip) and the only reservation I have is the decreased torque in Overdrive.......but the torque shown is close to my 350TBI engine so it should work.....
All of my torque numbers are based on a 283 with a 4 barrell. If your truck does not perform quite as well as it should in OD then you could change to 3:55 or 3:73 rear axle ratio at a later date. Kieth

[SMR]

Originally Posted by Captainfab

An '87 pickup dufferential will be the 8.5" 10 bolt. The 3.08 gears will definitely help your highway cruise rpm's, but your accelleration will suffer.

Leaving your current 3.38 geared differential in place, and changing to a 700R4, will give you both lower highway rpm's, but also improve your accelleration over the Powerglide. Plus it will make your truck, more enjoyable to drive IMO.

Couple of questions:
If I change to an 87 differential the setup is for a 5 lug wheel. I have really nice 6 lug wheels setup on the truck now. What would be the options dealing with this issue?

If I went the way of upgrading the transmission what are your thoughts on 700r v.s. 4L60E. I know the 700R is less expensive, but is there any advantage with putting in the 4L60E with my setup i.e. 283 / 3.38 rear end?

[chevyrestoguy]

Originally Posted by SMR

Couple of questions:
If I change to an 87 differential the setup is for a 5 lug wheel. I have really nice 6 lug wheels setup on the truck now. What would be the options dealing with this issue?

If I went the way of upgrading the transmission what are your thoughts on 700r v.s. 4L60E. I know the 700R is less expensive, but is there any advantage with putting in the 4L60E with my setup i.e. 283 / 3.38 rear end?

I recommend a 700r4 because you won't need a computer to run it. In the long run it'll be a lot less expensive to do a 700r4 versus a 4L60E. For your application, having the transmission computer controlled won't be any advantage at all. The 700r4 is a very reliable transmission, lots of great support info online, and the low first gear would be a plus with your 283.

If you decide to go with a 700r4, I also recommend keeping the lock-up feature operational. The lock-up will drop the rpm an additional 200rpm after it shifts into overdrive. In today's economy, every little bit helps.

Personally, I would keep the rear end you've got. You may find that the 3.38 might be just a tad bit too high of a ratio and the engine may be a bit lazy at cruising rpm. I wouldn't worry about it, though. With a 700r4, you're gaining TWO more gears (with the lockup, you could even argue THREE more gears), you'll be able to listen to the engine and tell where it's happy and keep it in the gear it likes.

Also consider a 200r4 as well. They resemble a th350 in dimension, and are a good transmission. Not as much info on line as the 700r4, not as popular, but still a good piece.

[SMR]

Hey guys...thanks for all your input and with helping me figuring out the best solution. Although I had really wanted to keep the powerglide, it's clear with my setup that even with a change to 3.08 rear gears the performance will be lackluster. Looks like the powerglide must go. I guess the big question is 700r4 vs. 200r4. Any thoughts on these two options?

[geezer#99]

Keep the powerglide! If you think a change to a 3.08 gear would produce lackluster performance then wait till you've got a 2.40 to 2.50 gear in o/d. That is if you got enough grunt to pull that o/d. Likely be spending most of the time in direct.
JMHO

[SMR]

Originally Posted by geezer#99

Keep the powerglide! If you think a change to a 3.08 gear would produce lackluster performance then wait till you've got a 2.40 to 2.50 gear in o/d. That is if you got enough grunt to pull that o/d. Likely be spending most of the time in direct.
JMHO

I agree if I leave the 3.38 gear set. If my understanding of Kieth's spreadsheet is correct, the torque levels produced will likely be insufficient for the 0/d i.e. (lackluster) with 3.38. However, if I switch to a 3.73 or 3.90 gear set, the torque levels should be at a level adequate enough to support the o/d…or am I missing something. I’m new to the game, so any “rubber meets the road” experience would be helpful on these setup variables.

[geezer#99]

If you're trying to have good to excellent power with the best mileage then lower rpm isn't the answer. You want to find the sweet spot where your motor is just singing along, not working hard to maintain speed. If you've got a good ear you can hear it quite well. You'll hear it just purring not grumblin'. As an example I ran a 74 chevy short step years ago that had a 350 3ott and 3.40 gear with 28" tires. A 2bbl motor, stock cam, stock single exhaust, points ignition too. The sweet spot was 3200 rpm which IIRC was about 75 mph. Pulled down 18 mpg (14 mpgUS) and it just purred down the road. At lower rpm you could hear the motor working harder and roaring a bit more. Probably due to need for a bit more throttle opening to maintain speed. I hotrodded it a bit with headers, an eddy intake and carter 600 and it still loved that 3200-3400 rpm range. Even ran a holley 800 double pumper on it for a while but it liked more rpm. 3500-3600 was the sweet spot. And still made 16 mpg. But fear of speeding tickets made for a swap back to a more sane holley 465 from a mopar 440.
It's all about your combination and listening to what your motor likes. Small motor, small cam, smallcarb, stockish exhaust and your motor might like 26-2800 rpm.
And the glide is the undisputed king of transmissions. We ran one for 2 years in a 59 Impala. 283 car. Every sunday on our favourite stretch of blacktop we popped it in reverse while cruising at 60 mph. Pedal to the medal smokin' the hides till she stopped going ahead, back up smokin' it a 100 yards ,back in below, back up to 60 and do it again. THat tranny outlasted the whole car. Tough tranny. After all they put them behind dual quad 409's in Pontiac's up here. Nothin' like 85-90 mph in first gear.
Those where the days!!

[Captainfab]

I agree that the 3.90 gear would likely be a better gear to use with the 700R4. What I would suggest, is try it with the 3.38's first, and decide for yourself. If you then decide you would rather have the 3.90 gears, you should have no trouble finding someone to trade you their 3.90's for your 3.38's.

[SMR]

I think that make sense. Will let you know how things go once I get a 700r4 installed..thanks for the input.