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

[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 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 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 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

[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.

[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.

[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.

[Clyde65]

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

[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.

[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.

[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!!