Rear axle ratio stuff ...

[Zone47]

I spun my wheels (limit slip - 12 bolt) one rotation and the driveshaft (marked with tape) spun 3 times even. Tranny is a T-400.

So I'm figuring it's got to be a 3:07 or 3:08. When I drive it though, it feels like a 3:73

At 40 mph, it's at 2200 rpm. At 50 it's 2600rpm. That seems a bit high for a 3:08, don't ya think?

[jocko]

what is your tire diameter?
EDIT - are you certain you are in 3rd ger at 40 mph?
If you are in 3rd, no, those numbers do not sound like a 3.08. Or a 3.73. or a 4.10...

[Zone47]

Originally Posted by jocko

what is your tire diameter?
EDIT - are you certain you are in 3rd ger at 40 mph?
If you are in 3rd, no, those numbers do not sound like a 3.08. Or a 3.73. or a 4.10...

Yes, 3rd gear. Tires are 29" tall, sorry I forgot to mention that. I've had Chevelles and such and that's what I'm comparing to. I guess I can pull the rear cover, but it seems that spinning the wheel and counting the driveshaft should tell me.

[jocko]

29" tire, TH400, and 2600 rpm @ 50 mph = 4.57 gear.
Are you using your speedometer or a GPS for the speed you think you're at? (use a GPS).

[Baime]

Originally Posted by jocko

29" tire, TH400, and 2600 rpm @ 50 mph = 4.57 gear.
Are you using your speedometer or a GPS for the speed you think you're at? (use a GPS).

Jocko,
I really like this chart you posted. It is very nice. Is this something you created, or is it from a website?

I'm trying to decide between changing out my transmission or my rear end gears. Are you willing to show another table based on my truck's configuration? I'm hoping you will.

- T350 or TH350 transmission. (Are these the same thing? If not, all I know is that mine is stock from a 1971 truck, and it has the rectangular pan with 45 degree notch in the rear passenger corner of the pan.)
- 4:10 rear end gear.
- 285/75/16 tires = approximately 33" diameter.

I'm thinking about a new positraction rear end with 3:52 gears. I'm very interested in how this would affect the engine rpms at highway speeds.

Thanks, JP

[57larry]

I pulled the rear end cover, the ratio is stamped into the gear. mine is a 307. why do a lot of people say 308?

[Killer Bee]

Originally Posted by 57larry

I pulled the rear end cover, the ratio is stamped into the gear. mine is a 307. why do a lot of people say 308?

different manufacturers have different actual tooth counts for the ring and pinion therefore ratios may vary slightly..

for instance;

my 72 K1500 has 3.08 GM 12 bolt rear and 3.07 Dana 44 front
my 71 K20 has 4.10 Eaton rear and 4.09 Dana 44 front..
my 65 C20 has 4.57 Eaton rear and my 02 C1500 has a 4.56 GM rear

hope that helps..

[jocko]

Originally Posted by Killer Bee

my 71 K20 has 4.10 Eaton rear and 3.09 Dana 44 front.

Rut Roh!

[Killer Bee]

alright, I just cracked myself up, that would be a problem

thanks Jocko, correction made

[geezer#99]

Originally Posted by Baime

Jocko,
I really like this chart you posted. It is very nice. Is this something you created, or is it from a website?

I'm trying to decide between changing out my transmission or my rear end gears. Are you willing to show another table based on my truck's configuration? I'm hoping you will.

- T350 or TH350 transmission. (Are these the same thing? If not, all I know is that mine is stock from a 1971 truck, and it has the rectangular pan with 45 degree notch in the rear passenger corner of the pan.)
- 4:10 rear end gear.
- 285/75/16 tires = approximately 33" diameter.

I'm thinking about a new positraction rear end with 3:52 gears. I'm very
interested in how this would affect the engine rpms at highway speeds.

Thanks, JP

T350 or th350 is normal. Same thing. Or turbo 350. Different names for the same thing.

Use this calculator to figure out the effects of different ratios etc.
http://www.advanced-ev.com/Calculators/TireSize/

[Baime]

Originally Posted by geezer#99

T350 or th350 is normal. Same thing. Or turbo 350. Different names for the same thing.

Use this calculator to figure out the effects of different ratios etc.
http://www.advanced-ev.com/Calculators/TireSize/

Geezer,
Thanks for confirming that T350 and TH350 are the same.

My cousin helped me determined my rear end gear ratio by jacking up the back end, and counting differential output turns to wheel turns. We came up with 4:11, but a lot of people keep mentioning 4:10 gears, so I got confused on that, but it sounds like they are both basically the same.

This calculator says 2950 rpm at 70mph with the 4:10 rear end, and 2500 rpm at 70mph with the 3:52 rear end. (33" dia. wheels). This seems to answer my question, but it doesn't factor in the gearing in third gear of my T350 transmission. If I had a newer 200R4 or 400R4 transmission with overdrive, the engine rpms would be lower, so I'm assuming the transmission high gear matters, or is their a minimal difference?

Thanks, JP

[geezer#99]

Originally Posted by Baime

Geezer,
Thanks for confirming that T350 and TH350 are the same.

My cousin helped me determined my rear end gear ratio by jacking up the back end, and counting differential output turns to wheel turns. We came up with 4:11, but a lot of people keep mentioning 4:10 gears, so I got confused on that, but it sounds like they are both basically the same.

This calculator says 2950 rpm at 70mph with the 4:10 rear end, and 2500 rpm at 70mph with the 3:52 rear end. (33" dia. wheels). This seems to answer my question, but it doesn't factor in the gearing in third gear of my T350 transmission. If I had a newer 200R4 or 400R4 transmission with overdrive, the engine rpms would be lower, so I'm assuming the transmission high gear matters, or is their a minimal difference?



Thanks, JP

Normal tranny in high gear is always direct drive or 1 to 1. Overdrive ones are usually near .70 to 1. Some are lower like 1.67 or higher like 1.8. You can calculate your effective drive ratio by multiplying your rear gear ratio by the overdrive ratio.
Your 4.10x.70 by example becomes a 2.87 gear ratio. Your 70 mph rpm would be near 2100 rpm. Quite a drop.
Not necessarily a gaurentee of better mileage though.

Apologies to the op for jackin’ His thread!

[jocko]

Originally Posted by Baime

Jocko,
I really like this chart you posted. It is very nice. Is this something you created, or is it from a website?

I'm trying to decide between changing out my transmission or my rear end gears. Are you willing to show another table based on my truck's configuration? I'm hoping you will.

- T350 or TH350 transmission. (Are these the same thing? If not, all I know is that mine is stock from a 1971 truck, and it has the rectangular pan with 45 degree notch in the rear passenger corner of the pan.)
- 4:10 rear end gear.
- 285/75/16 tires = approximately 33" diameter.

I'm thinking about a new positraction rear end with 3:52 gears. I'm very interested in how this would affect the engine rpms at highway speeds.

Thanks, JP

Hi Baime - yes, it's my doohicky, not an online thing. I just plug info in the yellow boxes and the rest is automatic. What can I say, I'm a geek, but it sure has come in handy. I can't post an excel file on here, but I may just post all the different possibilities in a separate thread, folks may find it useful, dunno. Your truck's config is in upper left corner of the chart - other transmissions and diff ratios shown for comparison. Not familiar with anyone that makes a 3.52 (3.54 available in a Dana 60), but used 3.52 as requested. 700R4/3.73 is a great baseline combination - that's why GM used it so much. Lots of bottom end with the 700R4's 3.06 1st gear and nice, low rpm highway driving courtesy of the 0.70 o/d. Lots of options though. If you know you DON'T want to change your rear gears for example - just look across Row 1 for the Trans options to compare. If you know you DON'T want to change your trans, then just look down Column 1 for the diff gear options to compare. If you want to change both, that's the rest of the table. To your additional questions - yes, 4.10 and 4.11 the same, 4.10 a truck ratio, generally speaking - and yes, trans high gear greatly affects highway rpm - as geezer pointed out. The "Final Drive" ratios for 1st gear in the boxes below are also a good number to compare between combos - they tell you how gear/trans changes affect your off the line acceleration. So, while a higher highway gear (lower numerical ratio) will decrease your highway rpm, it destroys off the line acceleration. Vice versa for deep (high numerical ratio) rear gears which have great acceleration off the line, but scream rpm at highway speeds. Thus the evolution of overdrive transmissions - they allow the rear gear to be middle of the road and the trans has the deep 1st and high highest gear to provide off the line performance and low highway rpm. One other note - 4L60E is basically an electronic 700R4 (same is true for for 4L80E and TH400 - except the 4L80E has an o/d gear - it's not depicted below since you have a TH350) - but the point is that the gear ratios in the trans are the same. TH350, 700R4, 2004R, and 4L60E are somewhat medium duty transmissions (that can be beefed up) - and are "generally" associated with 1/2 ton or sbc trucks. TH400 and 4L80E are all natural HD (and can be beefed up even more) - and are "generally" associated with 3/4 ton or bbc trucks. There are, of course, exceptions (for instance, all K20 auto trans trucks in this era came with TH350 only (but also no big blocks on Ks - but still, it is a heavy duty 3/4 ton truck with a TH350...). A Gear Vendors overdrive/underdrive is also an aftermarket option - it will not increase bottom end accel, but it will provide an overdrive function - it bolts behind the trans and provides gear splitting capability which include an overdrive high gear. It's expensive, but bulletproof. Turns a 3-Spd TH350 into a 6-speed o/d or 4-spd o/d, depending on how you choose to use it. My apologies to the OP also for yammering on like this. Hopefully some helpful info in here for you too Zone47

[Killer Bee]

Originally Posted by jocko

Hi Baime - yes, it's my doohicky, not an online thing. I just plug info in the yellow boxes and the rest is automatic. What can I say, I'm a geek, but it sure has come in handy. I can't post an excel file on here, but I may just post all the different possibilities in a separate thread, folks may find it useful, dunno.
<snip>

fairly sure I can share that file if you want to allow folks to download and run it themselves..

if you're interested, please send your email address or text number in pm and I'll reply with mine..

great tool btw, nice job!

thank you!

[Zone47]

Originally Posted by jocko

29" tire, TH400, and 2600 rpm @ 50 mph = 4.57 gear.
Are you using your speedometer or a GPS for the speed you think you're at? (use a GPS).

I'm using the speedometer, but checked it against one of those radar devices... it's pretty close. I'll try it with a GPS.

The thing is, I'll turn a corner and before I even straighten the wheels out, it's into 2nd and then hit third at 25-30. Seems to wind. I tried looking for the stampings on the rear axle but can't find any.... I thought that would tell me something.

That chart is great, so I have more work to do figuring this thing out. Why the spinning the wheels and counting the driveshaft rotations isn't working, I'll never know


*edit* I should mention too that I have a B&M 2000 to 2200 stall converter, however I'm taking the reading at easy throttle cruising speed, so any slipping should be minimal. It was pretty much the same with the stock converter.

[geezer#99]

Did you have both wheels off the ground when you counted the ds revolutions?
Best exact check for ratio is pull off the cover and check the ratio stamped on the ring gear or count pinion and ring teeth.

[rjs53]

Spinning the wheel one revolution and counting drive shaft rotations always worked for me. I had a 72 Nova with a ZZ4/th350 and a 308 gear. The trans and rear end were obviously stock, but it would shift right out of the hole also. I just changed springs and weights in the governor. Why yours is running such a high rpm at low speeds is Kinda weird. I ran the nova at the strip a few times, never made it out of second in a quarter mile, and would be doing 97mph. That was with a 28"tire and a 10" converter with a 2800 stall. When I put the car together I planned on changing gear ratio, so that's the reason for the higher stall. Your trans must have a special gearing for it to have that high of an rpm at that speed.

[LockDoc]

Originally Posted by Zone47

I'm using the speedometer, but checked it against one of those radar devices... it's pretty close. I'll try it with a GPS.

The thing is, I'll turn a corner and before I even straighten the wheels out, it's into 2nd and then hit third at 25-30. Seems to wind. I tried looking for the stampings on the rear axle but can't find any.... I thought that would tell me something.

That chart is great, so I have more work to do figuring this thing out. Why the spinning the wheels and counting the driveshaft rotations isn't working, I'll never know


*edit* I should mention too that I have a B&M 2000 to 2200 stall converter, however I'm taking the reading at easy throttle cruising speed, so any slipping should be minimal. It was pretty much the same with the stock converter.

Don't know if this thread will help or not.

http://67-72chevytrucks.com/vboard/s...d.php?t=578793

Check post #8 & #19 for code locations if yours has them.

LockDoc

[Zone47]

Ok, I cleaned the rear axle housing and finally found some numbers:

HA 0415 w and then in the upper right corner of the picture, there is another 1 stamped way out there.

I think C228 is the date code.

I don't know if any of this will tell the ratio or not... but it's worth a shot.

Originally Posted by LockDoc

Don't know if this thread will help or not.

http://67-72chevytrucks.com/vboard/s...d.php?t=578793

Check post #8 & #19 for code locations if yours has them.

LockDoc

Actually that is a very helpful source, thank you!!

There is some confusion however. The HA for 68 appears to be for an open rear end and mine is clearly a posi track. It even has the limited slip lube tag on there... so I'm guessing someone threw a posi carrier in there and then who knows what the gears really are.







ok, thanks everyone for the help. I'll pull the cover and figure it out from here since it's still a mystery.

[Mike C]

These "help me id my rear axle" threads come up all the time. And it seems the codes are right SOME of the time. The reality is since you don't know what is inside, and it is a used vehicle new(er) to you, and it's 50 years old... It's probably a good idea to change the diff fluid.

Nobody EVER wants to open that up so comes to the board to ask this questions. But in the truck's 50 years of driving, hauling, and parking who knows what sort of care or abuse has been heaped on it.

Change the fluid. Take pictures of all the numbers on the ring gear. Take a picture of the differential case as well.

Then you will know EXACTLY what you have and that it is in good condition with clean fluid.

[Baime]

Originally Posted by geezer#99

Normal tranny in high gear is always direct drive or 1 to 1. Overdrive ones are usually near .70 to 1. Some are lower like 1.67 or higher like 1.8. You can calculate your effective drive ratio by multiplying your rear gear ratio by the overdrive ratio.
Your 4.10x.70 by example becomes a 2.87 gear ratio. Your 70 mph rpm would be near 2100 rpm. Quite a drop.
Not necessarily a gaurentee of better mileage though.

Apologies to the op for jackin’ His thread!

Geezer,
Thank you for the very clear explanation. I'm actually learning something.

I always thought an overdrive was 1 to 1, and transmissions without overdrive where not quite 1 to 1 in high gear. I didn't realize that overdrives could be a ratio less than one to one (.70 to 1).
Take care,
JP

[Baime]

Jacko,
Thank you very much for all of this information. It is exactly what I'm looking for. I'm still undecided on what I'm going to do, but having this information will definitely help me make a better decision.

I thought I read 3:52 was the lowest gear number for a Dana 60 rear end. I'm sure you are correct that 3:54 is the lowest available.

I am a little disappointed that the highway rpms are not lower if I stay with the TH350 transmission, and change the 4:10 gears to 3:54.

Take care, JP

[jocko]

Originally Posted by Baime

Jacko,
I am a little disappointed that the highway rpms are not lower if I stay with the TH350 transmission, and change the 4:10 gears to 3:54.

I think you may be misreading the chart - staying with TH350 and swapping rear from 4.10 to 3.54 will absolutely drop highway rpm. (2713 to 2330 at 65 mph for comparison). Drops it by almost 400 rpm.
It does so at the expense of bottom end torque multiplication though (10.33:1 final drive changes to 8.87:1 - so less accel off the line). Not "that" noticeable, unless your favorite passtime is doing burnouts. I'd say the highway rpm reduction is worth the tradeoff. Keep in mind, it's ALWAYS a tradeoff, unless you upgrade to an overdrive trans - doing that (without changing the rear gears) normally means increased acceleration and decreased highway rpm because of the additional gear allowing for a more optimal spread.

[Baime]

Originally Posted by jocko

I think you may be misreading the chart - staying with TH350 and swapping rear from 4.10 to 3.54 will absolutely drop highway rpm. (2713 to 2330 at 65 mph for comparison). Drops it by almost 400 rpm.
It does so at the expense of bottom end torque multiplication though (10.33:1 final drive changes to 8.87:1 - so less accel off the line). Not "that" noticeable, unless your favorite passtime is doing burnouts. I'd say the highway rpm reduction is worth the tradeoff. Keep in mind, it's ALWAYS a tradeoff, unless you upgrade to an overdrive trans - doing that (without changing the rear gears) normally means increased acceleration and decreased highway rpm because of the additional gear allowing for a more optimal spread.

Jocko,
I agree, your chart shows the highway rpms dropping about 400rpms. I was trying to say that I was hoping they would drop more than that.

I'm concerned about losing the bottom end torque, but it sounds like that isn't too big of a concern for normal truck operation.

The message I get from your chart is to change out the transmission if possible. Keeps the three lower gears almost exactly the same, and drops the highway rpms way down toward 2100rpms. That sounds like the perfect setup. I need to look into the details of what is involved to change out the transmission.

Take care, JP

[jocko]

Swapping to an o/d trans, a 700R4 for example (12.55 in 1st with 4.10s), will provide a noticeable increase in acceleration in 1st gear compared to your current TH350 (10.33 with 4.10s) while also reducing highway rpm with its o/d high gear.