Posi track and gears question

[*71SWB*]

Is there a major difference in the 3.42 & 3.73 gears? Currently I believe I have the 3.08. Has anyone had both? I am trying to decide which to have put in

[zeldman]

A little more info please, such as transmission type and motor type. Are you a red light to red light person or are you looking for cruise RPM. For a big block with a standard tranny I would go with 3.43 gears. A small block with an auto I would choose the 3.73. If you have an OD tranny then I would go with 3.73 or maybe 4.11. For me it depends on what I want to do with it.

[*71SWB*]

The motor is SBC 350 and the tranny is TH350 (no over drive). I'm looking for cruising.
Would there be significant difference in RPM between the 3.42 & 3.73 ?

[Fitz]

Originally Posted by dossfatboy

The motor is SBC 350 and the tranny is TH350 (no over drive). I'm looking for cruising.
Would there be significant difference in RPM between the 3.42 & 3.73 ?

Go here:

http://www.grimmjeeper.com/gears.html

And before anyone whines "But it's a jeep calculator", yes it is a jeep calculator but it lets you do FWD's too! Use the pulldown menu to find your trans. Click no lockout torque converter, enter the number one in the boxes for high range, low range and stak 3rd range. Enter a rear end ratio, specify tire size and press calculate. It will display road speed vs RPM for three different values in each gear and other useful stuff.

Change the rearend gear ratio and do over.

Now you'll be able to compare any combination of trans, overdrive/no overdrive, tire size and rearend ratios for RPM ranges, crawl speeds and other cool stuff.

Have fun!!!

[68350]

My setup is 295/65/15 tires, 3.42 Eaton posi, th400 and at 70 its roughly 3000 rpms. My 496 with a holley double pumper will pull 10-12 at 70 cruising down the highway.

[54belair]

If youre into cruising-----without overdrive---Id stay with the 3.08's
These trucks dont get good mileage no matter what you do--but the 3.08's will sure help.
Im currently building my 68----I pulled a 700r4 and Im installing a muncie M 21----Just my personal preference-----With 3.08's Ill cruise at 65-ish @2400 (ish) difference between 2400 rpm and 3000 rpm will be significant to your wallet.
Here again man, its all personal preference. I built my engine for low rpm and higher torque to help with off the line, after I get into 4th the 3.08's will work for me.

[hugger6933]

I think one of the best all around gears seems to be the 3:42. I am building two 70's and one 68 and I feel that they all have the standard ratio rear [for the 70 it will be 3:73] I have not looked up what was standard in 68. Probably those gears will remain just because it is easier to leave them. One 70 will be a small block very mild, with a T-5 trans the other close to stock 454 T400. The 68 will have a slightly warmed 454/4L80E. So while I am all over the place on the power plants, unless the first or second get changed out for LS power [I need to clear out some drive train stuff] I have it laying everywhere around and in my shop, the ratios seem to be the only thing consistent in my builds at this point. Jim

[*71SWB*]

Man I hate to ask one more questions about which gear to choose. I guess my issue is I really like the thought of the quickness from light to light with 3.73 however I really only cruise around town and like that aspect of the 3.42. I have th350 and 275/60/15 and 23-2700 converter. The calculation shows 3.73 will be 3075 rpm at 65 mph and 3.42 will be 2819 at 65 mph. Will the converter make any difference between the two? AND my last question - is there a significant difference with the 3.73 vs 3.42 considering light to light aspect ?? Thanks everyone for bearing with me and my questions..

[Fitz]

Originally Posted by dossfatboy

Man I hate to ask one more questions about which gear to choose. I guess my issue is I really like the thought of the quickness from light to light with 3.73 however I really only cruise around town and like that aspect of the 3.42. I have th350 and 275/60/15 and 23-2700 converter. The calculation shows 3.73 will be 3075 rpm at 65 mph and 3.42 will be 2819 at 65 mph. Will the converter make any difference between the two? AND my last question - is there a significant difference with the 3.73 vs 3.42 considering light to light aspect ?? Thanks everyone for bearing with me and my questions..

Converters do slip so the rpm's will be higher than your estimates by 100-200. You are also about to discover what happens when you increase the stall speed of your converter. Unless you keep the rpm above the stall your trans is going to run hot. With the way you say you drive, loafing around town (with business and residential speed limits) and 3.42 rear gears you aren't ever getting above the stall speed of that converter unless you say in second gear all of the time. I would take that truck back to a stock stall speed converter or add a trans cooler or both!

[In The Ten Ring]

What about a 250 I6, 3 on the tree, positive traction, and 3:73 gears?

Right now I have a 4:11 posi trac and the gas mileage is crap and top speed is pretty low.

If I go to 3:73 will it go faster and get better mpg? Can I keep the posi trac?

[*71SWB*]

Originally Posted by Fitz

Converters do slip so the rpm's will be higher than your estimates by 100-200. You are also about to discover what happens when you increase the stall speed of your converter. Unless you keep the rpm above the stall your trans is going to run hot. With the way you say you drive, loafing around town (with business and residential speed limits) and 3.42 rear gears you aren't ever getting above the stall speed of that converter unless you say in second gear all of the time. I would take that truck back to a stock stall speed converter or add a trans cooler or both!

So if I'm understanding correctly the 3.73 is what the converter I have requires considering the tire size and th350

[Keith Seymore]

Originally Posted by zeldman

A little more info please, such as transmission type and motor type. Are you a red light to red light person or are you looking for cruise RPM. For a big block with a standard tranny I would go with 3.43 gears. A small block with an auto I would choose the 3.73. If you have an OD tranny then I would go with 3.73 or maybe 4.11. For me it depends on what I want to do with it.

Originally Posted by hugger6933

I think one of the best all around gears seems to be the 3:42.

I'm right there with you guys on this one.

We historically always started with 3.42 and moved off that depending on what we wanted.

More pep off the line? Adding an overdrive? Start going up (numerically) from there.

BTW - the 3.73 in my truck is perfect. 4.3L V6, MY6 overdrive (.73:1) trans, 3.73 posi, P235/75R15 tires.

K


K

[Fitz]

Originally Posted by dossfatboy

So if I'm understanding correctly the 3.73 is what the converter I have requires considering the tire size and th350

If it were my truck (I enjoy the stoplight wars) I would go with a 3.73 rear. If you drive like my girlfriend (130 on the interstate but sedate around town) then 3.42! Either one is a good choice. Use the calculator and you'll see that there is not a lot of difference and if you THINK you made a mistake shift the tire size for a test run! You can make a 3.73 act like a 3.42 and vice versa (almost) by changing tires.

[bigmac73]

why so big on the stall converter? unless you are doing some serouis drag racing anything bigger that a 2500 RPM for the street is over kill

[*71SWB*]

Originally Posted by bigmac73

why so big on the stall converter? unless you are doing some serouis drag racing anything bigger that a 2500 RPM for the street is over kill

I thought I was getting a small converter when I ordered it. I ordered it from jegs and the listing shows 23-2700. I thought that was what I needed, however I am the extreme non-mechanic.

[bigmac73]

back when i was a young man i had another 72 C-10, 307 3 on the tree, with 3.08 rear end, i put in a 350HP crate engine and a turbo 400 and a B/M Holeshot stall converter i think it was 2100RPMs with a Holley 650 DP and did some tweaking on my advance springs in my Dizzy and it ran great off the line, but since i had those 3.08's in the rear end i would give the 5.0 Gt's prettyboys a run for there money on the highway and it cruised great, this was back in the early 90's when every punk had to have a mustang

i would see if you can exchange the converter and get a 2100 stall

[oldspowered67C10]

If it helps, I believe if you want to run 3.73s you need to change the carrier.