What gear ratio do I need?

[A110235]

Hey all.  I am looking into getting my 8 3/4 rebuilt in my '71 Challenger and I am wondering what ratio gears would best suit me.  It has a 741 center section.  I will be using the OD 833 trans behind a mildly build 383.  I will probably be going with a 255/50 R16 tire (26"tall +/-).  I would like to drive my car a couple of times a week with a few long road trips a year.  I don't really plan on taking it to the track, but I would like a little bit of starting line grunt just in case of an impromptu stop light shootout with a ricer.  If I remember right, the first gear ratio in the OD trans is lower than the normal 833.  Would 3.55's be a good middle ground?  I think I remember reading that the 741 case is the weakest of the 8 3/4 center sections.  Do I need to upgrade to the 742 or 489, or will the 741 live up to what I have planned?

Kyle   

[72bluNblu]

For a mildly built 383 the 741 should be fine, if you were planning on racing it I would say upgrading would be a good idea. But for a street car you shouldnt have problems. As for gears, the OD trans has a 3.09 1st gear, but 4th is .73 or even .71 if it was a truck transmission, so a 3.91 will still be fine for the freeway. I just ran all the numbers for my 904 with 2.76 gears and 26" tires converting to a 833 OD and a 3.91, cruising rpm for the OD and the 3.91's is only about 15 rpm higher. Yes, 15. 2,497 with a 1:1 gear and 2.76's, 2,512 with a .71 gear and 3.91's. Neat huh? If you're really worried about it, you can run 3.73's, but I wouldn't worry about it. 

[Chryco Psycho]

the 3.55 is nice with the OD , 1st gear is OK with the 3.55 .
The 3.91 is ok on the highway as well but the 1st gear becomes almost usless

[cwestra]

I have 3:55's in my car with a 4-speed and, although I have only had it on the road for a little while, I am very happy with the ratio.  Orignially, I was convinced that I would be swapping it out for a 3:91 or 4:10, but I know now that I'd get way too much wheel spin off the line if I did that.  And as CP stated, 1st gear would be useless.

I'm not sure how much money the rebuild of your 741 will be, but it may be a good opportunity to put it towards an aleady rebuilt 742 or 489 instead.  You could keep your 741 chunk on the side and even run a different ratio on the 742 or 489, swapping them in and out depending on what you plan on doing (road trip or track).  Money invested in a 741 may not be recoverable, while the 742's or 489's should always hold their value.  Just a thought.

Here is something else that you might find useful.  I think someone on this site actually referred to it a while back.  That's how I found it.  Pretty cool way to configure your tire size, axle ratio, etc... and see the results.  Check the site out below.

http://vexer.com/automotive-tools/speed-rpm-calculator

[SBDave]

I like playing with that calculator but I really enjoyed reading this little discussion.  I've got a 318/904 with what I assume is a stock rear end (it's non posi and I'm guessing 2.76 rear end).  I don't know exactly what I want for an engine/tranny/axle setup but I'm thinking of staying small block with possible power adders (turbo?) and I want to change over to a manual transmission (5 speed w/ over drive?).  I think it would be really helpful if we had a thread where people posted their drive train setups, what they use them for and some stats IE rpm's at highway cruising, how long 1st gear lasts etc.  Or does this already exist?  Good luck Kyle! Let us know what happens!

-Dave

[ntstlgl1970]

Does someone offer gears for a 741? (the answer used to be no). The gear you currently have may be the gear you are stuck with.

[72bluNblu]

Does someone offer gears for a 741? (the answer used to be no). The gear you currently have may be the gear you are stuck with.

Yes. Not sure who makes them, but Dr. Diff lists 3.55 and 3.73 gears for the 741's, I believe I have seen other ratio's as well elsewhere. Not a great selection, but they're out there.

[73EStroker]

I now have my Passon Hemi OD in and running behind 3.73 gears. All I can say is fantastic. Had RPMs up to about 6000 in 2nd which is showing just over 70mph and had her up to 95MPH in 3rd at about 4500RPM. Passon recommends taking it easy for 500 miles to mesh the gears so I am not accellerating as hard as that 451 likes to do. The drop in RPM from 3 to 4th is very close at around 80MPH as the 4th is only 0.80 ratio in this setup. My older A833 OD was 0.73. Nets me about 3000RPM @ 80MPH in OD. I am very happy.