Looking for a little help on rear gear ratio and speed.

[green69rt]

Just starting to think a little about the final drive on my 69 Charger.   Current rear gear ratio is 3.91.  Just for starters I was assuming a 255X60X17 tire so maybe 29" tall?

Just trying to do some calcs and it looks like I'll be turning about 3170 RPM at 70MPH.  That doesn't sound very nice for long trips.  With a 22% overdrive (maybe a GV unit) that brings it down to about 2460RPM.  Is that reasonable or did I make a mistake??  Rear end is done and I like a 3.90 around town so not too willing to change that part.

[A383Wing]

go here, put in some numbers and see what happens

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

[green69rt]

Interesting.  I punched in the same numbers and got same as my calcs.   Then started fooling around with TC efficiency.  Does anyone have a number for a stock Torque Converter?   I stuck in 90% for laughs and came out with over 3500 RPM at 70MPH!!  I have to assume a higher than stock stall would have lower efficiency at lower RPM, right??

Reaching back in my old memory, it seems my old charger with 3.23 gears  turned close to 3000 RPM at 70 and the calculator shows 2900RPM at 90% efficiency.

Newer TCs any more efficient??

[1974dodgecharger]

I spin 3k going 80mph so its not bad for long trips.  I spun nearly 4k going 95 for roughly 50 miles going to Vegas since everyone else was also, but then they hit 100 plus i backed off to 80.

[fy469rtse]

gear vendors is the way to go , when i have the money thats the next part for me
i think the numbers are much better than that, 3.9 's in the rear will equat to 1300 1660 rpm , have to check with a mate of mines set up , and his car has 411's in it, and is just above 1200 at 60 , i was so impressed

[justcruisin]

I don't think that is correct. The GV has a 22% reduction, with 4.11 rear @ 60mph you would be spinning around 2300rpm in OD, presuming a 28" tire and manual trans, with an auto you may see 2500 - 2600 @ 60mph, depending on converter slip.

Green69 - your calculations seem good to me.

[Back N Black]

I'm thinking about the GV overdrive but seem everyone is having vibration issues. Don't want to pay big buck and have a vibration in the driveline.

[Paul G]

Vibration issues can be a b!tch to resolve.

[Brass]

The GV overdrive is .78:1. 

[Dino]

You made me think back to when my car had 3.55's.  Drove me insane even at 55 but that was with 26" tall tires.  With my tall tires now and 2.96 gears it's perfect for cruising but still I can't wait to get the 6 speed in.  3 speed...what the hell were they thinking...

If the GV was closer to .65:1 and a grand or two cheaper I would've gone that route.  As it is, I don't see it being worth it though.

[green69rt]

You made me think back to when my car had 3.55's.  Drove me insane even at 55 but that was with 26" tall tires.  With my tall tires now and 2.96 gears it's perfect for cruising but still I can't wait to get the 6 speed in.  3 speed...what the hell were they thinking...

If the GV was closer to .65:1 and a grand or two cheaper I would've gone that route.  As it is, I don't see it being worth it though.

Yeah, what you describe is exactly what I was concerned about.  Driving on city streets below 55 it will probably be tolerable but highway cruising would probably be miserable.  With 3.91 gears and a 22% OD that gets it down to approximately 3.05 which should be ok.  Trouble will be coughing up 4 grand for a GV,  maybe I can find a sale.  

[Dino]

Haven't seen a sale on those yet BUT I have seen one or two used ones for sale.  Maybe post a wanted ad?  Nothing to lose right?  It may make an owner decide to sell it so he can buy that 4 speed he always wanted.   

[randy73]

I put a 46RH behind my 340, easy install and did not take much to get the shifter linkage setup right and I have .69 and it looks stock, except the trans mount is a little butchered up.

[Dino]

I put a 46RH behind my 340, easy install and did not take much to get the shifter linkage setup right and I have .69 and it looks stock, except the trans mount is a little butchered up.

If I wanted to stick with an auto then that would be the way to go.  Better ratio and way cheaper.  If I recall you need to massage the tunnel slightly and as you said adapt the trans mount crossmember.

[green69rt]

I put a 46RH behind my 340, easy install and did not take much to get the shifter linkage setup right and I have .69 and it looks stock, except the trans mount is a little butchered up.

If I wanted to stick with an auto then that would be the way to go.  Better ratio and way cheaper.  If I recall you need to massage the tunnel slightly and as you said adapt the trans mount crossmember.

Actually, I think CDR did something similiar (maybe the exact same thing.)   It's in his thread.

http://www.dodgecharger.com/forum/index.php/topic,90903.200.html

[randy73]

I put a 46RH behind my 340, easy install and did not take much to get the shifter linkage setup right and I have .69 and it looks stock, except the trans mount is a little butchered up.

If I wanted to stick with an auto then that would be the way to go.  Better ratio and way cheaper.  If I recall you need to massage the tunnel slightly and as you said adapt the trans mount crossmember.

Had to make a small dent on the passenger side and I trimmed some off the tranny, hardest part was setting up the vacuum. tried 3 or 4 different ways till I found a fitting that would work with the room I had.

[fy469rtse]

You were right , I was thinking of tremec  manual

[fy469rtse]

Is Keisler still going, they were working on a modern 6 speed to fit, that could be the way to go Mitch
If they have made to take the sort of hp our cars have,

[troy.70R/T]

I have some 3:23 open gears in a 742 case I would like to trade if you interested.

[green69rt]

For the rear, I ended up with a setup from Dr Diff...3.91 gear/SG.  I like the idea of a low gear around town and overdrive for the highway.  Only choices seem to be a overdrive tranny or a GV add-on.