So if the wheels rotate once and the driveshaft rotates four times...

[bull]

... does that mean I have 4:10s? If so, WTH am I going to do with those friggin' things except top out at 60 mph and 5k rpm? Also the wheels are both turning the same direction so I obviously have 8 3/4 Sure Grip but the car didn't originally come with Sure Grip. Is it common for people to get in there and change the non-Sure Grip rear ends to Sure Grip and mess with the ratio? I can tell someone's been inside the pumpkin because there's some blue ATV that's been squeezed out along the seam. What do I look at to see if I have the original rear end in the car?

[Duey]

Bull, you could have a 4.10 or a 3.91...both are only about a tenth of a revolution (35 degrees) of a driveshaft revolution from four full turns.  If you're slightly "more" than four turns then yes, you're a 4.10....slightly "less", a 3.91.

Your fender tag or build sheet would tell you what the car originally had.  What was your car's original driveline again?  From that you could guess what was in there if you don't have the build sheet or didn't decode the fender tag(s).  e.g.  SG 3.23's were a very popular rear end on early 3rd gens with 440 or 400.

Re: revs on the highway...you either have to live with high rpms if you want the "giddy up and go" of a deep rear end or consider overdrive options (Gear Vendor O/D or a Keisler/Tremec 5/6-speed tranny).  I figure I'll live with life with 4.10's since my autobahn cruzer is my 6-speed 540i, not my Charger...its sole mission, to quote Cudaken   is to "sound like a can of rocks" in the parking lot or scare away all the 350ci-yada-yada-yada Chebby guys at stoplights without actually "launching in anger"...

Cheers,
Duey

[bull]

It came with 3:23s new. I was wondering if there's a way you can tell by looking at the differential if it's stock, in other words, is there a way I can identify my rear end as being original but modified or if it's been swapped out? Can it be returned to it's original ratio? It's an 8 3/4, that's all I know for sure. Are 3:91s much more bearable than 4:10s on the highway or can you even tell a difference?

[Duey]

Bull, I'm guessing your '68 would have come with the 742 casing (1 3/4" pinion) although 741's (1 3/8" pinion) were also an option in 68 (but for lower horsepower options, usually).  If you have a 489 case, it's likely not original, but an upgrade after the fact.  If you have a 742, it's likely original with different gears added at a later date.

Cheers,
Duey 

[cudaken]

 Bull, from the factory there should be a metal tag on the rear end. Tag is held on by one of the bolts that hold the pumpkin.

Now, if you want a 3.23 sure grip and want to dump that rear end we can talk. If you like me to do the math on RPM's I be happy to as well. I will need how tall your tires are to do it.


                           Cuda Ken

[bull]

Bull, from the factory there should be a metal tag on the rear end. Tag is held on by one of the bolts that hold the pumpkin.

Now, if you want a 3.23 sure grip and want to dump that rear end we can talk. If you like me to do the math on RPM's I be happy to as well. I will need how tall your tires are to do it.


                                        Cuda Ken

What do you need from me other than tire size? The car is off the floor at the moment. I've got it up on jack stands so I can get under it to unbolt the tranny and pull the engine. I'm also going to go out on a limb here and guess the little metal tag is gone because someone got in there and messed around with it. $20 says they tossed the tag.

[bull]

Yup, they tossed it. Rotten you-know-whats! Anyway, the rear tires it has at the moment are 255/60R15s and that's about 26" in diameter.

[Runner]

is this a surgrip rearend?.  if its not and you have both tires off the  ground you wont be able to get the currect gear ratio by spining the tire and counting drive shaft revalutions.

[694spdRT]

is this a surgrip rearend?.   if its not and you have both tires off the   ground you wont be able to get the currect gear ratio by spining the tire and counting drive shaft revalutions.

Bull mentioned it is a sure grip in the first post. I would guess they are 3.91's unless someone went to the expense of putting in aftermarket 4.10's.

[firefighter3931]

... does that mean I have 4:10s? If so, WTH am I going to do with those friggin' things except top out at 60 mph and 5k rpm? Also the wheels are both turning the same direction so I obviously have 8 3/4 Sure Grip but the car didn't originally come with Sure Grip. Is it common for people to get in there and change the non-Sure Grip rear ends to Sure Grip and mess with the ratio? I can tell someone's been inside the pumpkin because there's some blue ATV that's been squeezed out along the seam. What do I look at to see if I have the original rear end in the car?

Personally i like a 3.91-4.10 gear for the street, especially in a heavy beast.....really wakes the car up ! The hwy rpm will depend a lot on tire diameter. A 4.10 with 28in tires will turn 2950 rpm at 60 mph on the hwy. If you're planning on running a 275/60/15 tire then i'd consider keeping that gearset.  

Sounds like the previous owner did you a big favor  

Ron

[Runner]

is this a surgrip rearend?.   if its not and you have both tires off the   ground you wont be able to get the currect gear ratio by spining the tire and counting drive shaft revalutions.

Bull mentioned it is a sure grip in the first post. I would guess they are 3.91's unless someone went to the expense of putting in aftermarket 4.10's.

  my bad 

[cudaken]

 Bull, here is some math for you.

3.91 gears 65 MPH = 3284 RPM's
4.10 Gears 65 MPH = 3444 RPM's

At 5200 RPM's 3.91 gear's 102.9 MPH
At 5200 RPM's 4.10 gear's 98.14 MPH

Just for the grins, my Charger with 26.5 Tires and 2.76 Gears.

2.76 gears 65 MPH = 2278 RPM's
At 5200 RPM's 2.76 gear's 148.59 MPH.

Kind of makes you think about the guys that say they did 150 MPH with 4.10's to 3.23's.

Heck for the fun of it.

4.10 Gears 150 MPH = 7797 RPM's
3.23 Gears 150 MPH = 6143 RPM's.

Now, if you want the 3.23's out of the Runner, they are out of a 69 Charger R/T.

                                                           Ken

[bull]

... does that mean I have 4:10s? If so, WTH am I going to do with those friggin' things except top out at 60 mph and 5k rpm? Also the wheels are both turning the same direction so I obviously have 8 3/4 Sure Grip but the car didn't originally come with Sure Grip. Is it common for people to get in there and change the non-Sure Grip rear ends to Sure Grip and mess with the ratio? I can tell someone's been inside the pumpkin because there's some blue ATV that's been squeezed out along the seam. What do I look at to see if I have the original rear end in the car?

Personally i like a 3.91-4.10 gear for the street, especially in a heavy beast.....really wakes the car up ! The hwy rpm will depend a lot on tire diameter. A 4.10 with 28in tires will turn 2950 rpm at 60 mph on the hwy. If you're planning on running a 275/60/15 tire then i'd consider keeping that gearset.  

Sounds like the previous owner did you a big favor  

Ron

I was thinking of going with 295/50R15s on the rear. Would those be too short?

[bull]

Bull, here is some math for you.

3.91 gears 65 MPH = 3284 RPM's
4.10 Gears 65 MPH = 3444 RPM's

At 5200 RPM's 3.91 gear's 102.9 MPH
At 5200 RPM's 4.10 gear's 98.14 MPH

Just for the grins, my Charger with 26.5 Tires and 2.76 Gears.

2.76 gears 65 MPH = 2278 RPM's
At 5200 RPM's 2.76 gear's 148.59 MPH.

Kind of makes you think about the guys that say they did 150 MPH with 4.10's to 3.23's.

Heck for the fun of it.

4.10 Gears 150 MPH = 7797 RPM's
3.23 Gears 150 MPH = 6143 RPM's.

Now, if you want the 3.23's out of the Runner, they are out of a 69 Charger R/T.

                                                           Ken

Thanks Ken. Points to ponder.

[firefighter3931]

... does that mean I have 4:10s? If so, WTH am I going to do with those friggin' things except top out at 60 mph and 5k rpm? Also the wheels are both turning the same direction so I obviously have 8 3/4 Sure Grip but the car didn't originally come with Sure Grip. Is it common for people to get in there and change the non-Sure Grip rear ends to Sure Grip and mess with the ratio? I can tell someone's been inside the pumpkin because there's some blue ATV that's been squeezed out along the seam. What do I look at to see if I have the original rear end in the car?

Personally i like a 3.91-4.10 gear for the street, especially in a heavy beast.....really wakes the car up ! The hwy rpm will depend a lot on tire diameter. A 4.10 with 28in tires will turn 2950 rpm at 60 mph on the hwy. If you're planning on running a 275/60/15 tire then i'd consider keeping that gearset.  

Sounds like the previous owner did you a big favor  

Ron

I was thinking of going with 295/50R15s on the rear. Would those be too short?

The 295/50's are 26.5in tall....so yes there is a difference. With that tire and 4.10's you'll be turning 3120rpm at 60 mph.....almost 200 rpm more for the shorter tire. I find a 26.5in tire looks small on a second gen charger.....almost looks like a kids wagon when the tires don't vertically fill the wheel well. Personal preference i suppose but i won't run anything shorter than 28in on the back. I've got 295/65/15's for mine which are 29.5in tall which is ideal for the engine/stall/gear combo i'll be running. With the 295/65 tire combo and 4.10's the 60mph cruise rpm drops to 2800.  

Ron

[bull]

Thanks. I'm a math retard so I'm glad you guys are around.

[cudaken]

 Ron, I have 26.5 tall tires on the Charger and I like the look. I have had 28 tall tires on the Runner, well all I will say is lot of guys in POS Pick Up's give me thumbs up.   Felt like I was driving into the dirt all the time.

                                        Ken

[Duey]

... does that mean I have 4:10s? If so, WTH am I going to do with those friggin' things except top out at 60 mph and 5k rpm? Also the wheels are both turning the same direction so I obviously have 8 3/4 Sure Grip but the car didn't originally come with Sure Grip. Is it common for people to get in there and change the non-Sure Grip rear ends to Sure Grip and mess with the ratio? I can tell someone's been inside the pumpkin because there's some blue ATV that's been squeezed out along the seam. What do I look at to see if I have the original rear end in the car?

Personally i like a 3.91-4.10 gear for the street, especially in a heavy beast.....really wakes the car up ! The hwy rpm will depend a lot on tire diameter. A 4.10 with 28in tires will turn 2950 rpm at 60 mph on the hwy. If you're planning on running a 275/60/15 tire then i'd consider keeping that gearset. 

Sounds like the previous owner did you a big favor 

Ron

I was thinking of going with 295/50R15s on the rear. Would those be too short?

The 295/50's are 26.5in tall....so yes there is a difference. With that tire and 4.10's you'll be turning 3120rpm at 60 mph.....almost 200 rpm more for the shorter tire. I find a 26.5in tire looks small on a second gen charger.....almost looks like a kids wagon when the tires don't vertically fill the wheel well. Personal preference i suppose but i won't run anything shorter than 28in on the back. I've got 295/65/15's for mine which are 29.5in tall which is ideal for the engine/stall/gear combo i'll be running. With the 295/65 tire combo and 4.10's the 60mph cruise rpm drops to 2800. 

Ron

Ron, as a slightly O/T question...well lots O/T, how close to redline are you planning to trap through the 1/4 with your combo?

Cheers,
Duey

[firefighter3931]

Ron, as a slightly O/T question...well lots O/T, how close to redline are you planning to trap through the 1/4 with your combo?

Cheers,
Duey

Hi Duey, the 295/65's are going to be my "street" tires. I've got a set of 28in slicks for the track. At 6000rpm with the 4.10's and 28in slicks the mph is right around 120mph which is low 11's high 10's. That is assuming the motor makes enough power to accelerate the car to 120mph over 1320ft. Tire growth also has to be factored in as slicks grow with speed so this figure might actually be slightly low for the mph. I'm expecting the car to go mid 11's (116/117mph) given the power and weight....anything faster will be a bonus. The engine made peak power at 6100 with the 2in headers and was holding strong right to 6500 where the dyno runs ended. I'm gonna throw a 6200 chip in the msd and experiment with shift points to see where the car et's best. Like anything it'll be a trial and error deal. With the heavy piston/stock rod combo i don't want to risk windowing the block for the sake of a tenth or two on the time slip.

Realisticly, i don't expect it to hit 120mph in the 1/4...... but i could be pleasantly surprised.

Ron


Ps. Say hi to Paul (Dr. Pimp) for me if you run into him over there. Be safe !

[Duey]

Ron, just saw Paul two days ago...of all things our paths crossed in an airport in U.A.E. while we were criss-crossing all over the place.  He's looking forward to building the stroker 400...sounded like he was leaining towards a 451...we'll drop by and see you when we both get back.

Cheers,
Duey

['CUDA360]

... does that mean I have 4:10s? If so, WTH am I going to do with those friggin' things except top out at 60 mph and 5k rpm? Also the wheels are both turning the same direction so I obviously have 8 3/4 Sure Grip but the car didn't originally come with Sure Grip. Is it common for people to get in there and change the non-Sure Grip rear ends to Sure Grip and mess with the ratio? I can tell someone's been inside the pumpkin because there's some blue ATV that's been squeezed out along the seam. What do I look at to see if I have the original rear end in the car?

One thing I didn't see anyone mention is you can rotate the wheels twice to see what your gear ratio is.
It then becomes very obvious if your U-joint turns 7 3/4 times then it's a 3:90 and 8 1/4 it's a 4:10