Rear end Gearing

[dodj]

For an eye opener, (all you people driving with 3.91) get a pyrometer and check diff temps after a 100 mile trip. Or 40.

What kind of temps would you expect? What temp would you expect with a 3.23? I have no idea. I never thought the ring and pinion produced enough heat to be concerned about it?
Would using something like Amzoil Severe gear in the diff produce lower temps?

[Cudajason]

This is a subject I am very passionate about.
  Reading your post , the 1600 rpm at 60, is not possible with 28in tires and 3.23, according to the rpm/gear/ tire charts I have.
Even so, it brings up the camshaft speed question.  And converter flash issue.
Rpm is not the enemy.  Proper tune can be efficient even at higher speeds. When posting my mileage,  my average speed is 85, and I am traveling through mountains with 100 to 4000 ft elevation changes.
Driving at 70 with 3.91's  overdrive or not will probably ruin the gearset in short order if you were driving 600 or more miles a day, like on a road trip. For an eye opener, (all you people driving with 3.91) get a pyrometer and check diff temps after a 100 mile trip. Or 40.
I've stuck with 3.23 and built my car around them. Wide ratio 4 spd (3.09 first ) 245 60 15 at approximately 26.5 in tall. 340 , holley 780. Fun car.

Your chart does not factor in OD.

Seems like lots of folks out there with 3.91s or better putting miles on their cars with no issues.

[dodj]

Seems like lots of folks out there with 3.91s or better putting miles on their cars with no issues.

I'm thinking it unlikely you will be putting over 900km a day on the car? If you are, you will be passing my place on the second day...stop in for a beer.

But I am curious about the heat difference between 3.23 and 3.91 over, say, 500km?

[Brads70]

For an eye opener, (all you people driving with 3.91) get a pyrometer and check diff temps after a 100 mile trip. Or 40.

What kind of temps would you expect? What temp would you expect with a 3.23? I have no idea. I never thought the ring and pinion produced enough heat to be concerned about it?
Would using something like Amzoil Severe gear in the diff produce lower temps?

https://www.randysworldwide.com/blogs/tales-from-the-tech-line-keeping-gears-cool-during-break-in


170-220 according to" Randys"?   The factories have been installing 3.90-4.11 and more in stock  rear ends longer than I've been alive with no issues. Sure a 4.11 will make more heat than a 3.23 . My Ford service van has 4.10 in it with and OD tranny and has 406K on it so far. Yes the tires are bigger/taller.
For vehicles like ours that see limited mileage compared to our daily drivers I don't see an issue. If your really worried about it change the rear gear oil every 10,000 miles?
I've had 4.10 in my Challenger for almost 10 years now with no issues ( with a 4L60 OD transmission) I take many out of town trips, even been to Carlisle 2x with it ( thats 7 1/2 hours driving time , each way)   I'll have to take a trip then take a temp reading with the heat gun just for giggles....

[Mr Cuda]

For the sake of keeping the comparisons equal,  we should limit the discussion to 8 3/4 's.
Yes, I  have a dodge truck with 4.10's and it goes on the highway towing for hundreds of miles,  no problem.
Tire size and gear ratio  resulting in drive shaft speeds of 3500 are going to build heat.  Speed charts for driving at 60mph are great for in town , local show trips. I see you guys in the right lane.
Out west, everyone hauls ass, minivans stacked with kids, trucks with camp trailers in the left lane going 80.
So yes, overdrive will lower your engine to sane levels,  but your 3.91s are not going to like it.
So cudajason, I guess I didn't realize that you already had od, I  thought you were going to put in lower gears and offset the rpm with od.  And 28" tires are damn tall, what size are they

[Cudajason]

For the sake of keeping the comparisons equal,  we should limit the discussion to 8 3/4 's.
Yes, I  have a dodge truck with 4.10's and it goes on the highway towing for hundreds of miles,  no problem.
Tire size and gear ratio  resulting in drive shaft speeds of 3500 are going to build heat.  Speed charts for driving at 60mph are great for in town , local show trips. I see you guys in the right lane.
Out west, everyone hauls ass, minivans stacked with kids, trucks with camp trailers in the left lane going 80.
So yes, overdrive will lower your engine to sane levels,  but your 3.91s are not going to like it.
So cudajason, I guess I didn't realize that you already had od, I  thought you were going to put in lower gears and offset the rpm with od.  And 28" tires are damn tall, what size are they

Yep put the OD in two years ago.  Best upgrade yet!!!

I am running 255 60 R15s, which are actually 27 in tall, not that it makes that much of a difference.

[MoparCarGuy]

For reference on tire sizes...

[Mr Cuda]

Just got home from  a 100 mile trip in my 1/2 ton d100, WITH an 8 3/4.
2.76 gear ratio,  so the coolest running gear you are going  have. Plus,  it stopped raining,  but is still on 60° F, so that helps rear axle  cooling.
  145° was the  temperature
Still waiting for all you  performance gear ratio  drivers to take your  temperature.   Especially the 3.91 and 4.10 ratio  ones.

[dodj]

What do you use to read temp?  One of those infrared guns?

[one bad fish]

I'm running a 410 gear behind a 4 speed. 30 inch tire...I love it. 70 mph is 2700 rpm. the big 512 likes it there  . I don't like cruising at any higher speed. 70 is fine for me...