Differential Gear Decision help...
 

The short of it is I'm asking for advice on differential gearing. I'm currently running a 31" tire with a 700R4 that has a crazy 0.55 OD. I believe the current gear ratio is 3.73 and was thinking of going with 4:10.

Right now, at 80 mph I'm doing about 1700 RPM. With the lock up off, I'm doing around 2300 RPM at 80 (the gear is 0.70). With a 4.10 gear, according to this calculator I'd be running around 2000 rpm in OD and 2500 rpm with the lockup off.

The highway sounds fine, but I'm worried about first and second gear's manners on the streets.


For what it's worth, the only drivetrain item not new is the differential gearing. The POSI is worn out, needs to be fixed anyway. The motor is a 383, 10:1 compression, putting out 450 ft tq at 3,000 rpm (dynoed). The tranny is a 700R4 with that "double overdrive" gearing.

Thank you in advance.

Re: Differential Gear Decision help...
 

I was wondering how you got 0.55 OD in a 700R4.

From my RPM calculator I found that your final drive ratios would be very high. Not necessarily a bad thing, just means more tire smoke.

I looked at some shift points:

1st - 37 mph would be about 5000 rpm
2nd - 70 mph @ 5000 rpm
3rd - 100+ @ 5000 rpm
The rest really don't matter.

But for everyday driving, I can't imagine that it shifting a little earlier would bother anyone. It looks like it would probably shift at the points below for gentle driving.

1-2 - 13 mph @ 1600 rpm
2-3 - 25 mph @ 1800 rpm
3-4 - 45 mph @ 2000 rpm

I say go for the 4.10s. I really don't think you will notice a huge difference on the street.

Re: Differential Gear Decision help...
 

The 700r4 gives you the advantage of a lower 1st gear than most automatics. I think you have a good combo. Look at the numbers again and see if you like what 4.10s will give you in cruising RPM.

Re: Differential Gear Decision help...
 

6.0 4L80E with 4:10 gears and I'm cruisin at 2400RPMs @70MPH, tires are 29" tall. Truck gets about 16-17 MPG

I like the combo the truck moves good but I would like a little better mileage out of it.:chevy:

Re: Differential Gear Decision help...
 

Thanks for the input. Sounds like the 4.10 will make it zippier, which is good, but because of the 700r4, still will have decent manners.

Re: Differential Gear Decision help...
 

I don't know that you will be able to feel the difference between the 3:73 and the 4:10's. one thing to think about is the super low 1st gear 3:76 if I remember right That compares to the 400's 2:56 1st gear [all from memory] o5r maybe that was the th350 but you can see 1st gear is really deep on the 700 and the lower gears will highlight that.

Re: Differential Gear Decision help...
 

" The tranny is a 700R4 with that "double overdrive" gearing."


So you have a Gear Vendors under drive?

Re: Differential Gear Decision help...
 

I run 4.88s with a 0.67 OD. Puts along on the highway as if I had 3.26s, which is exactly what it works out to. Since 3.23 is a good stock ratio, I figured that'd be a good target.

With a big block or higher torque small block, one without a lot of cam, I bet 4.56 would be OK. At 4.10 you're at 2.25:1 if you -really- have a 0.55OD, which is too steep. If you were off on the OD ratio and it's more like 0.7, then you're be at 2.87:1 which I feel is still awfully steep, but if you're torquey enough it'd be manageable.

Re: Differential Gear Decision help...
 

I know, the 0.55 sounds ridiculous, but I have a programmable speedo and a new tachometer...I pull around 1700 rpm at 80 mph. Crazy.

Aside from that, I called a shop in Tucson about a new posi and gear swap and they quoted me around $1550 is all goes well. Does that seem steep? I know you get what you pay for, but not always!

Re: Differential Gear Decision help...
 

Again, do you have a Gear Vendor underdrive? That is the only way you are going to get .44 as an OD ratio. A normal 700R4 OD is .7

Re: Differential Gear Decision help...
 

The tranny was built by monster transmission. I figured out I had a .55 using an engine RPM calculator, plugging in my tire size, speed, and RPM. I played around with the various gearings available for a 700R4 and the .55 is the only thing close. The only other way to get to the RPM at speed I have is if I had a differential gear ratio around 2.8. That doesn't match the other known RPM/speed points I know for sure.

Regardless, in verifying the TV cable was correctly installed I have noted all of the shift speeds and also noted their RPMs at those speeds. All of the gears match the math. I'm pretty sure I have a 0.55 OD.

But it doesn't matter if I'm wrong about the 0.55, I do know I'm pulling around 1700 RPM at 80 mph.

Re: Differential Gear Decision help...
 

There is no optional OD for a TH700R4. The first gear is 3.06 and the OD is .70. If your numbers are working out funny it's because something else is wrong.

The 10% jump from 3.73 to 4.10 isn't worth it unless you had to buy new gears anyhow. Before you throw a whole lot of money at it, I would pull the diff cover and make SURE what ratio you have. Sounds like it's going to be 2.73 (unlikely) or 3.07 (likely).

Personal preference, but I dislike the drop on the 1-2 shift with the TH700 and prefer the TH200 4R ratio. 2.74 first gear and .67 OD. 3.73 is perfect ratio IMO and is what I will run in my short step with 29" tall tires.

Figure whichever you choose will be OK. I have 3.07 right now and the 4.10 with OD would give me effectively 2.73 and the 3.73 with the 33% OD would be 2.50.

Re: Differential Gear Decision help...
 

The posi is shot, so I need to go in there anyway. :(

I'll verify the engine speed at 20 mph with the known 3.06 1st gear. I'm sure it's not the 1800 rpm a 2.73 would give.

Re: Differential Gear Decision help...
 

I had a parts car with "4.10" gears that when I pulled it apart were 3.55. It's hard to calculate correctly with converter slippage, roll-out variance etc. Just pull the cover. Know for sure. Cost $3 for a gasket and $20 for lube. Skip the posi additive and maybe it will work better. If it's for sure still a posi. I've had a couple of trucks that said they were on the SPID but in actuality it was gone.

And 3rd gear is the one that makes the most sense to check rear ratio being it's 1:1. Eliminate all the other math except converter slippage.

Re: Differential Gear Decision help...
 

Yes I was hoping to hear about your planned gear change, I remember a discussion a couple of weeks ago.

Re: Differential Gear Decision help...
 

You can use this to calculate rpm at various speeds for various transmissions (all the below is for a 31" tire). No such thing as .55 o/d for 700R4, as was mentioned above.

Re: Differential Gear Decision help...
 

I checked 3rd gear at 50 and pulled about 2100 rpm, which matches with a 3.73 gear. I'll call the tranny manufacturer tomorrow and talk to them about the OD.

This weekend I'll pull the diff cover and physically count.

Re: Differential Gear Decision help...
 

3.07 rear end. As usual, I was wrong...it's my one trick. Either way, thanks for the input.

Last thing, is $1600 for parts (gear swap and posi) and labor a fair deal?

Re: Differential Gear Decision help...
 

Nice to know definitively. And it goes to show why the only real way to know is to physically look at it.

Re: Differential Gear Decision help...
 

I like the 3.42 in my trucks.

I run 275/60/15's and it runs well.

Smitty