Do I have the right gears for 4r70w swap? UPDATE

[bronco loco]

;DI have a 72 uncut Bronco with a C4 stock 3.50 gears and 235-75r15. I'm collecting parts for the 4r70w swap, so far I got the tranny of a 4WD 97 explorer, will I have to replace my differential gears in order for my OD to work properly?, looking forward to your responses, thanks.
UPDATE

I was about to order the 4.10 set up for my differentials, when I read the tech article of how to find out your r/p gear, by the way, this article just save me some $$$$$.
Before I bought this truck, the last po made it uncut again, replacing the 33's for 235/75/15. The speedometer always read faster than I was actually going, for example when the speedo read 60mph, my actual speed was 50mph. I assumed that one of the previous owners replaced the gear on the transfer case to try to compensate for the bigger tires.
Yesterday, after reading the tech article I found out this truck has actually the 4.10 gears, I checked 4 times!, so somewhere along before me a po replaced the differential gears;D. So one less thing to worry about during the swap.

 

[needabronco]

Unless you plan to drive at 90MPH you won't be using OD much if at all. I'd still install it, you can electronically lock out OD until you swap gears. What size tires do you ultimately want to run? It will have a huge impact on your gearing selection.

 

[bronco loco]

Unless you plan to drive at 90MPH you won't be using OD much if at all. I'd still install it, you can electronically lock out OD until you swap gears. What size tires do you ultimately want to run? It will have a huge impact on your gearing selection.

I like the way the truck looks and rides now, there is no plan to go higher than the 235's what gears will I need, thanks

 

[Viperwolf1]

Try it with the 3.50 gears. If OD is too steep then look at 3.92 or 4.11 gears.

 

[Broncobowsher]

So with stock tires, a good highway gear would be about 3.00:1
Maybe high two's. That would put you in around a 4.10 or 4.30 gear.

Another way to question it, what RPM do you want to run and what RPM are you turning now?

Example, you want 2300 and currently turn 2700, you are looking to reduce by a factor of .85 (therotical ideal OD ratio, 2300/2700=.85). 3.50 X .85 = 2.98, the new therotical overall gear ratio. Divide that by .70 (the real OD ratio of the 4R70W) Comes out to 4.25
That is based on my numbers, plug your into the equasions and see what you get.

 

[broncnaz]

I think you'd be ok with stock sized tires anything else and lower gearing would be better. I show the RPMs for that combo to be at about 2200 at 75 mph thats about what it was back when the bronco was made and the speed limit was 55. So the engine should be in its torque range. But I think I would want 3.93s or so. So you can cruise at less than 65mph in OD without the tranny hunting for gearing. Plus you just never know you may want some taller tires in the future.

 

[Amac70]

You are probably fine with what you have, being stock tires and lift your not pushing all the wieght of larger more aggressive tires and taller bronco. the motor will probably do just fine cruising in the 2000 rpm range.

 

[bronco loco]

I really appreciate your input, I guess I see if I"ll need to swap those gears once the
tranny is in.

 

[bronco loco]

A happy update;D;D

 

[broncnaz]

Happy that its in or happy that it works ok? expand please people like to know.

 

[bronco loco]

Happy that its in or happy that it works ok? expand please people like to know.

Happy I don't need to spend more money on the 4r70w swap. Where this gears to low for my C4 and the 235's
?

I was about to order the 4.10 set up for my differentials, when I read the tech article of how to find out your r/p gear, by the way, this article just save me some $$$$$.
Before I bought this truck, the last po made it uncut again, replacing the 33's for 235/75/15. The speedometer always read faster than I was actually going, for example when the speedo read 60mph, my actual speed was 50mph. I assumed that one of the previous owners replaced the gear on the transfer case to try to compensate for the bigger tires.
Yesterday, after reading the tech article I found out this truck has actually the 4.10 gears, I checked 4 times!, so somewhere along before me a po replaced the differential gears. So one less thing to worry about during the swap.

 

[Broncobowsher]

That is good news. Stock sized tires, 4.11 gears and that overdrive should make for a nice driving rig.

Now since you are dealing with what a previous owner did, do check that they did the fronts as well. There has been more then one case where only one axle got the new gearing. (lazy bastards!)

 

[bronco loco]

That is good news. Stock sized tires, 4.11 gears and that overdrive should make for a nice driving rig.

Now since you are dealing with what a previous owner did, do check that they did the fronts as well. There has been more then one case where only one axle got the new gearing. (lazy bastards!)

just check it and they are also 4.10;D

 

[Broncobowsher]

Golden!!