Best Over driver transmission

[psehring]

I know this surely has been posted but I'm not 100 percent how to search for it.
I finally got 1400 miles out of my bronco this summer after replacing everything to get it running right. But after 1400 miles on my rebuilt trans its done.... I need to upgrade to one with over drive I have a 351w. I have a the most expensive summer of my life and it doesn't seem to be getting better parts replaced this summer after full 2 year build by shitty shop. Fuel injection, All fuel lines pump and gas tank, Distributor, Electric fan, and now transmission I don't there is anything left to break after this....

 

[englewoodcowboy]

Best is always going to be an opinion. We need a lot more information. First off, how is the motor built, do you have specs? Cam profile etc. because going overdrive you will have to re-gear to take advantage of it and things that effect that is tire size, engine performance etc. and if you want manual or automatic. I will also need to know your planned use, if cruising down the freeway what speed etc. so you can select the correct ratio that puts cruise in OD where the engine is making enough torque that it won't hunt or constantly downshift going over overpasses or climbing slight grades etc.

 

[blubuckaroo]

X2 englewoodcowboy.
Too many choices and different applications to give that kind of advice.
Also, there are automatic and manual tranny overdrive choices.

Personally, I wouldn't try to match an overdrive to an existing gear ratio. That will seldom work out well.

Better to put in the overdrive tranny of your choice. Then with all the tranny gear ratio specs, along with the tire size, select the appropriate axle gear ratio for your expected use.

 

[Broncobowsher]

If you only got 1400 miles out of a transmission, I'm going to investigate why it died so soon before going to the different transmission options. If you are cruising around on 35s and still have the stock 3.50 gears, I understand why it died and will tell you that any of your picks to replace it will die as well.

So lets start with asking what size tires and what gears are in the axles? What transmission did you have that only lasted 1400 miles?

As for "best" there are too many best directions. A 4R70W can be a very good option, there are upgrade parts that can be tossed into it as well. 1,000 HP and survive is an option. 6 and 10 speed options also exist. I'm considering a 6R80 for a project pickup I am working on. But none of them will work good if the axle gearing and tire size are not well matched.

 

[1strodeo]

because going overdrive you will have to re-gear to take advantage of it and things that effect that is tire size, engine performance etc. and if you want manual or automatic. .

If you are cruising around on 35s and still have the stock 3.50 gears, I understand why it died and will tell you that any of your picks to replace it will die as well.
.

So in my 72 I've been riding around on 35's for a year or two with stock 3 spd and (if stock axle) 3.50 gears...am I heading for trouble? Would the attached calculation indicate I need to swap to 4:11? thanks

 

[englewoodcowboy]

So in my 72 I've been riding around on 35's for a year or two with stock 3 spd and (if stock axle) 3.50 gears...am I heading for trouble? Would the attached calculation indicate I need to swap to 4:11? thanks

Being a manual you will most likely just experience a shorter clutch life but your calculation is correct. If you want to regain the stock feel performance you need lower gears although I would go 4.11 unless your motor is strong I would look more at 4.30. You have to keep in mind those calculations do not account for engine health or strength as well as they don't accommodate for changes in unsprung weight. Going from a stock wheel and tire assembly to a larger one is adding to the unsprung weight and it takes torque to get that moving. Your calculator calls out 4.24 so I would go up just a tad and not down unless your engine is a healthy one and has been upgrade over stock.

 

[blubuckaroo]

Is that 35" tire measurement the advertised size, or is it the dynamic size?
It makes a big difference in your RPM calculations.

 

[1strodeo]

Thanks for the reply! I know you're a busy dude!

I plugged 31" into the calculator for my 66 with stock 4:11 gears, but it now has a mildly rebuilt 351W and 4 speed OD toploader (the one with shifter on top) and it says I need 4.41 gears. Guess I'll cross that bridge when I get to it, then again the tub is off so.......thanks for the help,

apologies to OP I have not yet been able to drive my replacement OD trans, but the long term member I bought it from said it was great and very fun to drive, it's a 4 speed out of an 85-87 F150 I believe

 

[1strodeo]

Is that 35" tire measurement the advertised size, or is it the dynamic size?
It makes a big difference in your RPM calculations.

the 35 is the number stamped on the tire

 

[blubuckaroo]

the 35 is the number stamped on the tire

You'll need to measure the radius of the tire to the ground with the vehicle weight on it. Multiply that measurement times 2. That's the effective diameter to use in your calculations. It will be considerably less than 35".

 

[1strodeo]

You'll need to measure the radius of the tire to the ground with the vehicle weight on it. Multiply that measurement times 2. That's the effective diameter to use in your calculations. It will be considerably less than 35".

in that case effective diameter is 32"

but this calculator asks for tire size, do you know of another calculator or a good formula? thanks

 

[englewoodcowboy]

You'll need to measure the radius of the tire to the ground with the vehicle weight on it. Multiply that measurement times 2. That's the effective diameter to use in your calculations. It will be considerably less than 35".

This is not entirely accurate. If you go to the tire manufacturers website you will get the correct measurement and revolutions per mile. This is the most accurate way and usually the difference in posted tire size vs actual is within 2". In the method described what you are not accounting for is for tire expansion when rolling at speed. Just like a dragster they do grow a little when the RPM's climb. The calculation above is more for figuring slow crawl ratio rather than highway speed and rpms etc.

 

[blubuckaroo]

Yes, the tire will expand slightly with speed, but not much. Look at radial tires on the freeway. They still have the same bulge at the bottom.
The revolution per mile would be the best, but I haven't seen a chart that used that in the calculation for gear ratio.

 

[englewoodcowboy]

The RPM usually comes into play when figuring out which speedo gear you will need, which with a gear swap is inevitable... unless he upgrades to a new speedo that has GPS. Those are very accurate and pretty sweet.

 

[Broncobowsher]

Still waiting for the OP to come back and tell us what HIS gearing and tire size are.

 

[psehring]

I was running 35s on the stock 3.5 gears. I planned on getting a new tranny next summer and thought it would last over this summer. I do enjoy driving it harder than most people but I don't drive it on the highway. I just like to step on it here or there. I am bringing to a tranny shop locally this weekend and doing whatever he suggests because I don't know as much as I should when it comes to this stuff. I was hoping there was maybe a go-to one. I will be keep it an auto because its just easier for what I use it for.

 

[psehring]

I Also didn't get any specs after my build or I would have alot more information im just slowing fixing all the parts to the bad build and unfortunately I only know when they stop or don't work at all.

 

[Broncobowsher]

I guessed right. Even guessing the tire size and gearing. Too tall of a tire with too tall of a gear. Add in liking to drive it hard. Failure was expected.

You MUST regear or failure will happen again and again.

A 4R70W is a good choice, but it needs to be matched with a set of 4.88 gears. Bonus, it will feel like you just got another 100HP as well. If you ignore the gearing, you will smoke the 4R70W. You will smoke any automatic.

 

[blubuckaroo]

It's to be expected Broncobowsher.
Changing the ratio in both axles is expensive and intimidating too. That's why owners try to rationalize living with it.
But it won't run right it if it not in it's torque band.

 

[DirtDonk]

in that case effective diameter is 32" but this calculator asks for tire size, do you know of another calculator or a good formula? thanks

Does it only accept metric sizes? Or would it accept something like "32x11.50x15" as a legit number?
Here's the calculator I like best (currently): https://www.yukongear.com/calc_rpm.aspx you just put in the real numbers and away you go.
If you notice they have several pages of calculators that come at the problem from different directions. If you're more comfortable calculating it differently, you can choose another method. But the one that shows up at that link is pretty straightforward.

Paul