Hello All,

Iv been using this site for a while getting ideas for my 91 4runner with 3.4 swap and 3.0 4x4 manual tranny, and I appreciate all of your guys' knowledge. You all have been very helpful. One thing I have not come across is t-case gearing intended for MPG. What I mean by that is gearing that allows me to go highway speeds with my engine in low RPM's. I'm assuming that if its possible to do the opposite, which is very prevalent for crawling, that it would be possible to achieve this as well. Any ideas? Or even if there is a post already in existence if someone could point me in the right direction that would be great.

Thanks a lot!

 

My 85 22R has Marlin dual transfer cases and she was only getting 8 or 9 mpg with the stock 4:10 gearing on 33"s but, after regearing to 4:88 a month ago my gas mileage doubled and now she is getting 17-18 mpg.
She can cruise 72 mph up the Grapevine now where as we could only manage 45-55 before the regearing took place.
I don't have that V6 power you have but, re gear that puppy and all will be better. 😉

 

Your assumption is incorrect.

It is not possible to use dual cases to achieve lower revs at cruising speed.

The transfer case either is straight thru ( 1 to 1) in high range, or reduced (more than 2 to 1) in low range.

Any overdrive gearing has to come from the transmssion itself, or you might reduce cruising revs with higher differential gearing.

 

Transfer case gearing does not affect 2 wheel drive final drive ratio. When you aren't in 4 low the gear ratio is 1:1 in 4hi it's 1:1 with your front axle parts turning and in 2hi its 1:1 with just your rear axle turning. So in other words the only time the transfer case effects gear ratio is in low range.

 

Ok, is this not possible because the t-case cant handle a gearing set up higher that 1:1? Is there not enough room in the housing?

 

There is no high range gearing, except to provide power to the front driveshaft when in 4WD, its just straight thru - 1 to 1.

I think if it was possible to provide overdrive by regearing a t-case, someone would offer kits for sale.

If you are a machinist with the skills to make such a thing, offer them up, I might buy one!!

 

Is it at all possible to use gears out of a transmission? Would they fit into the t-case? From what I'm hearing the problem is not that its impossible, just that nobody has ever done it. Is that correct?

 

Yes, it is impossible to use any existing tranny gears.

Anything is possible however, if you have a 6 or 7 figure machine shop and foundry, and the expertese and experience to match the tools.

If it were easy, or cost effective, several such mods would already be on the market. It does'nt pay to try and re-invent the wheel.

Back in the day, there were several behind the transmission type add-on overdrive units on the market made for Jeeps and such. You might do some research on those. Good Luck!!

 

Could run dual transmission. Have seen it on Pirate far less popular than dual t-case. No clutch between them so bit of a challenge but....

 

no one has done it because it doesn't make sense. If you want lower highway rpm then change your diff gearing or get an aftermarket overdrive.

 

Bah I'll bite..

Locate the most efficient rev range for the engine.
Next plot the torque curve.
Now locate the cross point of the torque curve relative to the efficiency curve.

OK now you have the ideal rpm for defeating wind resistance.

Using the gear ratio calculation (grim jeeper) find out what size tires and gear ratios get you there..

No machine work needed just a ton of math and some dyno time..

PS, you could apparently run the auto transmission with the Jeep stuff mentioned above since it's the same transmission..

 

dual trans would give you the overdrive your looking for. just leave the rear trans locked in 5th gear and you should be fine. guys who shift both trans tend to crap out the rear one in a few thousand miles. but what is the mileage gained vs the weight gained. and would it be worth it?

a better option would be to check through the W series and R series transmissions and see what has the overdrive ratio thats better than your current one.

 

Gear vendors makes overdrive units, which accomplish what you are trying to do, but i think they only make them for domestics.

Your best bet would be to change the gears in your diffs, and then the doubler allows you to make up for that gearing when offroad.

 

And that is pretty much what I said when I answered dude's question in the 1st response but, reinventing the wheel is being insisted on.
Just change the gear ratio in the differentials and be happy, 4:88 is what worked for me, maybe it or 5:29s will work for dude if he actually listens to the good advice freely given already.

 

Thanks a lot guys, I appreciate all of your help. Particular thanks to fasterspider and millball, wasn't trying to ignore your input. Just wanted to know why my idea was impossible.

 

If you took a t case apart youd see why its impossible. There are no gears involved with 1:1. The input shaft has a slide collar that locks onto the output shaft that runs all the way through the case to where the driveshaft flange attaches.

All the gears are for low range and front wheel drive.







output shaft is standing vertically on the far right.

 

That's the thing, I'm not trying to change the 1:1, that ratio can stay the same. Its the low range I had thought of changing. Though after thinking about it I can understand that a smaller gear may not have the clearance to fit. But when changing to another ratio how is it that can work? in reality aren't you putting in bigger gear(s)? And I just want to iterate that I'm not arguing, just trying to understand how it works since I don't have one in front of me to play with. I just cant wrap my head around the concept that a ratio can be changed one direction but not another. Again, thanks for everybody's help.

 

I want to make sure I understand what I'm looking at correctly. Picture one is high/low, and two is 2x4/4x4? When slow moving rigs are put together are the gears in the high/low actually being changed? Or is it just the affect of stacking t-cases? I always thought the gearing was being changed.

 

So you want to remove the low range gear set (yes, picture 1) and install an over drive ratio?

Its a non syncrod straight cut collar shift, so make sure to double clutch it. Haha. High speed shifting may not actually go so well.

 

I'm thinkin about beltin up a big propeller up top on my roof rack to get more MPG.