Manual Transmission Compatibility

[Brain75]

On 01/03 I went to the junkyard last and saw an F4 axle.. I considered getting it (solely for the locker and Colorado roads, but decided to at least run the numbers and see if I actually wanted that or something else)... so on the way home I ran exactly 75, set my cruise control and noted the RPMs... took a picture a mile later (when it was safe, long stretch of I 76, no cars around me etc...)

looks like slightly under 75 mph, slightly over 3k rpm.
stock 215/70R14 tires fairly new.
all other aspects pretty much stock.

back when I was doing body and bondo I noted the rear diff leaked so I put a new gasket in it, when I had it open I noted the stamp and quick checked the condition, took a pic so I would have the reference in case I decided I wanted to do something later.

The 3 is so lightly stamped as to be unreadable,but it says 3.45 and then later 11 38 (which is the pinion and ring gear teeth count), and ford oval... so tag matches gears matches door tag.

 

[Brain75]

I can get a junkyard axle (whole axle), or even a trans for less than what jegs/summit/yukon wants for gears, and I have never done a gear swap and understand they are a little technical. I have the skill but for less money I am inclined to just get a different axle / trans... I have started dreaming a little about this and considered the T56 (6 speed double overdrive) or 6060 (modernized T56), but those are big giant trannys and require a body lift or hammering out the trans tunnel bigger. The T56 has been done by a member here.
I re-read and re-read, and I think I get it now, he is not wanting to take a wide trans and convert it to narrow, he wants to take a narrow trans and make it wide - opposite of what I read the first time, right?

 

[sgtsandman]

On 01/03 I went to the junkyard last and saw an F4 axle.. I considered getting it (solely for the locker and Colorado roads, but decided to at least run the numbers and see if I actually wanted that or something else)... so on the way home I ran exactly 75, set my cruise control and noted the RPMs... took a picture a mile later (when it was safe, long stretch of I 76, no cars around me etc...)
View attachment 122725

looks like slightly under 75 mph, slightly over 3k rpm.
stock 215/70R14 tires fairly new.
all other aspects pretty much stock.

back when I was doing body and bondo I noted the rear diff leaked so I put a new gasket in it, when I had it open I noted the stamp and quick checked the condition, took a pic so I would have the reference in case I decided I wanted to do something later.

View attachment 122726

The 3 is so lightly stamped as to be unreadable,but it says 3.45 and then later 11 38 (which is the pinion and ring gear teeth count), and ford oval... so tag matches gears matches door tag.

Maybe bigger tires are the answer? The 1998 had 225/70R14s, if you want to stay more or less OEM. Later Rangers had 225/70R15 tires on RWD models. Of course, you'll need to fiddle with the speedometer gear to correct the speed indication if you do that. But it would help drop the RPM. Looking at the chart in the tech section, it would only be about a couple hundred RPM. About 300 if you went with a 235/75R15 but it would be a drop.

 

[sgtsandman]

I can get a junkyard axle (whole axle), or even a trans for less than what jegs/summit/yukon wants for gears, and I have never done a gear swap and understand they are a little technical. I have the skill but for less money I am inclined to just get a different axle / trans... I have started dreaming a little about this and considered the T56 (6 speed double overdrive) of 6060 (modernized T56), but those are big giant trannys and require a body lift or hammering out the trans tunnel bigger. The T56 has been done by a member here.
I re-read and re-read, and I think I get it now, he is not wanting to take a wide trans and convert it to narrow, he wants to take a narrow trans and make it wide - opposite of what I read the first time, right?

Changing gears is a frustrating pain in the butt. Not hard, just a pain in the butt getting the pinion and ring gear interface right and setting the back lash correctly. Doubly so if you have a 4X4.

 

[Brain75]

I'm a short guy and my shorty's tend to be short too... I would have to add running bars (nerf bars) or carry a step ladder if I went bigger... I would like the wheel wells slightly filled out more, but I have really really considered it. I have 16" wheels for when the shoes wear out and it is time to buy new, have not picked an exact size yet but have run the math...

Part of why I didn't start a thread is I don't have all the details, I "think" I have a 20 tooth speedo gear and with the tire size I am considering would need a 19.24 by calculation so 19 it would be... but I haven't gone out into the cold and looked to see if I do have a 20 tooth.

 

[sgtsandman]

I'm a short guy and my shorty's tend to be short too... I would have to add running bars (nerf bars) or carry a step ladder if I went bigger... I would like the wheel wells slightly filled out more, but I have really really considered it. I have 16" wheels for when the shoes wear out and it is time to buy new, have not picked an exact size yet but have run the math...

Part of why I didn't start a thread is I don't have all the details, I "think" I have a 20 tooth speedo gear and with the tire size I am considering would need a 19.24 by calculation so 19 it would be... but I haven't gone out into the cold and looked to see if I do have a 20 tooth.

This chart will help you some in that decision: https://www.therangerstation.com/how-to_pages/gear_ratio_tire_size_chart.shtml

And a tire size calculator like this one will help make the chart make sense: https://www.1010tires.com/Tools/Tire-Size-Calculator

 

[Brain75]

I actually already have worked (and superceded) that chart and referred many a 1 post user to the 1010tire page...

that chart is for 65mph and all the diff ratios are basically chevy ratios - close but not what our rigs carry... I looked up the formula and used it to build an interactive xl spreadsheet - was considering putting it into javascript and submitting it as a tech article for Jim..

at 75mph, with a 0.79 (M5OD) overdrive this is what the chart should look like

which really really confuses me why I am seeing 3000+ since the math says I should be 2660 (which is close enough to 2500 in my books to just live with)

 

[Curious Hound]

I guess I am still confused, if making a crawler, don't you want as wide a range (or as low a 1st gear) as possible - you said in this quote every bit helps, every bit lower right - he would want the lower gear of the wider transmission

If I understand your "wider/narrower" terminology, the 3.0l version is wider and better for rock crawling. I may not have been clear. But I was trying to say that is the better set of gears for Rock crawling and slow technical trail riding. The 3.72 ratio in first gear affords better mechanical leverage for the engine. Top end is the same in both transmissions. So, no difference at highway speeds.

 

[Brain75]

a "wide" transmission has a wider range/gap between 1st and last gear, a "narrow" trans has less difference. So for example the NP435 with it's gawd awfully granny 1st (6.68:1) and a top gear (4th) of 1:1 is an (ultra) wide transmission with a width of 5.68.... or at least that is the terminology as I was taught it.

Crawlers, fuel economy guys, high speed circle guys want wide, bout the only people who want narrow is 1/4 mile drag... or at least that was the fireside conversation consensus I've heard over the years.


I figured it out, I read the first post wrong but he wants to take the wider (3.0L std) gears and put them into the narrow (4.0L std) case to get the bellhousing.. which makes sense. But like someone else said, just find a trans from a 2.9L (same gears as the 3.0, and same pattern as the 4.0), whole lot less work unless you just can not for the life of you find a 2.9L trans (I can walk you to a couple in my yard but I think they are both 4x4 - don't know what tail he needs.)

 

[scotts90ranger]

If you were closer I'd swap you axles, if I remember right my ranger trailer has 3.08 gears, either that or the '92 that I have around does, don't remember...

The easiest way to lower rpm is to increase tire diameter, I think I've tried 4 diameters on my '97 before I was happy with the drivability and settled where at 72mph I'm at 3000rpm and with all the local speed limits topping at 65mph that's perfect for me...

 

[4x4junkie]

The 2.9L 4x4 M5OD transmissions are quite rare (AFAIK, only the last half of 1990-model production Supercab Rangers got them) so it's very possible to not be able to find one. Swapping the gears from a 2.3L or 3.0L unit into a 4.0L case then becomes the next practical option (especially for those folks that don't want to deal with shipping a trans across the country).

2.9L 2WD M5OD units are a bit more common (I think all of '89 & '90 had 'em).


I don't suggest changing a 3.45:1 axle with 215/70R14 tires for something taller. You're not likely to see an MPG increase with a 2.9L (the 2.9 has a short stroke and likes to spin fast), but you will kill what little responsiveness your truck has while accelerating. If it's better MPG you want, slowing down from 75 to 65 is good for a ~15% increase. Another ~10% on top of that can be had at 55MPH (there's valid science behind those old 55 speed limits we had that began in the mid-1970s). Of course you won't get to where you're going quite as fast, so you'll need to prioritize your time vs. your wallet.

Also, it's very possible your tach is off like the one on my '90 is. Mine seems to show about 10-15% faster that what is actual, so might be why the discrepancy you're seeing in your numbers (the tach on my '94 4.0L seems closer to what it should be).