Topic: 91 Pickup 4x4 High RPM/MPH Ratio

[head2847]

Hi all!

Someone at Toyotanation suggested I post a question on this forum as it might have a little better luck.  I hope that's the case so here goes: (Quoted)

Hi all,

First post here and hoping I'll get someone pointing me in the right direction, even if it's to another thread I just couldn't find (and I've searched everywhere for the past week).

I've recently come into a rather cheap but somewhat neglected '91 4x4 pickup w/ a 3.0. Among the other issues I'll be working on my own to troubleshoot (outside of the realm of this post/thread) one of the most mysterious to me has to be the incredibly tall gear ratio's I seem to have. As an example, to get to 65mph on the hwy I'm doing 4krpm exactly, in 5th.

I first checked the range selector to make damn sure I wasn't in Lo or 4x4 at all (wouldn't want my first post to be that ridiculous) which I wasn't. Then I looked up the tranny/axle codes (waste of time IMO as I knew they couldn't be right at that speed/rpm). They show the R150F and the 8" 4 pin 4.1:1. Since I suspected that to have been changed I jacked up one side of the rear end, taped the tire, taped the rear fender, taped the driveshaft, and spun the tire 10 times counting the driveshaft revolutions; which were 20.5 giving the correct 4.1:1 rear end ratio.

So...wtf!

If this is right, and given my 30x10.50x15 tires and accurate tach readings (could be askew but truck just barely stalls at appx 400rpm so not that much) the gear ratios through the tranny are (if my math isn't off completely);

Speed at 4krpm

1st = ~14mph
2nd = ~26mph
3rd = ~37mph
4th = ~55mph
5th = 65mph

1st = ~6.2198:1 should be 3.83 (+162.4%)
2nd = ~3.3491:1 should be 2.062 (+162.4%)
3rd = ~2.3534:1 should be 1.436 (+163.8%)
4th = ~1.5832:1 should be 1 (+158.3%)
5th = 1.3396:1 should be .838 (+160%)

But the math did give me some hope in that each gear seems to be about 160% taller than it should be.

Now to the question, wtf would give that universal reduction?

TL;DR

R150F tranny seems to be geared at about 160% taller than stock. What could do that?


Thanks in advance guys!

Mike

-------------- 2nd Post -----------------

So as I haven't really heard any responses nor found any other information I took it on myself to verify the only thing that could have been incorrect in my calculations, my truck's tach; as I was already using a GPS speedo app on my phone for the MPH. After grabbing a cheap multimeter/tach combo from harbor freight I can confirm my suspicions as the internal tach matches it.

Also, after looking around on the internet for a while the only thing I can possibly think of that'd create these problems would be that the Input shaft to Layshaft gear ratio has been tampered with.

My follow up question, as it seems I'm the only one interested, is if something like that is common for these transmissions, or even possible? I'd assume that the gears would have to be pressed on and off, no simple task, and likely not even close to as cheap as simply swapping the axle gearing.

Lastly, it's my understanding that the 4wd t-case is 1:1 input/output when in 2hi and 4lo with no reduction. That said, and if my understanding's correct, is there a mod that would change your t-case gearing even when not in 4lo?

I'm just spitting out questions hoping to get some input.

Thanks guys!

Mike

[ryantowry_81]

 

that is a strange problem you have there.

here is a good gear ratio calculator i use alot:
http://www.grimmjeeper.com/gears.html

really helps put the numbers you posted up above into a more comparative format.

personally i would pull your rear third and count the teeth on the pinion and ring gear. any time you have a consistent ratio difference like you have there it would lead to one torque multiplier being different than the control (the stock setup in this situation). I could see if someone put 5.29's or 5.71's it would be close to the amount off you are seeing.

glad to hear you checked the MPH and RPM with an external source too to eliminate that cause.

also, it cant be in low range or you would not get to 65 even in 5th at 4k rpm, the low range would put it over 200% off.

[HogCanyonHopper]

    thats some serious tech there 

[Dirt Mover]

I agree with pulling the 3rd member to see what you have. Huge difference between 4.11s and 5.29s with 30" tires.

[head2847 [OP]]

Thanks everyone for the ideas! 

If time permits I'll pull the 3rd member apart tomorrow night and count those gears for reference. 

Just one thing though; according to ryantowry_81's calculator, and my own math, the gearset I'd have to have to get the numbers I'm seeing would have to be in the realm of 6.6:1 which to my knowledge, feel free to add to that knowledge/correct it, isn't available.  So, assuming when I pull the cover and count the teeth I've got the 4.11:1 gears, is there any other cause anyone can think of that would give me the reduction I'm seeing?

Just trying to move the discussion along while I wait to pull apart the axle.

Thanks again guys!

Mike

[ryantowry_81]

remember the calculator assumes a actual diameter of 30" for a 30" tire, this is usually not the case. actual diameter is usually smaller than listed and that can have a big effect on your rpm numbers. if your actual diameter was 29" a 5.71 gear ratio would put you at 72 at 4000 rpm, or 27" with 5.29's would also put you there so that is not out of the realm of possibilities.

only other factor that i can think of is a slipping clutch but i dont think your rpm range would be off so uniformly.

[jssgbsn]

Pretty sure BigMike put this up a few weeks ago.

http://www.marlincrawler.com/differential/ring-pinion

I plugged in 5.71:1, 29" Tires, 4th Gear (1.00:1)-(4th gear should almost always be 1.0:1), when adding.....

 Speed 55mph - RPM came out to 3640

 RPM 4000 - MPH came out to 60

I don't believe you can get gears shorter than 5.71:1, I could be wrong about that though.  I would double check your actual tire height when your checking your tooth count.  If they are used, going bald, 30" tires, they could be 'smaller' than 29" which might be contributing to what you have going on and the number would prob get closer.

Maybe BigMike will eventually add a few smaller tire diameters to the drop down.


I used the grimjeeper calc and put in the following

Step 1: Gear Ratios
1st =  3.83
2nd = 2.062
3rd = 1.436
4th = 1
5th = .838

Step 2:Transfer Ratios
Hi Ratio
1.00
Low Ratio
2.28

Step 3: Underdrive
Blank

Step 4: Axle Ratio
5.71

Step 5: Tire Size
27"

Result Tables reflect at 4000rpm you should see the following speeds by gear.

1st - 15
2nd - 27
3rd - 39
4th - 56
5th - 67

Result Tables reflect at 55mph you should see the following rpm (where possible of course)

1st - 14968
2nd - 8051
3rd - 5612
4th - 3908
5th - 3275



That looks pretty close to what your describing.  So, I am guessing 5.71:1 with really small tires since you ruled out the possibility of the Dash Guages being incorrect.  Using 4th gear for your calculations and research will make things easier because it should be a 1.00:1 ratio across all transmissions (5Speed anyway).

[head2847 [OP]]

@jssgbsn

I never even considered my tires in this, good idea!  They're about 5 months/4,000 miles old BFG AT's but are most definitely worth a quick check with my measuring tape.  I've got the time/motivation to tear into the axle tonight so I'll be able to confirm shortly, and all fingers/toes are crossed hoping the gearing isn't stock (as I've got no ideas to the source of the problem otherwise).

Thanks everyone and updates tonight or tomorrow morning!

Mike

[jssgbsn]

im far from an expert.  all i did was play with the numbers to try and get them to match what your reporting.

Hopefully its short gears, small tires and some minor gauge discrepancies

good luck!

[head2847 [OP]]

Whelp, I hate to have to admit this, would rather just slink off and only admit my idiocy to myself, but too many have tried to give me guidance to just not say what the issue was.

It turns out that I've been using my external tach in 4cyl mode; shoulda RTFM....

Anywho, needless to say my numbers are skewed and after a quick calculation it seems that there isn't an issue with gearing in my truck at all, just that the tachometer pot within the gauge cluster needs to be adjusted to read correctly.  I think next time I'm measuring w/ new gear I'll either RTFM or try a control reading with another vehicle; either of which woulda' clued me in right away.

So, problem solved!

Thanks for the help guys, it's really appreciated!

Mike

[HogCanyonHopper]

 



                                    RTFM

[ryantowry_81]

well at least you figured that out before tearing the rear axle apart, and it is a cheap fix.

[jssgbsn]

I had a hard time believing you would have 5.71's anyway.  Especially with since you woulda had to have 27" tires to reach those numbers.

Glad you figured it out!