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RPM at a Given Speed?

1985xjlaredo

NAXJA Forum User
Location
Kansas City
Here is the equation I am working with: MPH x Total Gear x 366
Tire Diameter
= RPM at given Speed
I am turning about 4000 at 60 mph. My MPH is at 60 and my tire diameter is 28.28" (225/75/15). The only variable is my gear which I am trying to figure out. But something seems a miss. Am I doing something wrong? Does total gear mean my final ratio?
Using the above formula I should be turning 3250 with a 4.56 gear. It doesn't really feel like I have a 4.56 but that seems what it is when Im turnin that high RPM on the Highway. Im going to figure what gear I have by getting close to the 4000 RPM range by changing my gear ratio variable. Any other Ideas?
:cheers:
 
Well it should be a 1:1 ratio It is 3rd in the TC904. I figured out that for what RPM I am running at 60MPH the rear end gear ratio should be 5.67. I thought that the 35 could only go up to 4.88?
 
1985xjlaredo said:
Here is the equation I am working with: MPH x Total Gear x 366
Tire Diameter
= RPM at given Speed
I am turning about 4000 at 60 mph. My MPH is at 60 and my tire diameter is 28.28" (225/75/15). The only variable is my gear which I am trying to figure out. But something seems a miss. Am I doing something wrong? Does total gear mean my final ratio?
Using the above formula I should be turning 3250 with a 4.56 gear. It doesn't really feel like I have a 4.56 but that seems what it is when Im turnin that high RPM on the Highway. Im going to figure what gear I have by getting close to the 4000 RPM range by changing my gear ratio variable. Any other Ideas?
:cheers:

"Final drive" mean axle ratio and overdrive ratio combined. Are you turning 4000 in "top gear" (overdrive) or in "direct drive" (on gear lower - fourth on a five-speed, or third in on AW4.)

Also, it would help to know more about your vehicle - most automatic XJs left the factory with 3.55, and most manuals with 3.07 (assuming a 4.0L engine, anyhow. Four-cylinder gearing was a bit deeper.)
 
This is a 1985 XJ Laredo, Towing Package, Stock 225/75/15 Tires, 2.8L Punched out to 3.1 Mild Comp Cam, Roller rockers, TC904 3 speed With the Lock up converter, D30 Front D35 Rear, NP229 T case.
 
Blaine B. said:
4000 at 60?

I'm around 2000 at 65-70mph.
Do you have a 4.0 and a overdirve?
 
1985xjlaredo said:
This is a 1985 XJ Laredo, Towing Package, Stock 225/75/15 Tires, 2.8L Punched out to 3.1 Mild Comp Cam, Roller rockers, TC904 3 speed With the Lock up converter, D30 Front D35 Rear, NP229 T case.

I take it you've not found a tag on either differential, retained by a cover screw or two? The earlier Dana axles had the ratio on the tags "in the clear," at one end or the other of the bottom row (I don't recall which.) The tag proper is about 3/4" x 3" or so.
 
5-90 said:
I take it you've not found a tag on either differential, retained by a cover screw or two? The earlier Dana axles had the ratio on the tags "in the clear," at one end or the other of the bottom row (I don't recall which.) The tag proper is about 3/4" x 3" or so.
No no tags left on mine. I just had the covers off and didn't see anything stamed on the cover anywhere either. Guess I'll have to pull them off again and count the teeth. Is there any way this thing could be geared to 5.67?
 
Last edited:
1985xjlaredo said:
No no tags left on mine. I just had the covers off and didn't see anything stamed on the cover anywhere either. Guess I'll have to pull them off again and count the teeth. Is there any way this thing could be geared to 5.67?

You won't find anything for the ratio stamped on the cover, unless you stamp it there. Counting the teeth will be your best bet - mark one with a Sharpie pen first, so you don't overcount.

Geared to 5.67? I doubt it highly. 4.10 gears were rare, and I've heard tell that some rigs might have come with 4.55/4.56 from the factory, but nothing higher than that, I'm sure.
 
Since you have a three speed I would think that it is not shifting into third gear . Otherwise I see no reason that it would be running at 4000 rpms at 60mph ! ! Correct me if I am wrong .
 
REPOXP777 said:
Since you have a three speed I would think that it is not shifting into third gear . Otherwise I see no reason that it would be running at 4000 rpms at 60mph ! ! Correct me if I am wrong .

Either that, or the torque converter clutch is slipping. And so is the torque converter.

Check this - accelerate smoothly to 60MPH. You should feel two "actual" shifts - going into second, and then third. In third, you should feel a "soft" shift at cruising speed - that would be your converter clutch locking up.
 
5-90 said:
You won't find anything for the ratio stamped on the cover, unless you stamp it there. Counting the teeth will be your best bet - mark one with a Sharpie pen first, so you don't overcount.

Geared to 5.67? I doubt it highly. 4.10 gears were rare, and I've heard tell that some rigs might have come with 4.55/4.56 from the factory, but nothing higher than that, I'm sure.
Yea I didn't think that it would be stamped on there. I thought that only the D44's came with the higher gearing but who knows. I can't find anything aftermarket that is higher than 4.88 for a D35.
REPOXP777 said:
Since you have a three speed I would think that it is not shifting into third gear . Otherwise I see no reason that it would be running at 4000 rpms at 60mph ! ! Correct me if I am wrong .
That is what I thought at first but It is defenatly shifting 2 times. I have put it in first then second and then Into third and felt it shift. I dont think that the lockup is locking all the time some times I get about 250 RPM's difference in this little problem Im having.
5-90 said:
Either that, or the torque converter clutch is slipping. And so is the torque converter.

Check this - accelerate smoothly to 60MPH. You should feel two "actual" shifts - going into second, and then third. In third, you should feel a "soft" shift at cruising speed - that would be your converter clutch locking up.
Yes this seems to be the case. Is there anything that I can try to do to get it to stop locking up. THe PO had it rebuilt about 20,000 miles ago but he didnt say anything about a TQ converter and it wasnt done when the Tcase and engine were done.
Is there a band adjustment for 3rd in a TC904 and how hard is a TQ converter replacement. Do I drop the tranny or pull the motor?
 
1985xjlaredo said:
Yes this seems to be the case. Is there anything that I can try to do to get it to stop locking up. THe PO had it rebuilt about 20,000 miles ago but he didnt say anything about a TQ converter and it wasnt done when the Tcase and engine were done.
Is there a band adjustment for 3rd in a TC904 and how hard is a TQ converter replacement. Do I drop the tranny or pull the motor?

I'm fairly sure there is a band adjustment, but I don't recall what it is offhand. Torque specs (since that's how it's "adjusted") should be on my site, pulled from the AMC FSM.

Torque converters aren't rebuildable by the home shop - you first have to open the case, and you'll need a brazing furnace to put it back together. You usually pull the transmission to change the TC.

You don't want it to "stop" locking up - you want it to start. The idea behind the torque converter clutch is to get rid of TC "slip" at cruise, and that both increases fuel efficiency and reduces transmission internal heat - both beneficial. Check again for that "soft shift" once you get into cruising speeds - the TC should be able to lock up in second, as I recall, and damn sure should lock up in third at cruise. If your torque converter isn't locking up in cruise, that will affect your final drive ratio (no, I can't tell you how much - it varies, and is difficult to predict.)
 
Yes start locking up is what I ment to say. Is a new Tq converter going to be just popping out the tranny and pulling it off and putting a new one on?
 
Is there a significant rpm change as it shifts from 2nd to 3rd, if not, you may be interpreting TQ lock up as a gear shift to third, in which case the problem is no shift to third gear, which would explain the high rpms. Note that TQ lock up will drop the rpms a little.
 
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