Yet another rear end question?
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Laughing Gas
Joined: Jun 2003
Posts: 1,218
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From: Parkville, MO (KC)
Yet another rear end question?
I currently have a 240 with a Warner t-18 4 speed. Rear is the original stock 3.50 diff.
The Warner, for anyone who doesn't know, has an extremely low first gear. Something like 7.00 Usually, I start in second gear, unless I am towing or starting on a grade. My 240 runs like a top.
I have heard the following proposal posted elsewhere here. Instead of getting a overdrive tranny, put a much higher rear in. This would result in gears 1,2,3 being about where 2,3,4 are currently. And give me a fouth gear for serious cruising.
Obviously, my ability turn the tires or start with a very large load will be worse (no loss in my opinion).
But I have one concern. Is it hard on a manual transmission to run it extended periods of time in 3rd gear? The t-18 warner is a pretty tough tranny, and I have been told the anser is no. But I'm wanting to me more certain.
If I actually go for this, I am thinking of a 3.00 or 2.75 Sound reasonable?
later....
The Warner, for anyone who doesn't know, has an extremely low first gear. Something like 7.00 Usually, I start in second gear, unless I am towing or starting on a grade. My 240 runs like a top.
I have heard the following proposal posted elsewhere here. Instead of getting a overdrive tranny, put a much higher rear in. This would result in gears 1,2,3 being about where 2,3,4 are currently. And give me a fouth gear for serious cruising.
Obviously, my ability turn the tires or start with a very large load will be worse (no loss in my opinion).
But I have one concern. Is it hard on a manual transmission to run it extended periods of time in 3rd gear? The t-18 warner is a pretty tough tranny, and I have been told the anser is no. But I'm wanting to me more certain.
If I actually go for this, I am thinking of a 3.00 or 2.75 Sound reasonable?
later....
Posting Guru
Joined: Feb 2002
Posts: 1,115
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From: seattle us
Your gearing depends in large part on the tire size you run. As for running in 3rd at higher speeds, as long as engine speed is ok, shouldn't hurt the gears, but my question is why would you do this? All 4 speeds have 1:1 ratio in 4th and so you should be able to run in 4th no problem. When you come to a grade you may need to downshift, but normal running won't be a problem in 4th.
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Laughing Gas
Joined: Jun 2003
Posts: 1,218
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From: Parkville, MO (KC)
Comet -- I assume that if I go with a high enough rear, I may need to run in 3rd quite often when towing or going up hills.
Why do we have overdrive transmissions, as opposed to just regular trannys with a lower first gear, coupled with a higher rear end?
I mean, if you have a 3 speed tranny with OD, that ammounts to 4 possible ratios from the tranny.
What's the difference, then, between that and a 4 speed with a higher rear end? Obviously this ignores possible differences in the spread and spacing of the various gears. But, in principle, you should be able to get the same wheel speed at a given engine speed with either tranny.
later....
Why do we have overdrive transmissions, as opposed to just regular trannys with a lower first gear, coupled with a higher rear end?
I mean, if you have a 3 speed tranny with OD, that ammounts to 4 possible ratios from the tranny.
What's the difference, then, between that and a 4 speed with a higher rear end? Obviously this ignores possible differences in the spread and spacing of the various gears. But, in principle, you should be able to get the same wheel speed at a given engine speed with either tranny.
later....
Senior User
Joined: Jul 2003
Posts: 193
Likes: 1
From: BC, Canada
Your t-18 uses a 1:1 final gear ratio, so your effective final drive ratio is 3:50 to 1. With a 5-spd, the average OD runs about 0.75 to 0.8, which would lower your final drive to about 2.70:1
As for running in third, it makes no difference what gear you're in. As long as the tranny is in good shape it will be fine.
I think the reasoning behind OD trannies is to keep the gear ratios reasonable close to assist with driveability. As you well know going from 1st to 2nd with a t-18 takes some finesse to get a smooth gear change because of the large difference in ratio's.
There is one more possibility that you might want to consider, and that's an overdrive unit that bolts onto your tailshaft. This will in effect give you 8 forward gears, and 2 reverse. The downside of course is the need for a new driveshaft, but it opens up your options, especially if you like to haul or tow.
As for running in third, it makes no difference what gear you're in. As long as the tranny is in good shape it will be fine.
I think the reasoning behind OD trannies is to keep the gear ratios reasonable close to assist with driveability. As you well know going from 1st to 2nd with a t-18 takes some finesse to get a smooth gear change because of the large difference in ratio's.
There is one more possibility that you might want to consider, and that's an overdrive unit that bolts onto your tailshaft. This will in effect give you 8 forward gears, and 2 reverse. The downside of course is the need for a new driveshaft, but it opens up your options, especially if you like to haul or tow.
Senior User
Joined: Sep 2003
Posts: 497
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From: Houston TX
. But, in principle, you should be able to get the same wheel speed at a given engine speed with either tranny.
Yea, but the truck with the gear change will have less torque at lower speeds than the truck with the same lower gear and the o/d added.
Say you change the rear to a 3.08. Sure, your highway rpm's will drop a good bit, but you won't have near the low speed torque you did with the lower rear gear. With the o/d, you get the best of both worlds. You still have your low end torque, and low rpm's on the road. My plan for mine is to eventually change my 4 speed to an AOD with a high .67 ratio. That will drop your highway revs by a third. I considerd a manual, but most don't have as high a o/d ratio as the AOD. A whiz bang 5 or 6 speed manual-o/d would probably cost me more too. MK
Yea, but the truck with the gear change will have less torque at lower speeds than the truck with the same lower gear and the o/d added.
Say you change the rear to a 3.08. Sure, your highway rpm's will drop a good bit, but you won't have near the low speed torque you did with the lower rear gear. With the o/d, you get the best of both worlds. You still have your low end torque, and low rpm's on the road. My plan for mine is to eventually change my 4 speed to an AOD with a high .67 ratio. That will drop your highway revs by a third. I considerd a manual, but most don't have as high a o/d ratio as the AOD. A whiz bang 5 or 6 speed manual-o/d would probably cost me more too. MK
Elder User
Joined: Jul 2001
Posts: 602
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From: Huntersville
I've done the AOD swap in mine and I really do like it. I went from 3.25 ratio to a 3.70 ratio at the same time I swapped out a C-6 for the AOD. Got the best of both worlds. Great low end torque and low rpm freeway driving.
I know the AOD is'nt as strong as the C-6 but I pull very little and so far it is holding up. If it fails I will rebuild it with all the right stuff to make it mast. The AOD does have alot better shifting in my opinion. The c-6 was always wanting to stay in 3rd when I made a turn and did'nt like to kick down sometiimes. The AOD is great as far as that goes.
The OD such as a Gearvendors would be an ideal way to go if it was'nt for the high cost of the unit. At about $2500 it was way out of my range and hard to find used.
Clint
I know the AOD is'nt as strong as the C-6 but I pull very little and so far it is holding up. If it fails I will rebuild it with all the right stuff to make it mast. The AOD does have alot better shifting in my opinion. The c-6 was always wanting to stay in 3rd when I made a turn and did'nt like to kick down sometiimes. The AOD is great as far as that goes.
The OD such as a Gearvendors would be an ideal way to go if it was'nt for the high cost of the unit. At about $2500 it was way out of my range and hard to find used.
Clint
Post Fiend
Joined: Jun 2003
Posts: 8,209
Likes: 12
From: Black Hills of SD
cdherman, I see your point and I agree with you in principal. I never understood why they didn't just arrange all the ratios in the tranny and rear end to achieve the same result with the transmission running 1:1 in top gear. 1:1 ratio is allowing the power path to go directly through the transmission and is more efficient since the power path is not routed through the cluster gears with the accompanying parasitic drag of doing so.
I have 2 Fords with the AOD tranny and they have a nice low 1st gear for launching effortlessly but in overdrive they can smell a hill before you even see it. Both are 5.0 engines and when you leave it in overdrive the slightest incline requires a downshift.
I have 2 Fords with the AOD tranny and they have a nice low 1st gear for launching effortlessly but in overdrive they can smell a hill before you even see it. Both are 5.0 engines and when you leave it in overdrive the slightest incline requires a downshift.
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Laughing Gas
Joined: Jun 2003
Posts: 1,218
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From: Parkville, MO (KC)
Just out of curiosity and being a devils advocate, I have put the following together:
T-18 ratios
1st 6.32 2nd 3.09 3rd 1.69 4th 1.00
C4 with 0.69 AOD
1st 2.64 2nd 1.46 3rd 1.00 O.D. 0.69
Now, look at the RPM of the wheels in both instances at 2000 rpm engine speed, noteing the different rear diff numbers.
T-18 w/2.75 rear
1st 115 2nd 235 3rd 430 4th 727
C4 w/3.75 rear
1st 202 2nd 365 3rd 533 OD 773
With 29" diameter tires, assuming 0% slippage (OK not completely correct, but easier on the brain), the top end at 2000 RPM engine speed is:
T-18 -- 63 mph
C4 - 67 mph
People would think nothing of having an overdrive coupled to a 240 or 300 with a 3.75 rear. But everyone says I'm crazy for considering a t-18 with a 2.75 rear.
With the t-18 and a 2.75 rear I still can easily get started and through the first three gears. Its the shift to 4th that may be a problem sometimes.
The problem that I see with the Warner is that the jump from 3rd to fourth is a doozie. But the t-18 still retains significantly better towing characteristics than the C4 with OD.
What do ya think? Maybe I made an error in my calcs. Feel free to check if you wish.
cdhermann
T-18 ratios
1st 6.32 2nd 3.09 3rd 1.69 4th 1.00
C4 with 0.69 AOD
1st 2.64 2nd 1.46 3rd 1.00 O.D. 0.69
Now, look at the RPM of the wheels in both instances at 2000 rpm engine speed, noteing the different rear diff numbers.
T-18 w/2.75 rear
1st 115 2nd 235 3rd 430 4th 727
C4 w/3.75 rear
1st 202 2nd 365 3rd 533 OD 773
With 29" diameter tires, assuming 0% slippage (OK not completely correct, but easier on the brain), the top end at 2000 RPM engine speed is:
T-18 -- 63 mph
C4 - 67 mph
People would think nothing of having an overdrive coupled to a 240 or 300 with a 3.75 rear. But everyone says I'm crazy for considering a t-18 with a 2.75 rear.
With the t-18 and a 2.75 rear I still can easily get started and through the first three gears. Its the shift to 4th that may be a problem sometimes.
The problem that I see with the Warner is that the jump from 3rd to fourth is a doozie. But the t-18 still retains significantly better towing characteristics than the C4 with OD.
What do ya think? Maybe I made an error in my calcs. Feel free to check if you wish.
cdhermann
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Post Fiend
Joined: Jun 2003
Posts: 8,209
Likes: 12
From: Black Hills of SD
My only concern would go back to my opinion in my last post. If you have to run in third gear a lot then the power path is through the cluster gear which looses a little efficiency, but also may cause some premature wear since the transmission was designed to run direct at highway speeds. The T-18 may be tough enough to take it and maybe it wouldn't be a problem, just a concern. I guess Ford did that with its overdrive tranny of the early 80s. They just took a car transmission and made third gear into overdrive and flipped the shifter lever upside down so 4th(direct) became third and the reworked third gear became overdrive. Most of those trannys only came behind 6 bangers in F-100 pickups. I may have seen them behind a 302 but I can't remember.