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Gear Ratio
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New User
Joined: Aug 2009
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From: Southwest Oklahoma
Gear Ratio
Hi yall, I finally got the Dana rear end swapped out to a 71 model 9".Had the drive shaft rebuilt Redid the brakes ,wheel cylinders, new drums ,master cylinder. Any way I took it for a spin to check things out the brakes are fine ,But the rear end is too high for the truck. Granny grear or 1st is like first on a three speed, Second is almost too high to starte off in. I drove it around for a while.For highway speed it would work, but I need mine for pulling. The axle was pulled out of a 71 ford with a 4 speed which is what I was wanting since my 65 has one I thought it would be low geared I have the tag cleaned up,here's the numbers. WDM-AG 1AD 3 00 9 314A . Would someone tell me what this tranlate to .And what is a good gear ratio for pulling a car hauler loaded with about 3500-4000 lbs. Fifty mph+ Don't know for sure which trans. I have but reverse is over and up. I sure apreciate all the help you have given me.
Fleet Owner
Joined: Jun 1999
Posts: 24,552
Likes: 74
From: Creedmoor, North Carolina
Roundman, What we need here is a little edumacation. 
1- All transmissions that are not overdrive have a 1to1 output ratio 3spd or 10spd.
2- What's on either end of the tranny has to be factored next. 240 6 "little horse", has to have big numbers in the rear like 3.75 or 4.11 to get power to the ground.
A 390 "big horse" can do the same job with lower gears like 3.00.
Put a 3.00 behind a 240 and it's wimpy, put 375 or 4.11 behind a FE and it is screaming.
So matching the front & rear is most critical for a balanced truck.
I disagree with my friend Dave, a well built FE with a 3.00 rear will pull your tow just fine, example shown:
John
1- All transmissions that are not overdrive have a 1to1 output ratio 3spd or 10spd.
2- What's on either end of the tranny has to be factored next. 240 6 "little horse", has to have big numbers in the rear like 3.75 or 4.11 to get power to the ground.
A 390 "big horse" can do the same job with lower gears like 3.00.
Put a 3.00 behind a 240 and it's wimpy, put 375 or 4.11 behind a FE and it is screaming.
So matching the front & rear is most critical for a balanced truck.
I disagree with my friend Dave, a well built FE with a 3.00 rear will pull your tow just fine, example shown:
John
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Joined: Sep 2007
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Im glad you brought this up.. Im having a gear change coming my way... My pickup came with a 240 from the factory (po installed a 390-- it was junk when I got it) and 3.70 rear end... so I installed a 5.0 budget pavement pounder.. with a moderate cam.. and a c6 with TCI goodies for a stout build... but buddy at 55 with 28 inch tires Im pushin 3 grand or better. and dont even think about running the Interstate.. shish. thought I was gonna blow up 4 grand or so goin 70.. So looking to drop to 3.25 to 3.50.. I contemplated going 3.00.. but I dont want to loose my fun around town driving.
What do you guys recommend?? Id like to be able to run 70 the 2 or 3 times I go on trips.. but I dont want to lose my geddyup.
What do you guys recommend?? Id like to be able to run 70 the 2 or 3 times I go on trips.. but I dont want to lose my geddyup.
Post Fiend
Joined: Nov 2000
Posts: 6,060
Likes: 7
From: Glendale Arizona
John,
3.00 will tow fine behind a C6. But the poster has a Manual so second gear start out is too high to get everything going and will heat up the clutch.
For your 302 it will not like 3.00 gears as it needs rpm to make fun. 3.25 or even 3.50s will work. Or add an OD transmission and leave the 3.70s.
I have 2.75s in "Christine" and she never knew there was a trailer behind her, and still scooted real nice. However it is also a 472 cubic inch big bock.
Garbz
3.00 will tow fine behind a C6. But the poster has a Manual so second gear start out is too high to get everything going and will heat up the clutch.
For your 302 it will not like 3.00 gears as it needs rpm to make fun. 3.25 or even 3.50s will work. Or add an OD transmission and leave the 3.70s.
I have 2.75s in "Christine" and she never knew there was a trailer behind her, and still scooted real nice. However it is also a 472 cubic inch big bock.
Garbz
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New User
Joined: Aug 2009
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From: Southwest Oklahoma
Thanks for the help Guys, I should have mentioned my engine and tires size the engine is a 240 6, tires are 235 75R 15s Wimpys a good name for it. It sure has no pulling power except in 1st with an empty trailer. I"ll starte looking for another rear end to swap pumpkins or buy some new gears if I can't find one . Thanks agin guys
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Senior User
Joined: Jan 2010
Posts: 398
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From: AR
John I know we are supposed to be talking about old Fords here but I thought while we were educating I'd mention it.
I personally have never understood why manufacturers don't use a 1:1 top gear & faster axle ratio as opposed to a trans with overdrive & slower axle ratio. It would seem to me some transmissions could have less parasitic loss if it was "straight through" in top gear. Wouldn't it be a little more efficient to not have as many parts turning in the trans?
I understand if they want to have 5 or 6 speed transmissions but top gear could be 1:1 with very slow first gear & use something like a 2.20 or 2.50 axle ratio.
Does anyone get what I'm saying?
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Post Fiend
Joined: Nov 2000
Posts: 6,060
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From: Glendale Arizona
Any transmission with less than 1 to 1 final ratio is an overdrive. Be it an automatic or a Standard.
Manufacturers do not use 1 to 1 and 2.50 gears due to long acceleration times to 60 MPH, unless there is a good bit of torque available to overcome the gears. No use also running at 200 MPH either to really use the 2.50 and 1 to 1 ratio.
Garbz
The overdrive last gear is to bring the engine RPMs down to acceptable level and meet CAFE mileage standards.
Manufacturers do not use 1 to 1 and 2.50 gears due to long acceleration times to 60 MPH, unless there is a good bit of torque available to overcome the gears. No use also running at 200 MPH either to really use the 2.50 and 1 to 1 ratio.
Garbz
The overdrive last gear is to bring the engine RPMs down to acceptable level and meet CAFE mileage standards.
Senior User
Joined: Jan 2010
Posts: 398
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From: AR
Any transmission with less than 1 to 1 final ratio is an overdrive. Be it an automatic or a Standard.
Manufacturers do not use 1 to 1 and 2.50 gears due to long acceleration times to 60 MPH, unless there is a good bit of torque available to overcome the gears. No use also running at 200 MPH either to really use the 2.50 and 1 to 1 ratio.
Garbz
The overdrive last gear is to bring the engine RPMs down to acceptable level and meet CAFE mileage standards.
Manufacturers do not use 1 to 1 and 2.50 gears due to long acceleration times to 60 MPH, unless there is a good bit of torque available to overcome the gears. No use also running at 200 MPH either to really use the 2.50 and 1 to 1 ratio.
Garbz
The overdrive last gear is to bring the engine RPMs down to acceptable level and meet CAFE mileage standards.
Sorry I can't explain what I'm meaning but you completely missed what I was trying to say.
Just a bad example so these numbers aren't accurate, but I'm saying 5:1 first gear,4:1 second, 3:1 third, 2:1 fourth, 1:1 fifth. Pair this with a 2.50 rear diff & you would have good accel, cruise at a good low rpm & you would be straight through the trans.
You mentioned running 2.75 in Christine so whats with the 200 mph comment about 2.50 gears? If someone said they had a 3.50 rear with a .7 overdrive you wouldn't accuse them of trying to run 200mph even though they would both give similar top gear cruise rpm.
You could make two vehicles both cruise @ 1800 rpm, one with overdrive and one straight through, you could make them both accelerate identical with the proper trans ratios so why not turn less stuff in the trans?
Fleet Owner
Joined: Jun 1999
Posts: 24,552
Likes: 74
From: Creedmoor, North Carolina
Sorry I can't explain what I'm meaning but you completely missed what I was trying to say.
Just a bad example so these numbers aren't accurate, but I'm saying 5:1 first gear,4:1 second, 3:1 third, 2:1 fourth, 1:1 fifth. Pair this with a 2.50 rear diff & you would have good accel, cruise at a good low rpm & you would be straight through the trans.
You mentioned running 2.75 in Christine so whats with the 200 mph comment about 2.50 gears? If someone said they had a 3.50 rear with a .7 overdrive you wouldn't accuse them of trying to run 200mph even though they would both give similar top gear cruise rpm.
You could make two vehicles both cruise @ 1800 rpm, one with overdrive and one straight through, you could make them both accelerate identical with the proper trans ratios so why not turn less stuff in the trans?
Just a bad example so these numbers aren't accurate, but I'm saying 5:1 first gear,4:1 second, 3:1 third, 2:1 fourth, 1:1 fifth. Pair this with a 2.50 rear diff & you would have good accel, cruise at a good low rpm & you would be straight through the trans.
You mentioned running 2.75 in Christine so whats with the 200 mph comment about 2.50 gears? If someone said they had a 3.50 rear with a .7 overdrive you wouldn't accuse them of trying to run 200mph even though they would both give similar top gear cruise rpm.
You could make two vehicles both cruise @ 1800 rpm, one with overdrive and one straight through, you could make them both accelerate identical with the proper trans ratios so why not turn less stuff in the trans?
No woulda, coulda, shoulda, maybe, might be, allowed.
Stay in school.
John
Senior User
Joined: Jan 2010
Posts: 398
Likes: 3
From: AR
All I was saying to you John, is that there are vehicles made now that don't use a 1:1 trans ratio. Just makes no sense to me. I'm sure they have a reason for it just need an engineer to tell me why...lol
I edited the above posts now that I'm awake. I know what you were saying John was that it didn't matter if you had a 3spd manual or a granny 4spd, the top gear was 1:1.
I edited the above posts now that I'm awake. I know what you were saying John was that it didn't matter if you had a 3spd manual or a granny 4spd, the top gear was 1:1.
Senior User
Joined: Jan 2010
Posts: 398
Likes: 3
From: AR
Are you certified or what do you teach, John?
I used to be a ASE Master tech, of course all that meant was I could pass tests in a classroom....lol
By the way not being sarcastic if it sounds that way.
Senior User
Joined: Jan 2010
Posts: 398
Likes: 3
From: AR
The six speed auto in the new F150s have
4.17:1 first
2.34:1 second
1.52:1 third
1.14:1 fourth
0.86:1 fifth
0.69:1 sixth
The new GM trucks are very similar ratios with no 1:1.
Just strange to me that there is no "drive" or straight through the trans.
Post Fiend
Joined: Nov 2000
Posts: 6,060
Likes: 7
From: Glendale Arizona
.86 for fifth is an overdrive.
.69 is an overdrive in sixth.
the 1.14 is just about above the straight through ratio of 1 to 1
They use the overdrives to keep the RPM down on the engine in final drives.
1 to 1 is always the same if your gears are 2.5 or 4.11. This is along with tire diameter determines the given RPM of the engine. The factory strives to keep an engine in the meat and potatoes or sweet spot.
Straight through gears are 1 to 1. The crank and transmission are turning at the same rate.
Everyone wants overdrives today to keep engine RPM down at highway speed to create better fuel economy.
There is a little thing called the federal government and CAFE standards for fuel mileage that prohibits the use of 1 to 1 transmissions in today vehicles.
Garbz
.69 is an overdrive in sixth.
the 1.14 is just about above the straight through ratio of 1 to 1
They use the overdrives to keep the RPM down on the engine in final drives.
1 to 1 is always the same if your gears are 2.5 or 4.11. This is along with tire diameter determines the given RPM of the engine. The factory strives to keep an engine in the meat and potatoes or sweet spot.
Straight through gears are 1 to 1. The crank and transmission are turning at the same rate.
Everyone wants overdrives today to keep engine RPM down at highway speed to create better fuel economy.
There is a little thing called the federal government and CAFE standards for fuel mileage that prohibits the use of 1 to 1 transmissions in today vehicles.
Garbz