3 to 4 speed
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Elder User
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From: Fort Rucker, AL
3 to 4 speed
I've got a 1969 F-100, 390, 3:25 rear end, and 3 on the tree. My RPM's are 2500 at 55 mph. I'm guessing that a 4 speed manual would help my RPMS's and maybe give me more on the top end. I think the rear end is about right to give me good torque but I'd like to get a little more out of my 390. Know what I mean? Any words from the experts?
Postmaster
Joined: Apr 2002
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From: Coal country
Well, it depends on what kind of 4 speed you mean. Most transmissions have 1:1 output ratios regardless of how many gears they have.
The exception is those transmissions designated "overdrive" where the final gear ratio is usually between .7:1 to .8:1.
So, a regular 4 speed won't get you any advantage in gas mileage. You need to find an overdrive transmission.
Others can help you further, and you may want to try asking this question in the FTE transmisison forum.
The exception is those transmissions designated "overdrive" where the final gear ratio is usually between .7:1 to .8:1.
So, a regular 4 speed won't get you any advantage in gas mileage. You need to find an overdrive transmission.
Others can help you further, and you may want to try asking this question in the FTE transmisison forum.
Postmaster
Joined: Apr 2002
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From: Coal country
However, comma, those 2500 RPMs at 55 mph don't sound quite right for a 3.25 rear axle.
Are you sure you have a 3.25? What does your Warranty Plate say you should have, and have you checked it to be sure?
Are you sure you have a 3.25? What does your Warranty Plate say you should have, and have you checked it to be sure?
Postmaster
Joined: Apr 2002
Posts: 3,613
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From: Coal country
Do you mean the Warranty Plate or the axle tag? If it says 3_25 on the axle tag (and the rear chunk is original) then yes, you have a 3.25 rearend gear ratio.
If it is on the warranty plate, I don't know. I'm sorry, but my books start at '58 and only go up to '66.
Can anyone else help him out?
If it is on the warranty plate, I don't know. I'm sorry, but my books start at '58 and only go up to '66.
Can anyone else help him out?
Postmaster
Joined: Apr 2002
Posts: 3,613
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From: Coal country
Great! You have a 3.25! That ratio in my experience, tend to be highly desirable. For most situations, the 3.5 is usually used. People often want to go to a 3.0 or a 2.75, but that makes it too tall, so a 3.25 is a nice intermediate.
I had a 3.5 in my '66 3-speed, and tried a 2.75. I couldn't even pull away from a stop on a level road without feathering the clutch!
3.25 worked out real well for me, especially with the overdrive.
Good luck in finding the right gear ratio!
I had a 3.5 in my '66 3-speed, and tried a 2.75. I couldn't even pull away from a stop on a level road without feathering the clutch!
3.25 worked out real well for me, especially with the overdrive.
Good luck in finding the right gear ratio!
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From: Fort Rucker, AL
I've decided to go ahead and put in the 4 speed just to get some better mileage around town. I'm military and most of my driving is on and around post so I think that'll be better with a 4 speed. This may be a dumb question but is it necessary to change the pressure plate when I put a new clutch in her?
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Lead Driver
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From: Ohio
It is NOT necessary, but usually you can buy a "kit" with all the pieces, clutch disc, plate, throwout bearing. Usually not a bad deal either. I think just over $100 can get you a decent factory replacement piece.
Fleet Mechanic
Joined: May 2004
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From: Montana
The rpm's are strange because of tire size....I have the same ratio and I had 275-35 15's and thats normal.....I went to 31 10.5 15's and my rpms lowered alot on the highway my father changed them down one size smaller than stock and now at 55 mph the rpm's are screaming. So tire size has alot to do with it.
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From: Fort Rucker, AL
Originally Posted by Freightrain
It is NOT necessary, but usually you can buy a "kit" with all the pieces, clutch disc, plate, throwout bearing. Usually not a bad deal either. I think just over $100 can get you a decent factory replacement piece.
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From: Fort Rucker, AL
Originally Posted by EgoMan
The rpm's are strange because of tire size....I have the same ratio and I had 275-35 15's and thats normal.....I went to 31 10.5 15's and my rpms lowered alot on the highway my father changed them down one size smaller than stock and now at 55 mph the rpm's are screaming. So tire size has alot to do with it.