Taller diff for a 390
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Taller diff for a 390
I have a 67 F100 CS with a 390 and FMX tranny, and recently found out that I have a 9" rear with a 3.5 ratio. I saw a post about someone lowering the ratio to get better highway driving/mileage and am interested on how people's experience with this has gone. How much lower should the ratio be to put a slight emphasis on cruising on the highway over off-the-line acceleration? Where is a good website to buy the third member in different ratios?
Greg
1967 F100 CS 390 FMX
Greg
1967 F100 CS 390 FMX
Senior User
Joined: Jul 1999
Posts: 472
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From: USA
Taller diff for a 390
[font size="1" color="#FF0000"]LAST EDITED ON 11-Feb-01 AT 12:25 PM (EST)[/font][p][font size="3" color="#191979"]
I pulled the 3.50 gears out of my 1969 F100 and replaced them with a 2.75 gearset. The typical ratios available for the Ford 9" are 2.50, 2.75, 3.00, 3.25, 3.50, 3.70, etc...
My preference is the 2.75 - my 302 spins about 1900RPM at 60MPH with 29" tires on it, and gets about 16-17 MPG. I think your 390 should be pretty good with the taller gears if you are looking for better highway driveability without too much concern about launch.
As far as where to get the gears I recommend checking the boneyards, since most places that sell new gears carry just the steep ratios (4.10+). Look for a car or truck whose third member hasn't been tampered with (no silicone, hot rodding, etc...). The 9" in your truck is 99% likely to have 28 spline axles. 70's cars typically had tall (low numerical ratios) to try to help gas mileage.
Art
I pulled the 3.50 gears out of my 1969 F100 and replaced them with a 2.75 gearset. The typical ratios available for the Ford 9" are 2.50, 2.75, 3.00, 3.25, 3.50, 3.70, etc...
My preference is the 2.75 - my 302 spins about 1900RPM at 60MPH with 29" tires on it, and gets about 16-17 MPG. I think your 390 should be pretty good with the taller gears if you are looking for better highway driveability without too much concern about launch.
As far as where to get the gears I recommend checking the boneyards, since most places that sell new gears carry just the steep ratios (4.10+). Look for a car or truck whose third member hasn't been tampered with (no silicone, hot rodding, etc...). The 9" in your truck is 99% likely to have 28 spline axles. 70's cars typically had tall (low numerical ratios) to try to help gas mileage.
Art
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gadams
1967 - 1972 F-100 & Larger F-Series Trucks
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Mar 29, 2002 03:07 PM
