390 Fe 5 speed options
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3rd Gear
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390 Fe 5 speed options
I got a 1977 F-250 single cab short flat bed for Christmas one year and thought that it was amazing. Up until I flipped it and replaced the dry rotted 37s with brand new and wider 40s, however I've come to the realization that the 4 speed it has is hurting the engine with the large dip in rpms at every shift. I put a Holly EFI Sniper 2 on it and it helped wake up the engine a lot but I still feel like it would preform better with more gears where the dip in rpms are not as steep.
All this to ask what are the 5/6 speed manual options that will either bolt directly or bolt with minimal fabrication?
All this to ask what are the 5/6 speed manual options that will either bolt directly or bolt with minimal fabrication?
Fleet Mechanic
Joined: Jul 2000
Posts: 1,493
Likes: 346
From: SE Iowa
You're problem isn't necessarily the transmission. You put 40 inch tires on a truck designed for 32 inch tires. The solution is to regear the axles to compensate for the increased tire diameter. For that we need to know the current gear ratio. Decode the door tag or look for a tag on the rear axle assuming it's all original. I can venture a guess that it's 3.55, as that is common for a lot of 70s trucks. Some had 3.25:1. If 3.55 is correct you need at minimum a 4.10 ratio. Ideal would be 4.30 or possibly 4.88 if you have 3.25:1. Changing the ratio to a lower gear (numerically higher) will makes the drop in rpm less severe when shifting gears.
Super Moderator
Joined: Apr 2004
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Well, nothing cheap. IMO forget the old big truck transmissions. You'd be better off with your current transmission.
High dollar options:
https://americanpowertrain.com/shop/...paOVGLC2Lsq2U2
https://www.moderndriveline.com/shop...ern-driveline/
A fellow traveler:
https://www.ford-trucks.com/forums/1...behind-fe.html
But the prior post is correct--your axle ratio is the main culprit here.
High dollar options:
https://americanpowertrain.com/shop/...paOVGLC2Lsq2U2
https://www.moderndriveline.com/shop...ern-driveline/
A fellow traveler:
https://www.ford-trucks.com/forums/1...behind-fe.html
But the prior post is correct--your axle ratio is the main culprit here.
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3rd Gear
Joined: Jun 2025
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You're problem isn't necessarily the transmission. You put 40 inch tires on a truck designed for 32 inch tires. The solution is to regear the axles to compensate for the increased tire diameter. For that we need to know the current gear ratio. Decode the door tag or look for a tag on the rear axle assuming it's all original. I can venture a guess that it's 3.55, as that is common for a lot of 70s trucks. Some had 3.25:1. If 3.55 is correct you need at minimum a 4.10 ratio. Ideal would be 4.30 or possibly 4.88 if you have 3.25:1. Changing the ratio to a lower gear (numerically higher) will makes the drop in rpm less severe when shifting gears.
Fleet Mechanic
Joined: Jul 2000
Posts: 1,493
Likes: 346
From: SE Iowa
If you do indeed have 4.10s, the next option would be to source a close ratio toploader 4spd. Identified by a RUG-AD marking. The T18/T19 or NP435 that you have are wide ratios. Which is why you have the big drop in rpms between shifts.
You could also find a close ratio M5OD for the conversion, but that will not be an easy task as full size trucks got the wide ratio version.
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Tuned
Joined: Mar 2004
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All numbers here are arbitrary, for comparison:
6.56:1 final drive; shift @2500 RPM from 1.71:1 to 1:1 gear ratio. This drops you to 1461 RPM, around 20.2 MPH shift point
2.73:1 final drive; shift @2500 RPM from 1.71:1 to 1:1 gear ratio. This drops you to 1461 RPM, around 48.5 MPH shift point
Tuned
Joined: Mar 2004
Posts: 412
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You are out of power. Your fuel mileage is going to be garbage with tires that big, no matter what you do.
I'm calculationg that a change to a 4.56 ratio would give you a net of around 230 more RPM @60 MPH, around 2300 RPM. 4.88 gears would get you up around 2470 RPM, which would be a tad more than what a "stock" pickup would run @60 with 3.73 gears and normal height tires.
I'm calculationg that a change to a 4.56 ratio would give you a net of around 230 more RPM @60 MPH, around 2300 RPM. 4.88 gears would get you up around 2470 RPM, which would be a tad more than what a "stock" pickup would run @60 with 3.73 gears and normal height tires.
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Oct 24, 2011 04:25 PM