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Rear End Gears for my '85 F150

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Flareside Jeff

Rear End Gears for my '85 F150

Hey Everyone, I hope everybody is doing well!
So I think that I might be changing directions with my 85 Flareside build, right now she is a 2 wheel drive, and I just picked up a 83 F150 4x4 with good doors. anyway I thinking about swapping out my chassis for the 4x4 chassis, and I was wondering what rear end gear ratio I should use with my 300/6 4 speed combo?
I'm not going off roading, but I might use this truck to pull a trailer with my 8N Ford tractor. I was thinking something like a 3.55:1 ratio, any thoughts?

1- 1981 F100 Flareside,
1- 1982 F150 Flareside
1- 1985 M150 Flareside 4X4

Gary Lewis

Re: Rear End Gears for my '85 F150

Administrator

You say that you have a 4-speed, but lets make sure what one that is as there was the one with overdrive, the NP435, and the T-18. And they each have different gear ratios, as you can see on the page at Documentation/Driveline/Calculators.

And it makes a difference. The OD would allow you to run pretty low gears and still cruise with low R's when not towing. And you'd need the low gears to get the trailer rolling as the 1st gear is only 3.25:1.

But the NP435 and T-18 have 6.X:1 1st gears and then 1:1 4th gear, so you don't need as steep of gears. And, you don't want steep/low gears with the 1:1 top gear.

So what tranny you have makes a big difference. Do you know?

But with an NP435 or T-18 3.55 gears were common. However, going 65 is going to have you spinning 2400 RPM at 65, and that's pretty fast and will kill any MPG. But would be good for pulling.

Gary, AKA "Gary fellow": Profile

Dad's: '81 F150 Ranger XLT 4x4: Down for restomod: Full-roller "stroked 351M" w/Trick Flow heads & intake, EEC-V SEFI/E4OD/3.50 gears w/Kevlar clutches
Blue: 2015 F150 Platinum 4x4 SuperCrew wearing Blue Jeans & sporting a 3.5L EB & Max Tow
Big Blue: 1985 F250HD 4x4: 460/ZF5/3.55's, D60 w/Ox locker & 10.25 Sterling/Trutrac, Blue Top & Borgeson, & EEC-V MAF/SEFI

Pete Whitstone

Re: Rear End Gears for my '85 F150

In reply to this post by Flareside Jeff

Swapping out chassis's seems a much bigger job than just stripping the 4x4 stuff off the donor truck and putting on the 2wd chassis. Any reason for wanting to swap the chassis?

I just got done swapping in 4x4 suspension stuff onto my 81 2wd chassis, none of it was particularly difficult. Probably the worst part was getting the factory rivets out of the rear spring perches, but it's pretty do-able with a die grinder and an air chisel.

I have the rest of the conversion (transmission/transfer case) still to do, but that stuff is pretty straightforward and should not take long.

81 F150 Flareside, Edelbrock Pro Flow4 FI, hydraulic roller 351W, E4OD, 4x4, BW1356
92 F150 RCLB 351W E40D BW1356 mostly stock

Gary Lewis

Re: Rear End Gears for my '85 F150

Administrator

You actually don’t have to swap out the rear springs. Rusty was originally a 2wd truck and someone converted it to 4wd and left the original springs. They are 2 1/2” wide where the 4wd springs are 3” wide.

Gary, AKA "Gary fellow": Profile

Pete Whitstone

Re: Rear End Gears for my '85 F150

Right, I realize that I could have just run the 2WD springs. My concern with that was that the 4WD springs had more leaves and seemed to be arched more (and again I'm not sure the spring package on the 95 donor truck was stock Ford stuff). So if I would have kept my 2wd springs in the back and used the taller front springs, the truck would have had a taildragger look. I suppose of I would have kept the front springs too, I could have avoided that. But I couldn't see switching it to 4WD and not raising the ride height.

81 F150 Flareside, Edelbrock Pro Flow4 FI, hydraulic roller 351W, E4OD, 4x4, BW1356
92 F150 RCLB 351W E40D BW1356 mostly stock

Gary Lewis

Re: Rear End Gears for my '85 F150

Administrator

At least you'd need the spacer from the 4wd truck to lift the rear. That's what someone did with Rusty. But your approach is better all the way around.

Gary, AKA "Gary fellow": Profile

Flareside Jeff

Re: Rear End Gears for my '85 F150

In reply to this post by Gary Lewis

Thanks for the info Gary, I'm actually picking up the donor truck this Saturday, so I don't know exactly which tranny it has, but my thoughts were to swap the whole chassis complete if everything is good and straight on it, my 2wd chassis has a couple of issues, and I figured donor chassis might be a better choice.

1- 1981 F100 Flareside,
1- 1982 F150 Flareside
1- 1985 M150 Flareside 4X4

Flareside Jeff

Re: Rear End Gears for my '85 F150

In reply to this post by Pete Whitstone

Thanks for the input Pete, I'm picking up the donor truck this Saturday, so once I get the two frames side by side, I will pick the better of the two.

1- 1981 F100 Flareside,
1- 1982 F150 Flareside
1- 1985 M150 Flareside 4X4

Rembrant

Re: Rear End Gears for my '85 F150

In reply to this post by Flareside Jeff

Flareside Jeff wrote

I was thinking something like a 3.55:1 ratio, any thoughts?

I put a 1980 Flareside body with a 300 inline 6 on a 1995 4x4 Chassis with 3.55 diffs. I also used the M5OD 5spd that was in the 1995 chassis, and I actually really liked that combo. Carbed 300 with a 5spd and 3.55's. I was running 31" tires on that truck, but even still, first gear was pretty low and it cruised nicely on the hwy with the OD.

When the 9" rearend was still being used, gear ratios were all over the place, but by the time the 8.8 became the standard diff in the half tons, they almost all had 3.08's or 3.55's. You'll see a lot of 4spd's with 3.08's, and anything with OD using 3.55's. That went right up until 1996.

In any case, I really liked the combination of the 300 with the 5spd and 3.55 gears. I didn't keep the truck long after I finished it, but it was nice to drive.

1994 F150 4x2 Flareside. 5.0 w/MAF, 4R70W, stock.
1984 F150 4X2 Flareside. Mild 302 w/ 5spd. Sold.
1980 F150 4X4 Flareside. 300i6 w/ 5spd. Sold in 2021.
1980 F100 4X2 Flareside. 351w/2bbl w/NP435. Sold in 1995