change rear end gear ratio 4:11 to 3:78

[Jakno]

Interested in changing the rear end gear ratio on a 35 1/2 Ton which currently has 4:11 gears to 3:78 gears. Never have driven the truck yet, only have information gather thru the Barn, etc, regarding RPM's, over the road speeds and such.
If I do change the gears would the 3:78 set in the picture provide the bulk of what I would need other than gaskets and possibly replace the bearings?.

[Jakno]

here's the picture of the rear 3:78 gears

[Talkwrench]

In my 35 pickup I have a 37 21 stud a 39 toploader with '28' gears, a 3.78 rear with large 7.50 x 16 tyres. Pulls that along just fine.
If you where staying with 6.00 x 16 tyres Id even go for a 3.54 set.

[Admiral]

I upgraded the stock 4.11 rear in my '36 sedan to a set of 3.78 gears. I've never driven another flathead-powered Ford with a different ratio so I'm hardly an authority on this matter but with the 3.78s my car seems to cruise along just fine at 55 or 60 miles an hour. I don't think doing 65 or more on the highway would cause it much harm, either. I wanted a set of 3.54s but I couldn't justify the cost (they ain't cheap!), especially since my 3.78 gears only set me back about $50. The car still pulls nicely at lower speeds despite the slightly taller ratio. Also, if you do the swap make sure to get a new speedometer drive unit with the correct number of teeth so you know how fast you're going.

[JM 35 Sedan]

Are you certain that you have 4.11 gears in the rear now? How did you confirm this? The numbers stamped on the bottom rib of the banjo center housing would be 37/9 for 4.11 but the gears could have been changed over the years to some other ratio. One thing you could do to confirm this ratio, is remove the oil fill plug on the back of the banjo housing, mark one tooth, and count the total number of teeth on the ring gear. 4.11 gears would have 37 teeth on the ring gear. 3.78 would have 34 teeth and 3.54 would have 39 teeth.

Based on the pictures you have posted, it is difficult for me to see how many splines are on the pinion gear. If this is a 10 spline pinion these gears would probably drop right in place of the existing gears. If it is a 6 spline pinion a change to the female splined end of the driveshaft would be required

[Jakno]

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Thanks John, we counted the ring gear and it does have the 37 or 4:11 gears. I also appreciate knowing the 10 spline will drop in. We were looking at a 6 spline.

[jimTN]

My advice would be to find a 3:78 gear set that has not been removed from the housing. Everything will bold in and really no adjustment required.

[JSeery]

Or learn to install gears, not that difficult, just learning what your doing.

[rheltzel]

I've driven flatheads with 4:11's, 3:78's, and 3:54's...my current car, a '35 5-window, has 3:54's. Great on the freeway, but for all around use, I'd rather have 3:78's...best balance for acceleration and cruising.

[G.M.]

A 3:78 compared to a 4:11 is like going from 3rd gear to 4th gear. G.M.

[Brendan]

Originally Posted by JSeery

Or learn to install gears, not that difficult, just learning what your doing.

i learned on my Model A , just take your time and go slow!