66 F250 Rearend options 4.11 or 3.54
 

Hello All,

It has been a long time since I last posted on this forum. My 66 F250 has been in a shop for the past year. There were problems with the 428 build. It's too long of a story for this post.

I'm suposed to go get the truck back this weekend. The mechanic suggested that I swap out the 4.11 gear for a 3.54. The price would be $200 - $300 depending on the condition of the related bearings. The transmission is the original cruise-o-matic. The 428 has a solid lifter AA (427) Holman Moody cam, aluminum Ford PI intake, 10:1 compression tri-y headers, C6AE-R heads. The engine rebuild is from top to bottom.

What I'm wondering is how well he 3.54 gear will enable more pleasant (lower RPM / better gas mileage) driving on the highway at 65+ MPH without significant loss of towing capability.

i would do that swap, the new 428 engine will have plenty of power.

with 4.11 gears you will be hard pressed to run down the highway at 65+

what trans are you going to use?

3.54-1 ratio introduced in 1966, the pinion gear has 29 splines.

But, some 1966 4.11 pinion gears have 10 splines, some have 29 splines, as the 29 spline R&P was a "running change."

What this means is...if you have 10 splines now, you will need a 29 spline companion flange if installing 3.54 gears.

For what its worth (not much), back in the day (1966 to be exact), I had a stock 390/4bbl '65 Galaxie convertible with Toploader 4 speed and 3.25 gears. I got fed up with my roommate blowing my doors off with his '66 Belvedere 383 (beefed), 4 speed, and 3.90 gears. Not knowing my arse from 3rd base back then (or perhaps now, for that matter) I had a speed shop put a solid lifter 427 cam in my 390, thinking that would make things more equal. What it did was eliminate my previous low end torque, while allowing the engine to rev to 6000+. But the loss of low end torque was so drastic that it wouldn't launch. So I had to change to 3.89 gears to compensate for the loss of torque. I know this is apples to oranges, and the 428 will give you lots of displacement that my 390 lacked, but what I'm getting at is have a look at your cam specs and see what kind of low end torque it will give you - or not. Maybe the torque multiplication of your automatic will help there. The 3.54s will certainly make life nicer on the highway.

If you went with a rear end from a 1970-72 Camper Special, you could go for a 3:73 and also get
better drums (I think from 2" to 2.5"...). The yoke spline count is a bit different, I just did a conversion
U-Joint. With my 302/C4 and the Dana 60 3.73, I can easily cruise at 60-65mph at about 2300rpms. (16" Tires)
Towing is good and I also get off "the line" with good pick-up. The 3:54 will be great for cruising speed,
but personally, (FWIW) I think the 3:54 would be a dud when it comes to maintaining some low end torque.

1966: Ford introduced 12" x 2 1/2" brakes for F250's, but it depends on the F250's GVW if it has them or 12 1/8" x 2" brakes.

All 1966/72 F250 Camper Special's have the 12" x 2 1/2" brakes, as do F250's w/7500 & 8100 GVW's. But not w/the 6100 or some of the 6300 GVW's.

1968/72 F250 2WD's with optional discs, have 12" x 2 1/2" (rear) brakes as standard equipment.

My 66 F100 has the measly 240 I6, and 4 speed,, with 3.50's I would definately swap out the 4.11's,, you have plenty of engine to move that pig around!!

Thanks to all for the replies. As is usually the case after reading your posts I have more confidence on how to proceed. The cam is part number C3AZ-6425-AA. I'm trusting the mechanic on the torque output. He built the engine and has many years experience building FE's. This is the same cam used in some 427's. My understanding is that it was also used in some 66 police cruisers with the 428. I think that the 428 will still have adequete torque for my infrequent towing / hauling needs with the 3.54 gear and that the 3.54 gear will make for a better more frequent (lower RPM, perhaps better gas mileage) highway driving experirnce. I'll look into the torque numbers on the cam and report those here. Hopefully later today.

I'm in the same boat. I have a '65 F250 with a Dana 60 and 4.11's. I want to swap it out for a 3.54.

Did Ford produce any Dana 60's with 3.54's already in them?

1966 was the first year, but 3.54 is a hard to find ratio for a Dana 60. Everyone wants one, few find any.

And, since you have a 1965, the pinion gear you have is 10 splines, while all 3.54's have 29 splines.

So, you cannot use the companion flange your truck has now with 3.54's. You will need one from a 1967 or later Dana 60.

Just wanted to throw in my 2c on this discussion: I had a local shop do the swap from 4.11's to 3.54 on my 66 F250 and the difference is noticeable!

I can't tell exactly what speed I'm running at because the speedo does not work (another thing on a very long list of future fixes) Of course I still get passed up on the freeway but at least it doesn't sound like the engine is about to explode.

Also, I am running an old, tired 352 and it does not seem to have problems at low speed/low rpm's. On the other hand, I am almost never towing stuff so that is something to think about too.

I spoke with the mechanic doing the work a few minutes ago and he said it was a good thing I had him do this work. The drivers side brearing was seized to the housing and had buggered it up. In addition to the 3.54 gear change he cleaned up the housing on the drivers side and put new bearings in on both sides. He said if I had not had this done that more serious trouble was not too far down the road. My speedo works. However, I think it will now be off by a few mph on the plus side. I will need to replace it when time and funds permit. I've not driven this truck for about a year. So, I'm looking forward to seeing how she does with the 3.54 gear and the fresh / non-blue smoke 428. I still need to check on the camshaft torque numbers. Thanks again to all who posted on this topic. I'll let you know how it perfroms.

I guess don't know for sure what the torque and HP curves will look like on this cam in combination with the rest the 428 components. Perhaps I'll make a trip to a speed shop with a dyno some day.

Per FEmtnmax when I asked about the perfromance of this engine back in October of 2008 he said 346 HP @ 4500 rpm, and 454 ft lb TQ @ 2500 rpm
https://www.ford-trucks.com/forums/7...to-expect.html

Your engine will turn 14% slower with the 3.54 gear.. so your speedometer will be slower by the same percentage. To figure a new speedo gear, take the current tooth-count and multiply by .86

Example, if you now have a 21 tooth gear: 21 x .86 = 18.06    An 18 tooth speedo gear to get you back to where you were with the 4.11

(I got the .86 by dividing the new-ratio into the old-ratio... in this case 3.54 / 4.11 = .86 )


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jmf66. Thats great information. In addition to knowing about the number of teeth I need on the speedo gear I think it's helpful to understand that my engine will turn 14% fewer RPM at a given speed. Based on the 14% difference, if my engine was turning 3500 RPM at 45 MPH (I don't remember what it was turning before the gear change, but I can go the other direction on the calculation once I get my truck back and estimate what it was turning) then it will now turn at 3010 RPM. I'm pretty uninformed about rear end gearing. I think I will read up on the basics. My understanding is that the 3.54, 3.73, 4.11 numbers reflect some type of ratio.