Ok guys, here's the deal: I've got a '55 F250 loaded with a 302 Windsor, I THINK a C4, and no idea what's in the rear end. Problem is, I'm turning too many RPM's to cruise on the highway comfortably, which is something I find quite necessary in my day to day. As a matter of fact, I've found that the RPM's are uncomfortably high even cruising around town. It feels like I'm struggling to get over 35 mph. This is not a hot rod drag racer, nor do I want it to be. However I will occasionally introduce pedal to floor, though that's an entirely different topic.

Now the way I see it, I have two options to meet my current demands: I can either change out the gearing in the rear end, EDIT: it's a Dana 60 for sure, but I still don't know what the current gearing is. As far as I know, the previous owner used the truck in parades, I assume to pull floats around at low speeds. Probably why it's geared so low in the first place.

Or, I can swap in an AOD transmission, with matching peripherals (flyweight, tv cable, etc.). I have a friend who runs a transmission salvage and will sell me one DIRT cheap, so that is a contributing factor. It will need to be rebuilt, so I imagine the cost between a transmission rebuild/install will be similar to that of a rear diff. rebuild.

My question to you fine gentlemen: what would you do, and why? I'm planning on one or the other, and the goal is to be able to cruise at 65-70 mph under 3k rpm and also maintaining a very streetable ride.

The feel of the truck as it is, is pretty good. The first two gears shift very quickly, and then I'm in third. I can FEEL the untapped potential, she WANTS to shift and keep going, but that's all the gear she's got! So, what would YOU do?

Thanks in advance

 

OR is there an option 3, something I haven't thought of?

 

You probably need to change out the rear end or the gears if possible. Also, what is the diameter of your back tires? If they are small some taller tires will help a lot. The bigger the diameter the further you go with each revolution, like changing the gearing. The AOD tranny is not the most reliable one out there (depending on who you talk to). And if you do change it you may have to modify the driveshaft too. Try to post pictures of the rear end, people here can probably ID what you have to tell if you can change the gearing in it.

 

It's up to you! Personally, I would keep the C4 transmission as they are good transmissions and very reliable. Putting a AOD transmission will require some modification work including shorting the drive shaft, etc. The rearend is easy to change. It's quite possible you have a Dana 60 rearend which came in that year of truck. That would explain why your rpms are so high on the highway. The Dana 60 rearend came with 3.54 to 1 and lower gears. With the C4 tranny, you will definitely be in the 3K range at 60 mph. I believe there are higher gearing for the Dana 60's and any rearend shop will change over the gears and not break your wallet. I would look for something in the range of 3.00 to 1 or maybe 3.25 to 1 at the lowest for good highway driving. Taller tires will effect the rpm, but not as much as changing the gears. Your gas mileage will improve with higher gears as well.

 

Thanks for the quick responses guys. The one thing that concerns me about changing the gearing is going too low and affecting the "around town" drivability. Thoughts?

Also, is it normal for a C4 to shift so quickly? I honest-to-God thought there might have been a 2-speed Ford-O-Matic in it until I manually shifted through all three gears this morning! It's really just a blip of first gear, never would have guessed there was a second, and then pumping along in third. Maybe a symptom of the current rear end gearing???

 

It sounds like you've got much steeper gears than 3.50 in it. Unless you have a good load on the engine (foot in it), the vacuum modulator will call for a shift to the next gear. With steep gears and a moderate pedal, that can be very quick. I'd figure out exactly what gears you've got before going further. There are a couple of threads on the forum from about two weeks ago on the costs of re-gearing a Dana axle, look back at those. It ain't cheap.

Edit: Here it is https://www.ford-trucks.com/forums/1...ace-gears.html

 

Thanks for the link Albuq, I'll definitely give it a read

 

Well son-of-a.... After reading that I'm leaning towards changing the gears. I've got a whine that seemed like it was coming from the engine bay, but now I'm thinking it could be from the rear end (I've been riding Harley's long enough to know sounds can kinda "jump" from one place to another). Goes up and down with speed, not rpm. Thought maybe it was the alternator, but I don't think that's it. Also get a little bit of a clunk when turning corners above about 10 mph.

So I'm guessing the PO put some ridiculous gearing in it, like 5.38:1 for low speed trailer pulling, and whoever put them in did a weak sauce job. Sounds pretty accurate, no?

What's a good method to find out the ratio of the gears I've currently got? Pull the cover and count teeth?

 

The easiest way IMO is to put the whole rear axle on stands (front wheels blocked) and make a mark on one of the tires and on the pinion yoke. Count the turns of the yoke to get one rev of the wheel. Make sure both rear wheels turn freely (no brake drag) or that will mess up the calculation.

 

Albuq, you are a gentleman and a scholar, sir. Thanks for your help, I'll be back tomorrow with 900 more questions!

 

Also, some of the early c-4 transmissions start out in 2nd gear........but I would definitely check your rear gears.

Paul

 

I am having a similar issue. My truck has a rebuilt 289 with a Sizzler C4. From the numbers the rear axle is from a 64 Ford car and has 3.0 gears in it. Yet when I get the truck up to around 50 on the highway the cheap tach the previous owner put in the truck is reading about 4,000 rpm which seems really high for that setup.

I haven't had my truck that long and I am trying to work through all the problems and figure what all needs to be redone. The PO put 18" tires on the truck so they are fairly tall (30.5") which makes me question what if anything is likely the cause of the higher RPM and what type of RPM that combo should be turning at highway speeds.

The speedometer is slow due to the tall tires, etc. and I have only had it on the highway twice for very short distances but the other day I would guess I was running around 50 and the tach was at 4,000. I am now trying to decide the best way to try and approach this issue.

The first thing was to identify the gear ratio of the rear end which should not be the problem if it is what is stamped on the axle. Could the cheap Harbor Freight tach be off quite a bit? Transmission seems to shift fine but I wonder if it is really getting into third. Could it be a linkage issue or a vacuum issue? I am totally new to this type of stuff so any input on how to troubleshoot it would be appreciated.

 

Oldwi, You should check your tach first. If you have 3.00 gears there is no way that you would be turning 4k rpm at 50 mph unless your tranny isn't shifting into 3rd gear.

 

Most aftermarket tachs have a switch on them for 4, 6, or 8 cylinders. I would guess yours is set to "4"...

 

I wouldn't trust what's stamped on the exterior, you should verify what the ratio is for yourself using Albuq's method. Somewhere along the way there's a good chance at least one of the PO's changed the gears to suit whatever his needs were at the time, especially being that the axle came off of a car and is now on a truck.


Hey, I'm a quick learner.