I have a 48 M-68(F3). I plan on replacing the rear end with one from a 68-72 f250. I'm trying to figure out what rear end ratio I want, and in doing so, I keep reading how important the tire size is. What would be a normal tire/wheel size for this truck? Currently I have the stock WMs on the front and the PO put 235-85-16 on the rear. Eventually I'm going to want to have the same on the front and back, so I should plan for that now.
My second question would be what RPM should I shoot for while cruising highway speed. I know you don't want the RPM to be too slow, but you don't want it screaming either.
I am either going to go with a C4, or a AOD. I like the idea of having the over drive which will give me a better highway gear.

 

I always try to determine the final tire size that I'm going to run first and then choose a rear axle ratio while taking that and the rest of the powertrain into consideration. The tire size will be heavily dependent upon the look you want for the truck and how you plan to use it. Are you wanting to slam it, lower it a little, leave it basically stock, etc.? An F-3 has a pretty heavy suspension system and will require some pretty serious spring mods to lower and then you're talking about a whole different animal. I assume that since you're staying with a 3/4 tone axle that you intend to keep it relatively stock.

If you do want to try to keep a stockish look, then the 235/85R16 that you have on the rear now is 31.5" tall and would be a good choice. It was the stock size on many, many late '70's through mid '90's F-250s and is pretty common. The 16"x7" or 8" wheels that you'll need for them are too. Any 16" 8-lug Ford wheel will have the right lug pattern. I can pull these wheels out of my local Pull-A-Part all day long with good rubber on them for $25 apiece. The big thing to watch out for is whether or not they will clear your stock F-3 front brake drums. The F-3 trucks had bigger brakes and some guys have had issues with getting 16" wheels to fit over the drums. Some fit fine and others don't. It might not hurt to move one of the rears to the front and find out. The biggest problem with running later wheels is that the stock hub caps won't fit. You'd have to run some later model hub caps.

If you're really wanting to run stock hub caps, then you'd probably want to look into picking up a set of stock '48-'60 F-2/F-250 16" wheels. At 6" wide, they are narrower and won't properly mount the 235/85/R16 tire. A good tire for those would be the slightly shorter and narrower 215/85R16. That tire is about 30" tall. Again, you will want to double check to make sure that they will clear your brake drums in the front first.

Once you've nailed down what size tire you want to run, start plugging things into an RPM calculator. The one I like to use is here:

Engine RPM Calculator

You want to end up with something above 1500 RPM at 55 MPH and about 2500 at the maximum speed that you would expect to run for a long distance. I'll figure this with the taller tires. The AOD has a final drive ratio of .68:1 and the C4 is 1.00:1. With the OD transmission, it looks like you'd be turning 1488 RPM at 55 MPH and 2029 RPM at 75 MPH. That's actually a hair low for my taste although is certainly livable. A little shorter tire with the 3.73 or moving to a 4.10 gear might be a bit better choice with the OD but not really necessary. If you went with a C4, you'd be at 2188 RPM at 55 MPH and 2984 RPM at 75 MPH. That's not too bad either, although 70-75 will be about your top comfortable speed on the highway.

 

I'll agree with pretty much everything said except the highlighted sentences. The front drums on both F-2s and F-3s from 48 to early 51 were 11 13/16" inside diameter, and for late 51s and 52s they were 12". I've not heard of anybody having trouble finding 16" wheels to fit a front hub and drum. It's the 14" back drums of the 48 to 51s that cause the trouble. Stu

 

Thanks for clearing that up for both of us, Stu. I knew about the issues with the rear brakes and wasn't sure about the fronts. I thought I should mention it just in case. If he's swapping to a Dana 60 out of a later truck, then he should be able to run whatever 8-lug wheels he wants.

 

Is there any reason why I can't put a 1/2 ton rear in my truck? I realize that it would then be 5 lug, but I could also change the front, correct?

 

how are you going to deal with the offset in the newer rear end? I know its an old conversation but I thought I would ask.

 

I still have not decided what I'm going to do. I am now thinking about putting a ford 8.8 from an explorer in which is supposed to be 59". The stock width is 61", so will an inch on each side make a huge difference?

 

I can tell you from experience, if you were to run a 3.54 gear with a 235/85x16 tire, you'd be turning about 2600 rpm at 70 mph, assuming a 1:1 final drive ratio. (no overdrive). That's about as fast as you'd ever want to go an a M-68, I think. Just for simplicity sake, findiing a Dana 60 with 3.54's would be the easiest and cheapest way to go. No extra expense or fussing around with half ton parts or swapping front hubs and brakes, either, unless you really want to. This is the combo I have in my regular driver '85 F-250. It works well.

 

I won't be using the truck for a work truck, so the extra heavy duty axles won't be needed. It will be for cruising around and whatnot. I want the explorer rear end with the disc brakes, and will be converting the front to disc as well. I am going to be putting a 351 with and AOD.

 

Sounds like a great plan. You won't have any trouble fitting the Explorer axle in the truck. Being slightly narrower isn't an issue. Worse case is you'll have to pay a little closer attention to your wheel offsets. No big deal.

 

how do you deal with the 2 inch offset of a newer rear yoke when the older trucks run center doesn't it matter

 

I believe as long as they are parallel, it will be fine. Correct me if I'm wrong.

 

Wouldn't the drive shaft be running in a angle from side to side. I've never seen that before but I may be wrong.

 

I have an oppertunity to buy a Ford 8.8, so I guess I need to make a decision.
I need some opinions on the following things;
1) Cruising RPM. I got a few opinons already. From what I gather, anything from 2100-2700 at cruising speed(70 mph) should be accetable?
2) Is there any issue with the offset diff. I believe it's about 2 1/2 " offset. Am I correct when I say that as long as all the angled are parallel, there will be minimal vibrations?
3) Still not sure on the tire size. I've been thinking about going with a tire about 30". I just won't want them to look small.

Once I know these answers, I can calculate everything and see if this ford 8.8 with 4.10 gears will work for me.

Thanks!

 

Don't know about that offset, but now you have two planes involved: vertical, as in the height difference between the tailshaft and the pinion along with the up & down of the axle, and horizontal with your offset.

Make sure you get the shaft balanced when you have it made up...

Back to gearing:

70 mph
1:1 high gear
30 inch tire
4.10 axle
= 3214 rpm.

Figure your combos here:

Novak Conversions - Gearing & Gearing Math for Jeeps