F15 diff swap

gareth · #1 26-03-11, 10:52

Hi, is there a diff swap I can do on my F15?
Would like less revs and a touch more speed, what do you think? overdrive? gearbox swap?, Axle swap?.
When driving at about 40mph am always looking for an extra gear and with the price of fuel at the moment if I could drop a thousand revs it would make a big difference, regards, gareth.

Harry Moon · #2 27-03-11, 02:56

I sure found a difference using the 10.50X16's I got from Wallace wade. Seems to make a big difference on my chev.
I've also put 20 inch rims on my truck for one trip and with 1100X20 i think i hold the land speed record in class. I've thought of the 900X20's I have on my project C60L might come in handy.
There really is no way to change the diffs on an F15A or C15A, maybe an F15 or C15's. Thought of using a CCKW overdrive but that couldn't work out without moving the differential back on the Frame.

cletrac (RIP) · #3 27-03-11, 03:52

On a C15 the rear end is the same as the civilian 1 1/2 ton and different ratios are available. The F15A rear ends will be the same as either 2 or 3 tons so different ratios should be available but I've never looked into it. I'll have to check out some old farm trucks if all this snow ever goes away!

Lynn Eades · #4 27-03-11, 04:27

According to a book, your diff ratio is 6.66 to1. I wonder if the Universal Carrier crownwheel and pinion will interchange. they are 5.83 to1.
Someone might have done the exercise and might be able to post an answer.

Ganmain Tony · #5 27-03-11, 04:49

Howard can tell you the exact ratio - but the diffs in the 2wd Fords where taller than the 4wd's. Apparently the white scout car diff centres are taller again and can be used, but there is some fooling around with carrier bearings....

I think its a matter of changing the crown wheels. Obviously you cant swap diffs easily (2WD to 4WD) because the location of the Crown & Pinion is different (offset in 4WD axle and in the centre of 2WD axle).

Has your truck got 16 inch wheels Gareth? Changing them over to 20 inch will give you a better rolling diameter and therefore speed at lower rpm.

Failing that a good set of ear plugs....... but why would you not want to hear the roar of the fan and the mighty V8 engine?

gareth · #6 27-03-11, 11:30

It is fitted with 16" wheels, I have thought of fitting 20" if I could find a set. I absolutely love the sound of this engine when burbling through town or down slow country roads, but when you are trying to keep up with jeeps, dodges and gmc on a 6 hour journey on the motorway, it can be just a little tiring.
I always have a mp3 player going in my ears, it's not the noise so much, it just feels like I am pushing it to hard and i cannot relax and enjoy the journey!
Maybe I should sell and buy another dodge wc, Gareth.

Phil Waterman · #7 27-03-11, 17:06

Hi Gareth

The topic of changing the gearing to lessen the noise and lower the revs has come up before but I can seem to find the Thread.

Couple of the points that, stick in my head, from the earlier threads is that it really is more about slowing the engine and drive train down to reduce noise and possibly help gas mileage.

Those who have re-engined or reworked the engines will I think general say with more usable RPMs you don't really improve top speed much particularly with the shorter wheelbase 101" the trucks with a relatively high center of gravity. They really get squirrelly at anything much above 55 MPH. CMP brakes if properly setup seem to be much better than many trucks of the `40s and could handle the speed.

One of the things I've notice with CMPs is that the fans are basically designed for the truck to be running at full load up a 40% grade in 100F+ temperature so that a lot of the noise comes from the fan. So reducing noise from the fan, check out some of the Ford V8 Hot Rod catalogs to see if there is a solution that works.

Can anybody remember the old thread so we can tap into that knowledge as well.

Cheers Phil