About to buy a rebuilt 9" rear end... looking for a 2nd opinion

First off, I have a '66 240cid with 4.00 9" rear end. I think I want a 3.25 ratio, but I'd like a second, third, opinion before I blow $600.

I did little mileage calculation, which was made harder to do since my speedo is off by a few miles. Starting at a gas station where I just filled up, I did mix driving, a little city, but mostly highway. Charted my route on googlemaps and compared the miles to my odometer readings and determined my correction factor. 1 odometer mile = 1.137 actual miles. I filled up at my end point and used this volume to determine the gas consumed. Sparing the boring details, I got 14.7mpg.

Now, when on the highway, the van cruises comfortably at 57 mph (actual). It feels strung out at 70mph (actual speed). It will go a little faster, but not much. So I figure, if I'd like to be able to cruise comfortable at 70mph, then I would need the final drive ratio reduced by a factor of 57/70 = 0.814. I have a 4.00 so, 4.00*0.814 = 3.26. So, this points me toward getting a 3.25.

Yesterday, I was just about to buy a rebuilt positrac 9" 3.25 rearend, and I thought, man, I really better double check that I actually have a 9" ford rear end. So I crawled under the van. Sure enough, it looks exactly the same, then I noticed the tag. It reads: WCR-S1, 3.00 7HE 787.

I can only assume the 3.00 means a 3.00 ratio... it must be an old tag?

It was quite a while ago when I checked the ratio... I did it when I was changing my clutch. This is what I think I did: with one rear wheel touching the ground, and the other up in the air, I turned the yoke until the wheel made one revolution. The number of turns of the yoke was 2. So if both wheel had been turning, I would have had to rotate the yoke 4 times to get them both to turn 1 rev... so I have 4.00. Right?

I've got a stock 240 with 3.00 and it will cruise all day long at 70 MPH. Just got back from a near 800 mile trip with it and cruising at 80 wasn't a problem, and it didn't even feel 'strung out'! We are able to pull a small camper with this gear also. Hope this helps.

What kind of mileage do you get Carl?

Also I was wondering if there are different yoke styles, and what style the econo yoke would be?

Here's a tip...

8's & 9's look alot alike. If you can put a socket and ratchet on all the carrier to housing nuts, it's a 8 inch. If you have to use a wrench on the lower 2 or 3 nuts, it's a 9.

dakeddie wrote:What kind of mileage do you get Carl?

Also I was wondering if there are different yoke styles, and what style the econo yoke would be?

I can only answer the gas mileage question, last weekend we went 768 mostly highway miles and averaged just a little over 16 mpg.

remember you need a 28 spline not a 31

Carl, have you check your compression? I would've thought a 240 with a 3.00 would get around 18+mpg. I used to have a 61 econo with a '73 300cid, and what must have been a 3.00 diff, though I never checked. It got 18+mpg on the highway. I check my compression recently, it was low. Cylinders 1-5 were 90psi, cylinder 6 was 80psi.

Another question, should I get diff shimmed rigidly instead of with the crush tube? I've heard this will allow me to change the yoke.

Haven't checked the compression, but the motor is a little loose!
Maybe it takes just a little more fuel to help burn the oil!!!