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Old 11-15-2010, 09:22 AM
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Rear end and big tires

I have my 72 C20 running as good as possible. Timing, Qjet, headers, ignition and 350 transmission. It runs great but I need it to pull a trailer 3 to 4 times a year and it has problems with grades. I thought about a rear end gear change but I am not sure what I have in it now and it would cost a bunch of money. I was close to buying a 96 F250 Diesel to pull the trailer but could not bring myself to buy a second truck that would sit most of the year and the money for the Ford could be spent on the Chevy.

First question is about tires. I foumd a good deal on two new Uniroyals at a swap meet for $100 so I put them on the rear. They are 265/75/R16 and have a 32" diameter. The front tires I just bought from Les Schwab are 245/75/R16 and have a 30" diameter. How much gear ratio would I gain by moving the front tires to the back? Could I get the gearing I need by going to an even smaller tire? What was the stock diameter tire that came with the truck?

What was the stock gear ratio on a 72 C20? What type of rear end was used. I know mine is not an Eaton because of the bolt pattern. I haven't been able to get under it to find a number on it and there are not tags.

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Old 11-15-2010, 09:34 AM
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How many bolts are in the rear cover ?
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Old 11-15-2010, 10:07 AM
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Quote:
Originally Posted by Mertz
...have a 32" diameter. The front tires *snip* have a 30" diameter. How much gear ratio would I gain by moving the front tires to the back?
Going from 32" diameter rear tires to 30", a 3.42:1 rear gear ratio becomes 3.65:1

A 3.73 becomes 3.98.

Calculator HERE.

Check the glovebox for a sticker w/the RPO codes. If it's there, and the gears are original, you can decipher the codes to see what you have.

Or drop the cover. Or do the "spin the wheel and count the d-shaft revolutions" deal.
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Old 11-15-2010, 10:29 AM
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Cobalt gave you great info but this is another way saying the same thing but may be less confusing...The smaller the tire on the rear axle, the lower the gear ratio will be. In otherwords, if you are running a 30 inch tall tire and taching at 2500 rpm @ 60 mph, by putting 28 inch tires, you'll be taching at 2730 rpm @ 60 mph. It's not real accurate, but you can guess-timate your current gearing by raising the rear end and tires off the ground. Mark the tire and turn it one complete revolution so the mark is at the same place. While turning the wheel, look at the driveshaft and count how many revolutions the driveshaft makes. 3 complete turns would be 3.00 gearing. 3 and half turns, 3.50 gearing etc. The calculator that Cobalt put is a nice tool to use. You should have a 12 bolt rearend in the truck unless it was replaced. Here's some info on your rearend and specs.

http://www.drivetrain.com/parts_cata...d_pinions.html

Last edited by kleen56; 11-15-2010 at 10:52 AM.
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Old 11-15-2010, 10:52 AM
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Thanks This helps. I didn't count the bolts but it has an odd shape not round.
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Old 11-15-2010, 10:54 AM
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Quote:
Originally Posted by Mertz
Thanks This helps. I didn't count the bolts but it has an odd shape not round.
I posted this on my earlier post, but will post it again in case you didn't see it. Here is what you should have... Yep, it's odd shaped.

http://www.drivetrain.com/parts_cata...d_pinions.html
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Old 11-15-2010, 11:08 AM
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I found that site too and that is what I thought I had but it did not list the C20. It does say 3/4 ton. There is nothing on the glove box to indicate gear ratio that I could find. Since I can't seem to burn rubber with the truck I assume I don't have the 3.73 gears and the 32" tires make it worse.
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Old 11-15-2010, 12:00 PM
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Assuming you have a 10 bolt rear cover, you may have a Dana 60 diff. There was a 354 ratio that was pretty common in those years. Double check and See if there is still a tag on the cover bolts. If the tag is gone the same number is stamped on the rear of the pass side axle tube. Your looking for a 7 digit number that starts with a 6.

Example..... 603322-5 The first 6 numbers designates the make and model. The last number designates the gear ratio. Without knowing if anyone has done a ratio change over the years it might be a good idea to pull the rear cover and verify the ratio.
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Old 11-15-2010, 05:10 PM
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I didn't see a tag but will double check. What is the code for the last digit? Would 5 be a 3.54?

How much difference would I see going from 3.54 to 3.77? I could go down a tire size or two to get there.
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Old 11-15-2010, 05:55 PM
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Quote:
Originally Posted by Mertz
I didn't see a tag but will double check. What is the code for the last digit? Would 5 be a 3.54?

How much difference would I see going from 3.54 to 3.77? I could go down a tire size or two to get there.
The number I gave you was just a example to show you what to look for. That number does happen to go to a 1972 Dana 60 w/leaf springs,NON posi and a 354 ratio. Honestly I just picked one from that vintage, so If that's the correct number that would be a stroke of luck. Double check and make sure it has 10 bolts on the rear cover. Again if the tag is gone look on the back of the Pass side axle tube. The number will be stamped there also. I'm not aware of a 3.77 ratio.
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Old 11-15-2010, 07:30 PM
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I will check in the morning on the bolt count, to dark now. The 3.77 comes from a calculator that converts tire size and existing ratio to a new ratio with a new tire size. The tire size I am looking at adds .24 to the existing ratio.
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Old 11-16-2010, 08:51 AM
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Definitely a 10 bolt cover. Could not get under it to find a number.
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Old 11-16-2010, 09:45 AM
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If there is 10 bolts, you more than surely have a Dana 60 rearend in there. They did come with that truck originally and some had the 12 bolt as well. The gearing options for that year and Dana 60 rearend was either 3.54 or 3.73 gearing for those years. Did you ever try the turning the wheel and counting the revolutions of the driveshaft method?
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Old 11-17-2010, 09:11 AM
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I will look for the marking and/or turn the wheel this weekend. The truck is outside and it is dark when I get home.

Would a different torque converter help my pulling power? A new one was installed when the transmission was rebuilt but I think it is stock. I don't seem to have much power around 35 mph.
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Old 11-17-2010, 09:56 AM
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I believe a change to a 3.73 rear ratio will wake it up, w/o going overboard. Too much gear will have it screaming at highway speeds and using a lot of fuel, unnecessarily. If it already has a 3.73, you're options are few, w/o breaking out some cash.

But a gear swap AND a trans swap to a 4-speed w/OD would rock. Sure, w/4.11 rear gears, first gear becomes a granny gear (11.26:1 or 12.58:1 depending on if a TH2004R or TH700R4, respectively), but 4.11's become high-2's (2.75:1 to 2.88:1, respectively) in OD.

Personally, I don't see changing tire diameter being enough, at least not just a 2" difference. Now, if you can get some low aspect ratio rubber on the back- 35 to 40, say- this might give you enough. But tires and wheels for a deal like that can run into some serious coin, depending on what you opt for.

And the load ratings have to be considered as well. Going to a "shorter" tire, it won't be hard to get a speed rating high enough- but the load rating may be another thing.
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