1970 C20 geared too low
 

Hi all,

Can anyone tell me which differential this is? It has the large oval, smooth cover. Also, is there a way of changing to a higher gear ratio? Sorry--I tried rotating the photo before I uploaded it--perhaps it's too large?

Re: 1970 C20 geared too low
 

Like this. 11:41

41÷11=3.727. Commonly referred to as 3.73's

Or 40X13

40÷13=3.076. Commonly referred to as 3.07's or 3.08

Can you give us more information on the truck?

Have you looked at the SPID on glove box door? It may specify the rear end it left the factory with.

And Welcome Aboard!

Re: 1970 C20 geared too low
 

Looks like a Eaton ho52 rear I believe the highest gear was 3.91 my 68 has 4.57 gears . And from what I've read or heard finding a 3.91 is damn near impossible .

Re: 1970 C20 geared too low
 

I believe most of the 20's had 4:11 gears except the BBC trucks

Re: 1970 C20 geared too low
 

C20's with the H052 could get 3.54 with the BBC
C30's with H072 could get 4.10 with BBC. That's as high as they go.

Re: 1970 C20 geared too low
 

Pretty certain 3.54 is a dana 60 ratio not HO52.

Re: 1970 C20 geared too low
 

That is correct.

Re: 1970 C20 geared too low
 

About 20 years ago there was an aftermarket company that made a limited supply of 3.90 gears for HO52 rears, but they're no longer in business.

Re: 1970 C20 geared too low
 

Thank you all for the quick reply and great info. I will pull the cover and find out what I have.

Re: 1970 C20 geared too low
 

Both of my 20s are geared low . My 72 has 4.11 with the Dana 60 and my 68 with the Eaton ho52 has 4.57 . With 31.6 inch tires it's really not to bad with the 4.11 I can maintain highway speeds with no problem the 4.57 with th 6 banger is reving pretty good at 60mph but it's still doable. Much different than what my Chevelle was like with the 4.11 gears and smaller diameter tires I changed them out to 3.54s .


My 85 K20 has 3.23 gears and a 700r4 . That truck will cruise at 80-85 all day . Takes a bit to get to speed though

Re: 1970 C20 geared too low
 

So I assume that's why c20's used both H052 and Dana60? Some ratios weren't available in the other axle?

Re: 1970 C20 geared too low
 

Pretty sure all C/20 big blocks (after 1970 anyways) came with 3.54 gears.

Re: 1970 C20 geared too low
 

What a shame. Must have been poor marketing at the time, a lot of C 20's could have used the upgrade. I surely would have purchased them if I would have known about them. Never seen them advertised. Most '48-'68's 3/4 tons are 4.57 HO52's, '69-'72 I would say 50/50 4.11-4.57 for trucks that came with a H052. A 3.70 ring gear set would have been cool too, but probably never done.

Re: 1970 C20 geared too low
 

My 72 C20 Cheyenne Super had factory a 3.54 posi unit. But I changed it out due to the clutches burning up. I changed it out for a Yukon 4.10 and a locker. My 32" tires and Gear Vendors overdrive unit allows me to cruise at a nice 70 mph on the highway with no problems.

Re: 1970 C20 geared too low
 

If I’m not mistaken the reason the 3.90 was the highest gearing made for the 52 differential was that the differential itself couldn’t accommodate the larger ring gear than the 3.90

Re: 1970 C20 geared too low
 

That is correct.

Re: 1970 C20 geared too low
 

If you have a manual transmission, change away. If you have an automatic transmission, caution needs to be taken so as not to gear out of your stall speed range of the torque converter. Going to a lower numbered rear can put up you in a range that keeps you in the stall range while driving in town. As I found recently, this will cause the converter to run on the hot side of things. If you only monitor the tranny pan with a gauge, you will miss the hot converter because our radiators do pretty good at cooling tranny fluid.

Re: 1970 C20 geared too low
 

My 1972 402 Cheyenne Super 20 has an Eaton rear with 4.10 ratio as per the SPID.

Re: 1970 C20 geared too low
 

With the proliferation of overdrive transmission, it is common these days to change the transmission, not the differential ratio.

Almost impossible to reduce the motor rpm by 30% by changing the differential ratio.
Easy with the overdrive. Highway rpm drops from 3000 to 2000.

By changing the transmission, you retain the low-end grunt and pulling power and significantly reduce interior noise.
The newer 8 and 10-speed transmissions often have three overdrive ratios.

Re: 1970 C20 geared too low
 

More than likely you have 4.57 gears in that Eaton, as the 4.11's are harder to find. That's gonna make it harder to get highway cruising, but swapping the transmission is the easy choice if you want to leave the Eaton. The truck will be much cooler with that rear end, but make sure you know what you're getting into with brake, bearing, seal components. They are not prone to fail, just hard to find.

Re: 1970 C20 geared too low
 

I found this 10 year old thread that talks about the Eaton HO52 gears. https://67-72chevytrucks.com/vboard/...d.php?t=625981 It mentions the very, very limited production of "Randy's R&P 3.90 gear sets". That thread is a good read and offers some additional info.

My '67 already has a Dana 60 so a switch from 4.57 to 3.54 was relatively easy and junk yard cheap for a Dodge carrier and gears. The difference between Dodge and GM Dana is the yoke. Hoopers Gear in Sun Valley, CA mentioned in that thread, had a yoke to make the switch.

Most people seem to prefer to switch to an OD transmission. I have considered a 5 sp manual, but only because the 3 to 4 spread is high when towing on a hill with the sm465 and 3.54 gears. When not towing the 292 enjoys it just fine. Don't need OD with 3.54 gears, at least for my truck.

Re: 1970 C20 geared too low
 

Water under the bridge, but the real problem was GM, or the other big 3 auto manufactures. There should have been a 5 speed overdrive manual transmission offered in 3/4 tons and 1 tons back in the day. They only offered light duty 3 speed overdrives that weren't designed for heavier vehicles. International did offer a 5 speed overdrive boxes in their pickups. This would have been a complete bolt in, just a shorter driveshaft would have been necessary and speedo would have threaded right in, no electronic B.S. The .8 5th gear in the'69 Chevy C50-60 brochure charts is just right for a heavier vehicle. The .7 5th gear, that is common in most modern overdrives is, IMO, too high of a reduction in rpm. The bottom chart, the CH 465 and New Process offerings didn't help, 1:1 final drives.

Re: 1970 C20 geared too low
 

-
If you have an SM465 4 speed just do this.

https://www.67-72chevytrucks.com/vbo...d.php?t=673588

You can do it to replace an SM460 too but you have to use the SM465 bell housing.

That way you still have your low 1st gear but also overdrive.
.

Re: 1970 C20 geared too low
 

Thank you all for the great ideas. I was looking at the price of the NV4500's and I think I am going to go with larger wheels. I saw where 72c20customcamper has 31.6 inch tires on his C20. Will that make a noticeable difference? have 8 lug steel wheels on it now. I don't have the truck here now and I don't remember the tire size. They are Bridgestone Commercial. Also, I just rebuilt the SM 465 and it works flawlessly so I'm not ready to give up on it yet.

Re: 1970 C20 geared too low
 

265/75/16 is slightly larger than stock.