Rear gears

[Andyboy]

Ok I know the answer is on here but can't find it. I drove the 68 c20 today and was at 3500 rpm at around 55. It has a 350/350 combo in it. Whats the best way to fix this, swap rearends, and if so which one? Or can I change the gears?

[special-K]

Is the center of the axle housing round? if so,you have the HO52 which allows simple gear changes. You'll need the complete differential assembly. the best hiway ratio is 4.10. It's just a matter of sliding the axles out,unbolting/removing the assembly,replacing with new one,and sliding the axles back in.

[cdowns]

factory service manuals have complete instructions and tools required and specs to change gears properly

[special-K]

Absolutely. But,here on this website where we come to ask or give advise to our fellow members we can give general information painlessly and willfully. I was not offering thorough and complete instructions on how to do this,which is what manuals are for,but simply explaining how simple the full-floating Corporate rears are to do gear changes on compared to most other rears due to the "drop-out" design. There is no "set-up",special skills,or special tools required. Anyone with a lick of mechanical skill can do this. If it were a half ton or Dana60 I would have recommended letting a professional handle it or buy a complete assembly in the desired ratio. I never used a manual to swap my gears because it's so simple all you need is common sense and some degree of mechanical aptitude.

[Andyboy]

Is this the HO52. The SPID says it came with 4:10 ratio

[dmack91]

Looks like the same on in my 68 C20. I was told they are the HO52. Didn't think they cam with 4.11 gears. I heard they came with something in the range of 4.56-5.13 or so.

Great for pulling trains, but a bit low for freeway speeds.

[So.Cal.Super]

I would go here

http://www.richmondgear.com/101032.html

input your information and figure out what you want.

I run a 4.10 with a 33" tire and am about 3k @ 70 on the freeway.

but I can tow a house

IMO it's based on what you want to do with the truck. I don't believe in a perfect gear.

[Stocker]

Originally Posted by Andyboy

Is this the HO52. The SPID says it came with 4:10 ratio

Yup, looks like an HO52... and yes, they came with 4.10 gears. Problem is you can't go much higher (if at all). I think maybe 3.73s but that's about it. To gear it taller, you'll either need to change the rearend or swap out the TH350 for an OD trans.

[special-K]

Originally Posted by Andyboy

Is this the HO52. The SPID says it came with 4:10 ratio

yes,that's it and 4.10s is as good as you'll do. dana 60s are 4.11 and Corporate axles are 4.10. No biggy,but that's where you get two ratios so close. The same for 4.56 for Dana and 4.57 for Corporate.
Anyway,you either have really short tires or your numbers are off. I can run 60 or faster at around 3,000. You can find a Dana60 from these years with 3.54s or make the later 14-bolt work by moving spring pads and shock mounts. Or,just drive at 60-70 and listen to her sing. It won't hurt. I like 4.10s and run my trucks at 75-80. Afyter running overdrive vehicles for a while it does feel like it's being pushed. But,that's all we had for years and never thought a thing about it. My '92 with overdrive (5spd) has 4.10s and I love it. I've caught myself on the hiway,after stop and go,running most of the way home in 4th without thinking about it because I'm used to the old stuff.

[Andyboy]

Well if it won't hurt the engine or blow up the rearend, then I'll run it. Thanks special-k

[Longhorn Man]

a 350/TH350 should already have the 4.10 gears (assuming still stock) and as said, that's about the best you'll get. There used to be an aftermarket gear, I think it was a 3.90, but it's safe to say you'll never find one.
The RPMs do seem high, and while Tim is right, you can just live with it, and they were in fact built to run on the freeway @ 70MPH... however, it does wear the engines faster, and does consume more fuel on most vehicles.
I would recomend either adding an overdrive trans, or swapping to a later rear end with more reasonable gearing. The reality is, putting a 3.73 gear rear end will drop the RPMs quite a bit and you can still haul/drag anything you want. If you have the truck grossed out or overloaded more often than not, then you should probably keep the gearing, but if running empty/lightly loaded, then I would recomend an upgrade. My daily driver C/30 has the same gearing, and I am swapping the rear end for a 14 bolt with 3.73 gears, with any luck, it'll be in there this summer.

[factorystock]

Originally Posted by Andyboy

The SPID says it came with 4:10 ratio

4.10 being on your spid means this ratio was an option. 4.57 was the standard ratio. Your Vin will tell if your truck was a CE=V8 or CS =6cyl originally.

[mr48chev]

That sounds pretty close to what I think my 71 runs at 70 when I run it at 70 which isn't often because there is a big difference in gas consumption between 60 and 70 mph in my truck.

This chart http://www.csgnetwork.com/multirpmcalc.html can be used to calculate the differences in engine rpm with different gear ratios and tire sizes.

I'm running 4.11 gears now with 245 70 16 tires and plan to go to the 3.56 rear end that I have waiting to go in. With the jump in gas prices it may go in sooner rather than wait until I put the Cad 500 in the truck. You don't really need the 4.11 gears unless you pull loads all the time or haul heavy loads or a camper a lot.

I checked mine and with 4.11 gears 30 inch tall tire and 70 mph I would be turning 3309 rpm with 3.56 gears it would turn 2866 which is a 443 rpm drop at 70 for changing rear ends.

[factorystock]

Originally Posted by Longhorn Man

The reality is, putting a 3.73 gear rear end will drop the RPMs quite a bit and you can still haul/drag anything you want.

Good all around ratio for a 3/4 ton that still occasionally hauls. But, the 64,000K question is why GM never offered this ratio on C 20's during this era? Ford did offer this ratio on Dana 3/4 ton rear axles during this era. Its water under the bridge now, but it does make you wonder.

[piecesparts]

Can you provide the diameter of your rear tires. There are calculations that will tell you what your rear end gear is from the info that you already have given.

If you cannot change the rearend gearing, then an OD is the other approach. Just thnk of your RPMs cut down by 30% and that is where you could live.

[Andyboy]

the tires are 9.50R 16.5lt do you need inches from the ground?

[Stocker]

That tire is about 30.6" diameter. If your speedo & tach numbers are accurate, you have some incredibly low gears. Is it possible the trans could be slipping?

http://www.richmondgear.com/101032.html

Known RPM: 3500

@ Known MPH: 55
Tire Diameter (Height) in Inches: 30.6

Estimated Rear End Ratio: 5.80

[Jimmy72custom]

Your tires are the last gearing to the road. The bigger the tire the higher the gear. A larger tire (bigger diameter) is a cheap way to lower RPM. But your speedo will be off. :-) my 2 cents

[Longhorn Man]

probably either an inacurate tach, or a guestimate on RPMs.

[piecesparts]

If the tires are truly a 30.6" tire and you have a TH350 tranny, then according to my calculations, you are running a set of 5.79 gears in the rear end as stated above. I would also question the RPMs from the tach to make sure.

[special-K]

Originally Posted by factorystock

Good all around ratio for a 3/4 ton that still occasionally hauls. But, the 64,000K question is why GM never offered this ratio on C 20's during this era? Ford did offer this ratio on Dana 3/4 ton rear axles during this era. Its water under the bridge now, but it does make you wonder.

GM didn't offer the 3.73s but did offer 3.54s in Dana60s and they haul just fine behind an automatic. My '72 was used as a 5th wheel hauler by the original owner I got it from.

[Andyboy]

Sorry guys I got under the truck and the tranny is a th400, dont know if that makes a difference. I'll run it tomorrow and check the RPM's again. Thanks for all this info guys.

[Stocker]

No problem, and no difference..... both have direct 1:1 output ratio in high gear.

[Blazer1970]

Originally Posted by Longhorn Man

probably either an inacurate tach, or a guestimate on RPMs.

If it is a stock tach it is definitely not accurate. Pretty much all of those old tachs read at least several hundred RPM high.

[Longhorn Man]

Originally Posted by Blazer1970

If it is a stock tach it is definitely not accurate. Pretty much all of those old tachs read at least several hundred RPM high.

they sucked when they were new, add 40 years to them and they are craptastic at best.
I still don't get it why people go ga-ga over them.