C-20 highway rpm's

[Indy72]

With the original set up on a 72 C-20,350-TH350-Eaton rear end.What rpm's are you pulling on the highway going 60-65mph?I just drove mine the first timesince getting it yesterday on county roads running about 40-45mph and it was running right under 2000rpm.Have any of you just left yours stock and run it that way or did you change the rear end gears or upgrade to an overdrive transmission? Thank's

[Longhorn Man]

you should have a 4.10 gear, which is the best you'll find for that rear end. The only other common ones are 4.56 and 5.13 There were some aftermarket 3.90, but odds are you'll never find one.
You can run it 2800 to 3200 RPM on the freeway for as long as you want assuming the engine is in good to decent shape. If the engine is already on its last leg, then maybe you should keep it on the surface roads.
You can swap the trans out for a 700R4 or a 200 4R, to do it right, you'll spend some decent money, or you could swap out the rear. The big block got a dana rear with 3.54 gears, both leaf and coil style. My burb had this and it cruised freeways at a decent RPM. (no tach, couldn't say, but I would guess 2600-ish. This rear is also quite lame and weak with axleshaft that can break if big tires, or some good engine mods done. Stock setup though, it is a great rear that is easy to swap in.
If you can weld, or are willing to pay/trust someone to do it right, the later 14 bolt rear is all but indestructable, is as common as parking lot gravel, and has a wide range of gearing both stock, and aftermarket.

[bobs409]

I think mine runs low 2K's (22-24?)but I have an SM465 trans if that makes any difference. Mine has the 4.10 gears and I have tall tires, I think 9.75 X 16.5's.

My C20 loves the big highways! Much nicer than taking my C10 with the 3.73 rear.

Seems odd that a 4.10 rear in the C20 runs lower RPMS than the C10 with a 3.73?

[cdowns]

the heck with the board rules about double postings//post it in as many places as you can you're special

[Indy72]

Originally Posted by cdowns

the heck with the board rules about double postings//post it in as many places as you can you're special

My posts ask two different questions in what I thought was the proper places.My first one is in the engine and drive train section asking about the Eaton rear end,the second is asking others who have a similar setup what they did.But if I did break any rules I have no problem with them being deleted.Thank's to all the replys.

[Mike C]

Originally Posted by bobs409

Seems odd that a 4.10 rear in the C20 runs lower RPMS than the C10 with a 3.73?

Not really. It's all about the tire circumference. That and assuming your C10 has an automatic, the converter slippage can add as much as 20% to the rpm depending on the converter.

[treveiger]

I have a sm465 with a rear end like yours and I ran mine 80mph for 350 miles the day i got it without a problem.
Posted via Mobile Device

[bollybib]

From what I've read here, a 0-4000 rpm tach would be OK if I want to monitor my engine RPMs ?

I rarely get on the freeway so I probably won't be going any faster than 65 mph most of the time. I was thinking about an Autometer 2890 http://www.summitracing.com/parts/ATM-2890/

I have a C20/350/TH400/H052@4.10

[webfoot]

Maybe OK for a diesel, probably a lot more choices for one that goes to 8K or so.

[Slowtruk]

My C20 is an 85, but same setup nontheless. 350/400.4.10's.

It runs at 3,000 rpm at 70. No problems. I drive it everyday. But the gas mileage is 11 average. The engine is freshly rebuilt by me 10,000 miles ago and the trans is a AC-delco crate.

I personally didnt want to castrate it by different gears and ovedrive. It wasbuilt this way, It could tow a house and I want it that way! lol. I just fill up both tanks and dont thing about it!

[bollybib]

I've been getting a little over 11 mpg so far. Sounds like 4k rpm is an appropriate range for what I have. With an 8k, the needle would stay in a very narrow range at the bottom of the gauge most of the time.

[treveiger]

Get a 8 with a full sweep

[special-K]

I've been driving C and K20s for over 35 years anywhere I need to go. No one had overdrive on anything until more recent years and that's back when speed limits were generally higher. At least here in Maryland it was 70 and now no more than 65. I've driven on interstates all day long at 75-80 mph and the truck was very happy. People are just used to having overdrive and are accustomed to hearing a lower rpm drone. With overdrive now,people are also driving faster. I was caught up in wanting o/d and have pretty much decided that non-o/d transmissions are more reliable,don't cost a cent to leave alone,and if I want to save on fuel I'll stay in the right lane and drive a safer more sensible speed and feel good about not being on pace with what seems to be an impatient and rushed world. It's very relaxing . I will only go with an o/d transmission in an old truck if it's built specifically for traveling. I don't have time to travel more than once or twice a year so I guess that'll be when I retire.

Oh,just for an example...I had a '71 C/20 and drove it from Maryland to The Grand Canyon,as well as up and down the east coast on camping trips for 18 years. The truck is gone and I still have the unrebuilt motor.

[RUSHNBOBO]

Originally Posted by bobs409

I think mine runs low 2K's (22-24?)but I have an SM465 trans if that makes any difference. Mine has the 4.10 gears and I have tall tires, I think 9.75 X 16.5's.

Nope: the sm465 makes no difference neither do any other non o.d. transmissions
"drive" is 1:1 ratio in autos and manuals

I'm running 4:56 Eaton ho72 (load bolt model) in my dually 31.7 inch tall tires w/turbo 350 TCI super street fighter trans and it runs right about 3200 at 70. In my situation this is fine because the truck only sees less than 1000 miles a year.

If your going to be putting on a lot of miles look into the od auto or if you got the bucks the ultimate is a Gear Vendors od behind a strong turbo 350 or 400.
This way you get gear splitting capabilities ...which makes the 3 speed in to a virtual 6 speed on demand. The unit is small, light and ultra strong.

One last bit of interesting info to remember about the turbo 350 is that it is exactly the same length as the 200r4 and uses the same yoke, so it's a simple swap. Don't be fooled about the 200r4 vs 700r4 as far as strength goes, gm used these behind the turbo grand national buicks and gnx's. ....they can be built to handle huge horsepower and have more performance oriented close ratio gears than the 700r4.

I still love the Eaton rear "banjo" axle, they just look cool and beefy. You can still get all the replacement parts except for ring and pinion sets and dually 13 inch brake drums. Seals, bearings, gaskets and rebuild kits are avail. Unless you have to have those highway gears I'd leave the Eaton right where it is.

[sawyerrm]

Originally Posted by special-K

I've been driving C and K20s for over 35 years anywhere I need to go. No one had overdrive on anything until more recent years and that's back when speed limits were generally higher. At least here in Maryland it was 70 and now no more than 65. I've driven on interstates all day long at 75-80 mph and the truck was very happy. People are just used to having overdrive and are accustomed to hearing a lower rpm drone. With overdrive now,people are also driving faster. I was caught up in wanting o/d and have pretty much decided that non-o/d transmissions are more reliable,don't cost a cent to leave alone,and if I want to save on fuel I'll stay in the right lane and drive a safer more sensible speed and feel good about not being on pace with what seems to be an impatient and rushed world. It's very relaxing . I will only go with an o/d transmission in an old truck if it's built specifically for traveling. I don't have time to travel more than once or twice a year so I guess that'll be when I retire.

Oh,just for an example...I had a '71 C/20 and drove it from Maryland to The Grand Canyon,as well as up and down the east coast on camping trips for 18 years. The truck is gone and I still have the unrebuilt motor.

Read this old thread and thought I would check for my understanding. . .

Picking up a C20 next week in Fresno and driving back to Redwood City. Truck has 327, granny-4speed, p265/75 R16 and I assume the stock rear end 4.10?. The drive home is 180 miles of all interstate and flat except for the Altamont Pass (Rolling Stones / Hell's Angels ) which is 6-8% grade for three miles. From what I've read, I can stay to the right and if I stay up with traffic ~65 mph, I won't do any harm to the engine and just burn a lot a gas. Fill up every 100 miles and I should be good, right?

After getting home, I can tweak the drive train to get it suited for a daily drive commute truck.

[SS Tim]

If that is a stock 68 C20 it could very well have the standard 4.56/4.57 gears (see the are power train charts linked in my signature).

[sawyerrm]

Originally Posted by SS Tim

If that is a stock 68 C20 it could very well have the standard 4.56/4.57 gears (see the are power train charts linked in my signature).

actually the truck is a 1972 but same possibility applies. So if it is a 4.56 the rpm would be even higher, right? So if I were uber cautious I should trailer it back to RWC? I was hoping to drive down with a friend and then just drive home.

[sawyerrm]

Originally Posted by SS Tim

If that is a stock 68 C20 it could very well have the standard 4.56/4.57 gears (see the are power train charts linked in my signature).

I found the bare bones SPID



so if 4.10 was standard, and there's no mention of any other rear end, it's probably a 4.10? Of course, after 40 years who knows what has changed. After all, the 350 is gone too.

so back to my original question, driving ~65 mph on the road home won't hurt the engine?

Rob

[SS Tim]

So wish people would not use off site hosted images.

Anyway since all I see is a red X I have to go on what you wrote.
Yes 4.10 gears would be in a factory 350/four speed 72 C20.
The RPMs are not a problem at reasonable speeds, it is more a matter of the trucks condition. If is was well maintained and driven you should be OK. On the other hand its foolish to expect a truck that has been sitting for a year or two to be problem free. Success all depends on what you start with.

[sawyerrm]

Originally Posted by SS Tim

So wish people would not use off site hosted images.

Is this better?

Rob

[Alex V.]

With 4.10's, no. 4.56's will have it wound pretty tight at 65 but if the engine is healthy it won't phase it. Manufacturers didn't start installing faster gears and overdrive transmissions for durability, they did it for fuel economy - any of the Chevy small-or big-block V8's will hum along at 2,700-3,000 RPM for thousands of miles. I've personally put somewhere between 15 and 20,000 miles on my '85 C3500 on interstates doing 60-70 MPH for 70-400 miles at a time, and the truck runs better after such a trip than when I started. That's with a TH400, 4.10 gears (in a 14 bolt), and 32" tires (235/85-16). My '67 Suburban's RPM/speed ratio is nearly identical, but it's 3.73's with 30" tires and an NP435. The 'Burb feels ever so slightly less strained at 65, but both really hit a sweet spot past about 55 and feel well within their limits to 70+. I've spent some time in two non-Vortec pickups with overdrive transmissions (both TBI 350/700r4 K20's) and they pale in comparison. Prior to the refinement of the Vortec-era trucks, 3.73's or 4.10's is where it's at for any 3/4 or 1-ton.

[GTSideSwipe]

Originally Posted by Indy72

With the original set up on a 72 C-20,350-TH350-Eaton rear end.What rpm's are you pulling on the highway going 60-65mph?I just drove mine the first timesince getting it yesterday on county roads running about 40-45mph and it was running right under 2000rpm.Have any of you just left yours stock and run it that way or did you change the rear end gears or upgrade to an overdrive transmission? Thank's

I have a 69 C/20 350/TH400 with the HO52 4.10:1 rear end and every day I go down the same road and I'm at a solid 40mph @ 2000 rpm. I don't recall what rpm I'm at going 60. I'll have to check tomorrow.

[Alex V.]

Originally Posted by GTSideSwipe

I have a 69 C/20 350/TH400 with the HO52 4.10:1 rear end and every day I go down the same road and I'm at a solid 40mph @ 2000 rpm. I don't recall what rpm I'm at going 60. I'll have to check tomorrow.

Theoretically it'd be an even 3,000.

[Layne]

My 68 C20 350/400 with 30.5" tires runs 3000 rpm @ 50mph
I assume 4.56 gear. Pretty snappy on the bottom end, but highway is a killer. Takes a while to get a decent distance @ 50 mph.

[GTSideSwipe]

Originally Posted by Alex V.

Theoretically it'd be an even 3,000.

True but if I recall correctly it was more like 3300 rpm...