1970 C20 cruising speed???

[65standard]

What are you guys with stock C20s able to cruise at? Mine with a 4-speed and 4.57 gears with 32" tires seems to be peaked at 50-55 mph. I don't have a tach, so I really can't tell how much the motor screams. And I can't see this truck ever going over 70mph before it blows up.

[rustrunner]

My dad had documented the Speed/RPM on the truck I have. 70 C20 Custom Camper, 4 speed, with 4.10 gears, 9.50 16.5 tires (I think are 31.5" Not sure)

60 - 2780
55 - 2650
50 - 2500
45 - 2350

You should be able to get close

[65standard]

Thanks for the information. I could live with those numbers. But my 4.57 gears are going to be spinning the motor a bit more.

[special-K]

I had a '71 with 4.10s and 33s on the rear. I've run that from MD to SC 75-80 mph the whole way down I-95. I took it to Grand Canyon with the camper on. I probably ran 65-70, maybe 75 that trip. I assume it had 4.10s since it was a small block automatic. It had leaf rear/Dana60, but only 402s were supposed to come with 3.54s

[swamp rat]

350/350 K20 with 4:10's 60mph is about 2300rpm, i have cruised the freeway at 75 mph many times.

EDIT: I stand corrected, 60 is closer to 2600, 65 is about 28 or 2900..

[Mmorgan32]

My 70 c20 at 70-75 is at about 3 grand. It's a turbo 350 tho I've gotten it to about 90-95 trying to pass a big rig to make my exit

[Ol Blue K20]

I would hook up a after market tach just to see your actual rpm. If it's too high, check the parts board for some 4:10 gears. Another option is to find a Dana 60. You have more gear options with the Dana.

[BlacklandCamper]

I'm at 2700rpm at 60mph, it will go faster, but it doesn't feel like it enjoys it. 60 is about the sweet spot for cruising. That's with a TH350, and 4.10 geared Dana 60, with 285/75-17's.

[nevada]

I will get a tach when I getting a new wiring harness installed but my 72 c20 350/th350, dana 60 with 235/85/16 tires, it sounds fine at 70 mph, doesnt sound like its revving but hell, with all the wind and cab noise I could be wrong?

[davepl]

That's when the 4L80E eases itself into fourth like an old man slipping into a warm bath, and the motor settles down to about 2000rpm for a nice cruise...

...or it keeps screaming because there is no fourth gear! All of which is to say "4.56s without overdrive is a lot of gear". Mine's a 1970 C20 as well, Dana 60, 4.10s, but overdrive.

[In The Ten Ring]

My '72 with 250 and 4.11 3OTT is happiest around 45 mph. At 65 mph, you can actually watch the fuel hand go down. No joke, I did it once.

Tachometers are nice but unless one is driving a race car or flying an airplane, it really isn't necessary.

[jeffahart]

Also, are you running a flex fan; correct schroud. With 4.56 your getting into high Rs for sure. But make sure you look at other things that make noise at high Rs too. I'm running 4.56 with 33s I can run 60mph real smooth. I've slowly worked out issues that make the thing noisy at speed. And getting a new fan clutch and schroud helped me considerably. I moved from the 4.10s to 4.56 with my axle swap, besides higher Rs I also feel like the harmonics are worse with 4.56s. If I insulate and sound proof, I think I could quite possibly live with the lower gears and 33"s now. Plus I have 4" lift and could go 35"s if I want.

I know this does not answer your RPM issue, but it's easy to equate some of the noise at high Rs with... "She's gonna blow".

That's said... I threw a rod through the side of the block on this exact truck back in the 80s (she did blow). I ran it, and beat it, like a rented mule. It still has that re-manufactured engine with over 100k of 4.10 miles

[nevada]

I've only put on a few hundred miles since purchase and only plan to drive about 2-3 thousand per year so dont know if I will change anything just yet, first tank check was 10.5 mpg but I was driving hard putting it through its paces since purchase. going to keep the 350/TH350 configuration and will continue running tall tires.

[jkeady]

This guy...

http://www.crawlpedia.com/rpm_gear_calculator.htm

...says you're probably turning around 3300 RPM at 70. Not destructive, probably drinking plenty of fuel. In my experience, if you have a loud exhaust and / or a cab that has a lot of wind noise, it can make it seem like you're turning a lot higher an RPM than you really might be.

EDIT: Just realized I replied to the 2015 first post. Whoops. Leaving the link for reference.

[Jarvis-69 GMC 1500CK]

No idea my gears in my 69, but when i am saving gas/don't need to go anywhere fast 50 - it does cruise decently at 60-65, anything over 70 sounds like ( and you probably are) murdering it.

I also don't know what rpm - no tach but At 50 it purrs (20-24 mpg) in 3rd, 60-65 is 15-18mpg it is pretty screamy -65-70 my secondaries open on my qjet and probably about 2 mpg

Pretty sure I am very low geared though, my backend torque bounces when you step on the throttle with a tired old 305 lol

I also got no muffler so my sounds are off compared to other trucks. High idle the thing is comparable to a large diesel in volume. At 50 it is just quite enough to yell at your passenger, once those QJet secondaries open up so do the gates of hell and it is as loud as a top fueler lol

[factorystock]

Old post, but same old problem with C 20s. 4.56-4.57 is a pulling ratio, was the standard axle ratio on pre '68 C 20's. Great for low speed hauling or driving in San Francisco. It all boils down to what you want to spend vs. acceptable crusing speed. Besides taller tires and overdrive transmissions, the cheapest simpliest minor improvement is a 4.10- 4.11 ratio pumpkin swap ( if you have round Eaton type 3rd member). Second simpliest better improvement is a Dana 3.54 complete axle swap out of a big block '68-'72 C 20. If you have a coil rear, find a donor with coil springs set up, you'll need the panhard bar style Dana 60. If you have a leaf spring C 20, look for a leaf spring C 20 donor. Any other setup with newer axles will require welding skills.