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Old 11-23-2019, 12:02 PM   #1
mattshea125
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78 K20 towing with 454

Hello I have a 1978 Chevy K20 that I plan on swapping a big block into with a TH400 transmission. I would like to hear from anyone with some experience on what final drive ratio to go with as I would like it to be able to tow some weight and not be a dog, but have a final drive ratio that isn't screaming too bad a 75 MPH on the highway. The truck has a 2" lift with 265/75R16 tires. I realize that I'm not going to get good gas mileage no matter what, but that's why the truck has 2 fuel tanks right ?

Truck is currently set up with a 350 SB with th350 trans and 4.10 gears and it is really buzzing that 350 at highway speeds.
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Old 11-23-2019, 12:37 PM   #2
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Re: 78 K20 towing with 454

With your current setup (4.10ratio,,31.6tall tire,,1:1trans,,75mph)..your turning around 3270rpm at 75mph
Gear swap sounds like it's in order....or get a 4l80 trans and have the best of both worlds...low ratios and od....the od would get you down to 2289rpm at 75mph
Bad part of gear swap is cost x2 for a k series..
Heres a rpm calculator thatll help you....http://www.csgnetwork.com/multirpmcalc.html
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Old 11-23-2019, 12:43 PM   #3
mattshea125
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Re: 78 K20 towing with 454

I currently have a TH400/208 that I got for a song and dance. Does anyone here have experience with fitting the 208 to a 4L80E? Seems to be lots of questionable information on the web regarding this swap.
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Old 11-23-2019, 01:00 PM   #4
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Re: 78 K20 towing with 454

If... and I mean if...you can find a 90-91 3/4 suburban 4x4...it will have a 4l80/np241 pass side drop..
The 208/241 is a light casting transfer case...I would look for a 205 if it were me.....sometimes cheap ain't the best way
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Old 11-23-2019, 01:46 PM   #5
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Re: 78 K20 towing with 454

Are you towing at 75 mph ? (Gooseneck?)

Can one tow with a 4L80 in overdrive? (not supposed to with 4L60/700r4)
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Old 11-23-2019, 03:30 PM   #6
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Re: 78 K20 towing with 454

https://www.richmondgear.com/wp-cont...alculator.html

This will help you choose a ratio that you feel is the best of both worlds.
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Old 11-23-2019, 08:24 PM   #7
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Re: 78 K20 towing with 454

I have a 82,k-10 with a 454,t-400,373 rear gears. On 12.50x33x15 B.F. Goodrich tires.
I towed a 28' travel trailer all over the country,including the Smoky Mountains, east side of the Rockies and over the continental divide.
Got about 8 miles to the gallon average.
And,yes I towed at 70 mph.
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Old 11-24-2019, 01:54 AM   #8
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Re: 78 K20 towing with 454

I tow with an 8.1/ 80e in my k30 with 4:10 gears, and i'm wanting to go deeper (have an extra set of 4.56 CUCV axles), but I am running 315/75 tires.

with your tire and no OD, 4:10 will pull awesome, but pull at 65mph... at least that's my advice. Even though I can pull 80-100 with my setup, it's stupid to do so. My experience is that trailer tires stay together better when ran under 70mph.

For optimum pulling with a big block, it probably needs to be in the range of 2500-2800rpm in cruising gear while at pulling speed. 3:73 gears would be about perfect for your setup if you plan to stay on those smaller tires. If you go up to a 285 or bigger (rubber overdrive), then the 4:10's you have will be about perfect.
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Old 11-24-2019, 01:48 PM   #9
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Re: 78 K20 towing with 454

From my experience and observation, 70 mph with a bumper pull is really up there at the ragged edge. With a gooseneck or fifth wheel it's really just how fast can your truck go

There seems to be a lot more trailer wrecks in the past few years. I attribute it to trucks that can actually pull a trailer 80 mph. In the past you might be lucky to get up to 65.

Passing a semi going 65, when that bow wave hits the tail end of your trailer well, you better have a really steady hand and tap the trailer brakes to straighten out or you're going to end up in the ditch with a big pile of splinters. Lots of truck dash cams on youtube if you don't believe me.

We were passing a pickup and camper trailer in our little car down by Crow Agency last summer, when he blew a trailer tire. It scared the sh1t out of me! Fortunately, it was his right rear tandem so it didn't swing over into our lane, and he managed to slow it down without going into driver induced yaw. We were going the posted 80 so he must have been doing 70-75. Too hot, too fast, and maybe too old of a tire ?
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Old 11-25-2019, 09:13 AM   #10
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Re: 78 K20 towing with 454

The truck will be used for hauling small construction equipment ( Kubota L47TLB ) around town to different job sites, sometimes on the highway but no long hauls. After reading the input on this post I will probably do the engine and trans swap and I have a GM14 bolt 11.5 rear with 3.73's ill throw in to see if that works a little better on the highway
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Old 11-25-2019, 11:09 AM   #11
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Re: 78 K20 towing with 454

Quote:
Originally Posted by 1976gmc20 View Post
From my experience and observation, 70 mph with a bumper pull is really up there at the ragged edge. With a gooseneck or fifth wheel it's really just how fast can your truck go

There seems to be a lot more trailer wrecks in the past few years. I attribute it to trucks that can actually pull a trailer 80 mph. In the past you might be lucky to get up to 65.

Passing a semi going 65, when that bow wave hits the tail end of your trailer well, you better have a really steady hand and tap the trailer brakes to straighten out or you're going to end up in the ditch with a big pile of splinters. Lots of truck dash cams on youtube if you don't believe me.

We were passing a pickup and camper trailer in our little car down by Crow Agency last summer, when he blew a trailer tire. It scared the sh1t out of me! Fortunately, it was his right rear tandem so it didn't swing over into our lane, and he managed to slow it down without going into driver induced yaw. We were going the posted 80 so he must have been doing 70-75. Too hot, too fast, and maybe too old of a tire ?
I posted that I've towed at 70 mph.Not something I would do all the time,but there are times when you don't realize how fast you are going!
BUT,you must have sway control and weight stabilizers installed and adjusted correctly! Tires with stiff side walls are required.Mine weren't till later after the BFG's wore out. I also had a helper spring on the front axle to help with the weight.
Many times big trucks would fly past,trying to blow me off the road and did it too! I have a C.B. in the truck and could listen to them laughing at me! Manually over-ride the trailer brakes!
Mike.
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Old 11-25-2019, 11:40 AM   #12
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Re: 78 K20 towing with 454

Matt...are you just gonna try the 3.73s to see how you like it? ....if so, just remember to not use 4wd until the front ratio matches..

Towing at 70mph is not a big deal...if your equipped correctly..I regularly pull a 9500lb gross loaded trailer..its a bumper hitch and I pull it with my C30..and yes I also run 65-70...never had any issues..
Towing around town like you'll be doing you should be fine...
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Last edited by mongocanfly; 11-25-2019 at 11:46 AM.
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Old 11-25-2019, 12:18 PM   #13
mattshea125
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Re: 78 K20 towing with 454

Mongocanfly, yes haha. I understand the front axle will need to be re-geared which is no problem. It currently has a 14bolt 9.5 rear which is a semi-float light duty axle.

I got the 454 TH400 and 14bolt 11.5 with 3.73 out of a C20 suburban for free. The engine has less than 4k miles on it. Guy put a bunch of money into the burb and then a tree fell on it during the Sandy hurricane. "get it off my lawn and you can take it " deal.
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Old 11-25-2019, 02:00 PM   #14
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Re: 78 K20 towing with 454

Quote:
Originally Posted by mattshea125 View Post
The truck will be used for hauling small construction equipment ( Kubota L47TLB ) around town to different job sites, sometimes on the highway but no long hauls. ...
Well, that's a lot different than a big boxy camper trailer, assuming you balance it right.

Quote:
Towing at 70mph is not a big deal...if your equipped correctly..I regularly pull a 9500lb gross loaded trailer..its a bumper hitch and I pull it with my C30..and yes I also run 65-70...never had any issues..
It helps a lot if you don't think you can counter steer a sway situation. Lots of people get scared because their trailer wig-wagged and then wreck it when they could have just stayed steady and let it straighten out.

I used to purposely whip my trailer back and forth now and then just to see if there was someone hiding directly behind me in my blind spot. I've pulled everything up to a 10x50 mobile home, but that was with the old V-6 GMC and couldn't get it going that fast, and it was still 55 limit back then. Once I got used to the size, that was about the easiest trailer to pull.
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Old 11-27-2019, 11:09 AM   #15
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Re: 78 K20 towing with 454

Quote:
Originally Posted by 1976gmc20 View Post
Are you towing at 75 mph ? (Gooseneck?)

Can one tow with a 4L80 in overdrive? (not supposed to with 4L60/700r4)
Yes you can tow with a 4L80E in overdrive. That is what is was made to do.
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Old 11-30-2019, 06:29 PM   #16
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Re: 78 K20 towing with 454

The big thing with any A/T trans overdrive is to keep the speed/RPM over the up shift or down shift points. If your going drive just at the speed/RPM where it down shifting every few seconds it will kill any A/t Trans in time. So you should not drive in over drive un till you can keep the over drive speed/RPM in the gear. get a good cooler!
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Was a 77 454 w/308 gear. Taken out 550 lbs. up front with motor clip change.
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Old 11-30-2019, 07:10 PM   #17
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Re: 78 K20 towing with 454

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The big thing with any A/T trans overdrive is to keep the speed/RPM over the up shift or down shift points. If your going drive just at the speed/RPM where it down shifting every few seconds it will kill any A/t Trans in time. So you should not drive in over drive un till you can keep the over drive speed/RPM in the gear. get a good cooler!
I haven't even seen that much mpg difference with OD. One time had to drive up to the RR station in Malta about 200 miles each way and about halfway up there discovered I was still in drive. Still averaged almost 16mpg at ~65mph round trip, which is pretty okay for a 4wd Suburban.

Pulling a trailer over McDonald Pass, I found that it would stay in Drive/high until it slowed down to about 30 mpg then finally shift into 2nd and roar back up towards 50 then shift back into drive and start slowing down again. Enough with that, I would just manually drop into 2nd about 45 mph and hold it there all the way up with no extra throttle.

Even if it wasn't verboten, I can't imagine towing in OD with my transmission. I wish I had a 2500 with the better automatic ...
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Old 12-04-2019, 01:01 PM   #18
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Re: 78 K20 towing with 454

My 85 Suburban C20 454/400 turbo updated to a 4L80E netted about 25% improvement in Fuel mileage towing at highway speed of 65 MPH. Rpm went from 2750 to 1950 with 3.73 and 235/85/16 tires.
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Old 12-04-2019, 01:41 PM   #19
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Re: 78 K20 towing with 454

My old tow rig. 454, TH400, 3.73s, rear helper bags, and 265 tires.
I towed with this combo for years with similar or heavier loads shown at 65-70MPH. Never checked mileage but the motor was in the sweet spot at those speeds.
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Old 12-04-2019, 04:50 PM   #20
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Re: 78 K20 towing with 454

We had a 1977 C-10 Suburban with a small block and SM465. No idea the exact axle ratio, but it was fairly low. Still we towed a camper and a big u-haul trailer up to 65mph on the interstate. As I recall it got about 13 mpg towing and about 18 mpg solo, which seems pretty reasonable.

Now I'm almost sorry we traded it off because it always ran so good and pulled so strong. But it was a rugged brute: manual steering, no headliner or insulation, and the rust that had been painted over before we got it was coming through pretty bad. The 2wd did fine on snow and ice but not so well on muddy Montana back roads.
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