4.56 gears K2XX 4x4 L83

jumpinjoe · October 26, 2019

2 hours ago, Jacoby said:

I’ve got 4.56 gears with 35” tires and at 80mph I’m at 2500rpm

I can believe that. Is there any OD involved or are you at 1:1 in high gear? The biggest thing is the tire dia ..... I'm sure those tires we had back in the day were probably like 28" dia max if I had on some 8.25/15's. With 8.00/15's or even 7.80/15's dia would have been even less. Without doing some calcs, I don't know rightly just how much difference it would be from a 28" dia tire to a 35" dia tire, but probably substantial.

And I'm trying to remember car facts back to the early 1970's when I typically have a hard time remembering what I had for breakfast yesterday. But it seems like the RPM would be up near or better than the 4K mark at highway speed. Now that I think about it, "highway speed" might have been a little more than 55mph too. It's just really hard for us "old farts" to remember exact details sometimes. LOL! But I enjoy talking old times and comparing notes to the new.

Regards,

jumpinjoe

300 Blackout · October 28, 2019

I can believe that. Is there any OD involved or are you at 1:1 in high gear? The biggest thing is the tire dia ..... I'm sure those tires we had back in the day were probably like 28" dia max if I had on some 8.25/15's. With 8.00/15's or even 7.80/15's dia would have been even less. Without doing some calcs, I don't know rightly just how much difference it would be from a 28" dia tire to a 35" dia tire, but probably substantial.
And I'm trying to remember car facts back to the early 1970's when I typically have a hard time remembering what I had for breakfast yesterday. But it seems like the RPM would be up near or better than the 4K mark at highway speed. Now that I think about it, "highway speed" might have been a little more than 55mph too. It's just really hard for us "old farts" to remember exact details sometimes. LOL![emoji6] But I enjoy talking old times and comparing notes to the new.

Regards,
jumpinjoe

I’m at 2250rpm at 70mph with 34s. That’s in 6th gear which is an OD gear at 0.67.

I broke down the numbers you’re curious about below. To get the overdrive RPM for 6th gear in a 6L80 just multiply whichever engine RPM you are curious about by 0.67.

———

28” tires need 720 tire revolutions per mile. 50,400 tire revs to go 70 miles in an hour.

3,830 engine rpm with a 1:1 trans gear and 4.56 rear gears to travel 70mph.

3,133 engine rpm with a 1:1 trans gear and 3.73 rear gears to travel 70mph.

———

35” tires need 576 tire revolutions per mile. 40,320 tire revs to go 70 miles in an hour.

3,064 engine rpm with 1:1 trans gear and 4.56 rear gears to travel 70mph.

2,506 engine rpm with 1:1 trans gear and 3.73 rear gears to travel 70mph.

KARNUT · October 28, 2019

I can believe that. Is there any OD involved or are you at 1:1 in high gear? The biggest thing is the tire dia ..... I'm sure those tires we had back in the day were probably like 28" dia max if I had on some 8.25/15's. With 8.00/15's or even 7.80/15's dia would have been even less. Without doing some calcs, I don't know rightly just how much difference it would be from a 28" dia tire to a 35" dia tire, but probably substantial.

And I'm trying to remember car facts back to the early 1970's when I typically have a hard time remembering what I had for breakfast yesterday. But it seems like the RPM would be up near or better than the 4K mark at highway speed. Now that I think about it, "highway speed" might have been a little more than 55mph too. It's just really hard for us "old farts" to remember exact details sometimes. LOL![emoji6] But I enjoy talking old times and comparing notes to the new.

Regards,
jumpinjoe

It wasn’t unusual to cruise around near 3K RPMs back in the day. Even as late as 1988 I can remember a diesel one ton topping out at 74 mph, and later adding gear vendors. I had several cars and trucks with 373s and 410s. I never left 273s in some of the Impalas I owned back in the day. I learned to set up a Chevy ten bolt long before I rebuilt my first engine.

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M-Wheeler · October 28, 2019

On 10/23/2019 at 7:13 PM, Kmbz71 said:

I got a quick question. I recently bought a 2016 Silverado 1500 z71. Its got all the bells and whistles but after doing some digging i found out the previous owner had 4.56 gears installed. I bought this truck from a Chevrolet dealership and i think they didnt know about the gear swap. Im speculating but do you think the dealership would have wiped out the aftermarket tune for the gear swap and reset it back to default? The truck will get up and go but there are times that if i really hammer on it especially when i go to make a pass the rpms get so high so quick the engine will cut off. Its happened more than id like to admit. The truck has 35's on 22's and drives like a dream otherwise but sometimes i wanna let it sing! Do you think i need to take it to a tune shop? Is this normal with 4.56's? First photo is without tows second photo with tows.

I had a 17 5.3 6spd with 4.56 gears. 6" lift, 20's with 35's. Traded it for a Callaway SC560

Mine used to cut off in between 1-2 shifts using a handheld tuner.

Once I had a professional tune done to the ECM and TCM, the shifting was perfect and never cut out again.

I do not pretend to know more than I think I know, but I think if the speed calibration in the tune does not pair indentically ECM to TCM it might be causing the shift cutout at very high RPM. But I will leave that there for the more experienced tuners to chime in.

jumpinjoe · October 28, 2019

15 hours ago, 300 Blackout said:

I’m at 2250rpm at 70mph with 34s. That’s in 6th gear which is an OD gear at 0.67.

I broke down the numbers you’re curious about below. To get the overdrive RPM for 6th gear in a 6L80 just multiply whichever engine RPM you are curious about by 0.67.

———

28” tires need 720 tire revolutions per mile. 50,400 tire revs to go 70 miles in an hour.

3,830 engine rpm with a 1:1 trans gear and 4.56 rear gears to travel 70mph.

3,133 engine rpm with a 1:1 trans gear and 3.73 rear gears to travel 70mph.

———

35” tires need 576 tire revolutions per mile. 40,320 tire revs to go 70 miles in an hour.

3,064 engine rpm with 1:1 trans gear and 4.56 rear gears to travel 70mph.

2,506 engine rpm with 1:1 trans gear and 3.73 rear gears to travel 70mph.

That sure sounds about right. Even with the 28"'s, your calcs show about 3800rpm at 70 and we might have run just a tad over 70mph back then. LOL. And also likely is that I probably had even 27"'s on as often as not. I know it wasn't really smart but I sure did love hearing that little small block scream for mercy. But back then I was probably too stupid to know just how bad I was hurting it. That was the motor that finally changed me from a big block fan to a small block lover. Before the 302ci Z28 we put in my '55, I had a '63 409 w/2 4bbls @ 425 hp. Had a good bit of fun with that too, but think most of that time was with the 3:73's. Rpm was reasonable most of the time with that combo. Damn, hard to believe just how far back that goes.

Thanks for the numbers and conversation. Man, sure love me some of those old 'fart' muscle cars from back in the day.

Regards,

jumpinjoe

jumpinjoe · October 28, 2019

7 hours ago, KARNUT said:

It wasn’t unusual to cruise around near 3K RPMs back in the day. Even as late as 1988 I can remember a diesel one ton topping out at 74 mph, and later adding gear vendors. I had several cars and trucks with 373s and 410s. I never left 273s in some of the Impalas I owned back in the day. I learned to set up a Chevy ten bolt long before I rebuilt my first engine.

Sent from my iPhone using Tapatalk

Fond memories of all those GM 10 bolt's we tore up and scattered all over the roads back then. (Ha Ha). Not fun putting everything back together, just fond memories. LOL!

Ridintheberm · May 29, 2020

On 10/23/2019 at 7:13 PM, Kmbz71 said:

I got a quick question. I recently bought a 2016 Silverado 1500 z71. Its got all the bells and whistles but after doing some digging i found out the previous owner had 4.56 gears installed. I bought this truck from a Chevrolet dealership and i think they didnt know about the gear swap. Im speculating but do you think the dealership would have wiped out the aftermarket tune for the gear swap and reset it back to default? The truck will get up and go but there are times that if i really hammer on it especially when i go to make a pass the rpms get so high so quick the engine will cut off. Its happened more than id like to admit. The truck has 35's on 22's and drives like a dream otherwise but sometimes i wanna let it sing! Do you think i need to take it to a tune shop? Is this normal with 4.56's? First photo is without tows second photo with tows.

Did you ever get this fixed? I just did 4.56 gear swap and mine is doing the exact same thing!