Author Topic: Need confirmation on rear end ratio  (Read 2447 times)

Offline jfkelley

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Need confirmation on rear end ratio
« on: December 08, 2020, 06:52:12 PM »
I have a 1976 Chevrolet Scottsdale 4x4 I was under it for about for an hour scraping the axle tubes looking for factory # s and I wanted to see if someone could confirm my axle ratio for me Thanks in advance

KAH W159 2
« Last Edit: December 22, 2020, 09:26:06 AM by jfkelley »
1976 Chevrolet 4x4 ,K10,Stepside, 400/350/NP203, 35 BF Goodrich All-Terrain, Factory A/C

Offline bd

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Re: Confirmation of rear gear ratio
« Reply #1 on: December 08, 2020, 08:30:11 PM »
In 1976, KAH was originally configured as a 3.07:1 ratio.  Based strictly on the axle code, no one can assure you that the ratio, or for that matter, the entire axle assembly has not been changed.  Your safest bet is to remove the cover and count the teeth.  Divide the ring gear tooth count by the pinion gear tooth count or inspect the gear stamping to determine the ratio.
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline jfkelley

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Re: NEED Confirmation of rear gear ratio
« Reply #2 on: December 08, 2020, 08:46:08 PM »
Bd ..Thanks I also got 3:07 ratio after I researched also ..I was hoping it would be a better ratio so that I could install Overdrive , but with 35 tires and 3:07 ratio might be impossible..I drive it all over the state for hunting was hoping for overdrive 🤦🏼‍♂️
1976 Chevrolet 4x4 ,K10,Stepside, 400/350/NP203, 35 BF Goodrich All-Terrain, Factory A/C

Online zieg85

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Re: NEED Confirmation of rear gear ratio
« Reply #3 on: December 08, 2020, 10:15:38 PM »
3.07 with 35" tires would almost be like an OD ratio.  Your limiting factor is your take off.  2063 RPM @ 70 mph with your set up.  My truck has roughly a 31" tire with a 5 speed with 5th gear a 0.73:1 OD and at 70 is 2270 rpm. https://purperformance.com/p-29669-rpm-calculator.html
Carl 
1985 C20 Scottsdale 7.4L 4 speed 3.21
1986 C10 under construction
https://www.facebook.com/groups/248658382003506/

Offline jfkelley

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Re: NEED Confirmation of rear gear ratio
« Reply #4 on: December 08, 2020, 10:45:31 PM »
So if I calculated correctly with my 400 engine 3.07:1 rear gear ratio Mated to an overdrive transmission ..My final crushing Rpm on the interstate @ 70-75mph would be near 1440 RPM ..Im a noob but thats too low correct ?
« Last Edit: December 08, 2020, 11:21:08 PM by jfkelley »
1976 Chevrolet 4x4 ,K10,Stepside, 400/350/NP203, 35 BF Goodrich All-Terrain, Factory A/C

Online zieg85

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Re: NEED Confirmation of rear gear ratio
« Reply #5 on: December 09, 2020, 06:21:55 AM »
So if I calculated correctly with my 400 engine 3.07:1 rear gear ratio Mated to an overdrive transmission ..My final crushing Rpm on the interstate @ 70-75mph would be near 1440 RPM ..Im a noob but thats too low correct ?

IMO yes.  Your transmission would be searching.  The 700R4 came with 2.73 back in the day with tiny P195 tires.  A 3.73-4.10 would be a better ratio especially with 35" tires.
Carl 
1985 C20 Scottsdale 7.4L 4 speed 3.21
1986 C10 under construction
https://www.facebook.com/groups/248658382003506/

Offline jfkelley

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Re: NEED Confirmation of rear gear ratio
« Reply #6 on: December 09, 2020, 05:01:55 PM »
Pulled the rear cover and came up with these #s
14:43
1976 Chevrolet 4x4 ,K10,Stepside, 400/350/NP203, 35 BF Goodrich All-Terrain, Factory A/C

Offline VileZambonie

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Re: NEED Confirmation of rear gear ratio
« Reply #7 on: December 09, 2020, 05:07:29 PM »
Driven over drive = 3.07
,                           ___ 
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74 GMC, 75 K5, 84 GMC, 85 K20, 86 k20, 79 K10

Offline jfkelley

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Re: NEED Confirmation of rear gear ratio
« Reply #8 on: December 09, 2020, 05:16:06 PM »
Thats too low for overdrive ..Im gonna have to go 3.73 or 4:10  if I want overdrive
1976 Chevrolet 4x4 ,K10,Stepside, 400/350/NP203, 35 BF Goodrich All-Terrain, Factory A/C

Offline TexasRed

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Re: NEED Confirmation of rear gear ratio
« Reply #9 on: December 09, 2020, 05:16:56 PM »
What kind of mileage are you getting with it now? I assume that's what you're trying to get a little more out of? Maybe some work in tuning (and maybe a tune UP) will help?

Offline jfkelley

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Re: NEED Confirmation of rear gear ratio
« Reply #10 on: December 09, 2020, 05:22:43 PM »
Im getting 10-12mpg with it now on interstate ..Yes looking for better MPG a lot of the places I go hunting are 3/4 hrs away and on the highway with 3speed automatic pushing 70mph is hard on the old girl with All time 4wheel drive
1976 Chevrolet 4x4 ,K10,Stepside, 400/350/NP203, 35 BF Goodrich All-Terrain, Factory A/C

Offline SilverMiner

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Re: NEED Confirmation of rear gear ratio
« Reply #11 on: December 09, 2020, 08:06:35 PM »
Im getting 10-12mpg with it now on interstate ..Yes looking for better MPG a lot of the places I go hunting are 3/4 hrs away and on the highway with 3speed automatic pushing 70mph is hard on the old girl with All time 4wheel drive

10-12mpg with the NP203 full-time case is great, and quite likely near the top of the range possible with our non-aerodynamic bricks and modern gas. I have a vast collection of magazines from the era and none of the contemporary driving tests exceeded that value even with higher octane leaded fuel of the day. The 4" lift generally required to run 35" tires will degrade what little aerodynamic value these trucks have to begin with - an overdrive MIGHT get you back to your original numbers.

I have no doubt a more modern combination including an LS engine plus 4-speed overdrive could do better, but the financial calculations don't make sense. The best and only reason to go through the expense and work is simply because you can and you want to. As for myself, I drive these trucks because I enjoy it not because I think I'm saving money.

Offline TexasRed

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Re: NEED Confirmation of rear gear ratio
« Reply #12 on: December 09, 2020, 11:36:26 PM »
I'd ask, what carburetor are you running? Is the HEI in good shape? What kind of timing are you running? It may be worth it to advance the timing a little to see if you can get a little more economy out of it. Is it stock cam? Is the truck running good?

If you're getting like 2100 rpm at 70 MPH, that 400 should just be loping along, well somewhat. Obviously the tires and 4wd and such are somewhat of a power sucker.

What do the plugs look like? May check at least #1 plug and see if it's time to change plugs, get new wires, etc. Rockauto usually has fairly decent prices. Also may drop by the autoparts store and see if they can test your module. If its bad, I'd consider changing the coil too. I doubt you're having this issue, but maybe, and it should be free to get it tested (other than the ride to the autoparts store).

Offline jfkelley

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Re: NEED Confirmation of rear gear ratio
« Reply #13 on: December 10, 2020, 12:00:20 AM »
I have an Eldebrock 1406 carburetor.. I put a tuneup on it about a year ago, plugs , distributor cap , module , It could use some new wires.. dont have a TH 400 transmission I have a TH350 ..
1976 Chevrolet 4x4 ,K10,Stepside, 400/350/NP203, 35 BF Goodrich All-Terrain, Factory A/C

Offline TexasRed

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Re: NEED Confirmation of rear gear ratio
« Reply #14 on: December 10, 2020, 12:13:44 PM »
If the edelbrock is still set from out of the box, it may be worth it to potentially do some extra tuning on it.

For hotrodding the HEI, there are some good info out there. I'd tend towards Dave Ray (ignitionman)'s info.

Why did you replace the module? Usually those don't go bad unless something else goes bad. Like high resistance forces the coil to overwork which forces the module to overwork and cook itself.

I know you have a TH350 transmission, I was referring to the 400 engine that should be loping along. That's a lot of cubes cruising at an rpm that should be in the torque curve. But you say the speed is hard on the old girl, but that's a pretty good RPM I would think? Is it struggling?