another rear-end ratio question - The 1947 - Present Chevrolet & GMC Truck Message Board Network

willett · 01-18-2009, 02:27 PM

hey guys

i'm running a lifted '78 lwb with a 454, NP205, and a turbo 400 on 35's
i have 3.73's in a 14 bolt and i'm spinning about 3200 RPM at 110 km/h ( i think about 65 MPH)

if i were to throw some new hardware in my diff do y'all think 3.42 would be reasonable?

any other suggetsions?

i don't do alot of wheeling in my truck, but i am known to get her stuck in the mud on occasion

**thinair** · 01-20-2009, 01:34 AM

whats wrong with what you are running right now?

willett · 01-20-2009, 01:45 AM

i work up north,

the highways are long and straight up there, and it erks me a bit to be confined to 65 mph

overdrive would be nice, but i'm trying to go faster for cheaper.

**thinair** · 01-20-2009, 01:49 AM

then 3.42s would probablly be good for you

slimneverdies · 01-20-2009, 01:52 AM

If your worried about those rpm's being so high I would keep the 3:73 and do an overdrive transmisson. Since its lifted with big tires I would go with 3:23 or 3:42

James McClure · 01-20-2009, 02:14 AM

Willett, If I'm not mistaken your truck came with 235-85R16 tires. They are 31" tall. Going to 35" effectivly got you 3.42 gears. Something is wrong here. I am running 3.08 gears and turning 2300 to 2400 at 65 on 31" rubber. That about fits what I was getting with 4.10 gears, 60 MPG at 3000 RPM (speedo was calibrated and was using a Sun super tach). I think you had better check and see where your going wrong. Are you sure of the rear end ratio? Did you check your speed over a measured mile or just use the speedo? Lastly, is your tach correct? You might just be going faster than you think. You won't feel the difference between 65 and 75 and 3200 RPM should put you about 75.

jim

willett · 01-20-2009, 02:28 AM

perhaps i don't have 3:73's

the guy whom i bought the truck off of told me that that is what was in there, but the more work i do to the truck, i realize the less that the previous owner knew about it!

James McClure · 01-20-2009, 04:27 AM

Willett, pop the cover off the rear end, look for the part number. Near the part number will be 4 numbers seperated by a ":" divide the larger number by the smaller one and that will give you the ratio. 15:41 would be 2.73 as an example. Yours will be different, like 62:15 would be 4.13 ect.

willett · 01-21-2009, 02:10 AM

thanks james, i will take a looksee when i get a chance!

Bowed · 01-21-2009, 02:51 AM

With 35' tires your spedometer is reading low , you are doing more than 110 KM/HR at 3200 RPM .

b454rat · 01-21-2009, 03:37 AM

With 4.10s and 35" tires, 70 should be 26/2700-ish RPMS.....

01-18-2009, 02:27 PM	#1
willett Another Day, Another Dollar Join Date: Nov 2008 Location: North Vancouver, BC Posts: 1,316	another rear-end ratio question hey guys i'm running a lifted '78 lwb with a 454, NP205, and a turbo 400 on 35's i have 3.73's in a 14 bolt and i'm spinning about 3200 RPM at 110 km/h ( i think about 65 MPH) if i were to throw some new hardware in my diff do y'all think 3.42 would be reasonable? any other suggetsions? i don't do alot of wheeling in my truck, but i am known to get her stuck in the mud on occasion __________________ -78 K20 Cheyenne 454 long box, 4" lift, 35" MT's, '84 cab, '80 box "its hip to be square"

01-20-2009, 01:34 AM	#2
thinair thinair Join Date: Jun 2004 Location: omaha,ne Posts: 1,001	Re: another rear-end ratio question whats wrong with what you are running right now?

01-20-2009, 01:45 AM	#3
willett Another Day, Another Dollar Join Date: Nov 2008 Location: North Vancouver, BC Posts: 1,316	Re: another rear-end ratio question i work up north, the highways are long and straight up there, and it erks me a bit to be confined to 65 mph overdrive would be nice, but i'm trying to go faster for cheaper. __________________ -78 K20 Cheyenne 454 long box, 4" lift, 35" MT's, '84 cab, '80 box "its hip to be square"

01-20-2009, 01:49 AM	#4
thinair thinair Join Date: Jun 2004 Location: omaha,ne Posts: 1,001	Re: another rear-end ratio question then 3.42s would probablly be good for you

01-20-2009, 01:52 AM	#5
slimneverdies Registered User Join Date: Apr 2006 Location: Miami now in Vegas Posts: 828	Re: another rear-end ratio question If your worried about those rpm's being so high I would keep the 3:73 and do an overdrive transmisson. Since its lifted with big tires I would go with 3:23 or 3:42 __________________ Long live SLIM ------------------ At least try to buy American

01-20-2009, 02:14 AM	#6
James McClure One shot, one kill. Join Date: Dec 2007 Location: Saratoga Springs NY Posts: 859	Re: another rear-end ratio question Willett, If I'm not mistaken your truck came with 235-85R16 tires. They are 31" tall. Going to 35" effectivly got you 3.42 gears. Something is wrong here. I am running 3.08 gears and turning 2300 to 2400 at 65 on 31" rubber. That about fits what I was getting with 4.10 gears, 60 MPG at 3000 RPM (speedo was calibrated and was using a Sun super tach). I think you had better check and see where your going wrong. Are you sure of the rear end ratio? Did you check your speed over a measured mile or just use the speedo? Lastly, is your tach correct? You might just be going faster than you think. You won't feel the difference between 65 and 75 and 3200 RPM should put you about 75. jim __________________ 1984 K20 350M engine with 465,000 miles. Well, it's finally done!! Almost 2 years of work, but it was really worth the effort. Little stuff left to do is mount winch, wet sand & buff out and build belly pan\running boards. Body work takes lots and LOTS of beer!!!! God, Guts and Guns made America and God, Guts and Guns will preserve it! The worst thing you could do is get into my sights, but that don't matter, you'll never know it, cause you'll never hear the shotJim or Paladin whichever you choose.

01-20-2009, 02:28 AM	#7
willett Another Day, Another Dollar Join Date: Nov 2008 Location: North Vancouver, BC Posts: 1,316	Re: another rear-end ratio question perhaps i don't have 3:73's the guy whom i bought the truck off of told me that that is what was in there, but the more work i do to the truck, i realize the less that the previous owner knew about it! __________________ -78 K20 Cheyenne 454 long box, 4" lift, 35" MT's, '84 cab, '80 box "its hip to be square"

01-20-2009, 04:27 AM	#8
James McClure One shot, one kill. Join Date: Dec 2007 Location: Saratoga Springs NY Posts: 859	Re: another rear-end ratio question Willett, pop the cover off the rear end, look for the part number. Near the part number will be 4 numbers seperated by a ":" divide the larger number by the smaller one and that will give you the ratio. 15:41 would be 2.73 as an example. Yours will be different, like 62:15 would be 4.13 ect. __________________ 1984 K20 350M engine with 465,000 miles. Well, it's finally done!! Almost 2 years of work, but it was really worth the effort. Little stuff left to do is mount winch, wet sand & buff out and build belly pan\running boards. Body work takes lots and LOTS of beer!!!! God, Guts and Guns made America and God, Guts and Guns will preserve it! The worst thing you could do is get into my sights, but that don't matter, you'll never know it, cause you'll never hear the shotJim or Paladin whichever you choose.

01-21-2009, 02:10 AM	#9
willett Another Day, Another Dollar Join Date: Nov 2008 Location: North Vancouver, BC Posts: 1,316	Re: another rear-end ratio question thanks james, i will take a looksee when i get a chance! __________________ -78 K20 Cheyenne 454 long box, 4" lift, 35" MT's, '84 cab, '80 box "its hip to be square"

01-21-2009, 02:51 AM	#10
Bowed Registered User Join Date: Oct 2002 Location: where it's hot Posts: 1,059	Re: another rear-end ratio question With 35' tires your spedometer is reading low , you are doing more than 110 KM/HR at 3200 RPM .

01-21-2009, 03:37 AM	#11
b454rat Senior Member Join Date: May 2004 Location: Binghamton, NY Posts: 3,454	Re: another rear-end ratio question With 4.10s and 35" tires, 70 should be 26/2700-ish RPMS..... __________________ 2000 GMC CCSB 454/4L80 6” lift SAS 1999 Chevy CCSB 454/NV4500 6” lift 1999 GMC Yukon 350/4L60 4" SAS



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