New Diff for my Big Block - The 1947 - Present Chevrolet & GMC Truck Message Board Network

**Already Gone** · 09-20-2012, 12:50 PM

Maybe this will help somewhat with my gas mileage. With my 3:73's and TH400 the 396 turns 3000 RPM at 70 MPH and with this 3:08 (and bonus,its a posi) my RPMs should go down to about 2400 which I hope will get me 2-3 MPG more which will bring me to the 15/16mpg instead of 12 mpg.

northerngmc · 09-20-2012, 03:28 PM

Greg, paint it gold and it might even get better!

I measure fuel mileage of my 396 by tankfulls/year!

AusTx68 · 09-20-2012, 10:50 PM

Sell the 3:08 posi and get you an overdrive tranny so you can keep the 3:73s. Just a thought.

**special-K** · 09-21-2012, 06:45 AM

Greg,good move. Most BBCs came through with 3.08s anyway. You know it will perform right. That said,I love me some growlin' gears. I like to feel the pull of the motor. If I had my drothers,I'd have 4.10s with overdrive...or at least keep the 3.73s. I know I need to change with today's fuel costs. I like to drive them and it's getting expensive to be hard headed. I've been scoffing at those downsizing and going for mileage for decades and just forked out the few extra bucks to have what I want. Times is a changin',though (or did they already and it's finally sinking in?

)

rsavage · 09-21-2012, 08:02 AM

Don't think you are going to be at 2400 at 70 mph. My Blazer has 31" tires and 3.07's and at 24-2500 rpms I am at 55-56 mph. I have a TH350 but it is one to one in drive just like your TH400.

**Already Gone** · 09-21-2012, 10:56 AM

Quote:

Originally Posted by rsavage

Don't think you are going to be at 2400 at 70 mph. My Blazer has 31" tires and 3.07's and at 24-2500 rpms I am at 55-56 mph. I have a TH350 but it is one to one in drive just like your TH400.

Here is a link to a RPM/Gear Ratio Calculator. Plug your numbers in and se what you get. Are you certain you have 3:07's?

http://richmondgear.com/101032.html

rsavage · 09-21-2012, 11:14 AM

Yes, I have had both covers off . Have a tach dash and have checked speedo with my Garmin as I had to buy a new adapter for the taller tires, so was interested in checking accuracy.

rsavage · 09-21-2012, 11:25 AM

I see what you mean with the calculators - just not what I am seeing in the real world use? My Burb also had 3.07's in it and with a TH350 was also right around 24-2500 at 55mph.

**Already Gone** · 09-21-2012, 11:37 AM

I too have a dash tach and have checked my speed with my Garmin and with my speedo at 95 MPH the Garmin was reading 93 MPH which matches using my trans, rear end and tire factors.

If somone has any ideas why there might be discrepencies here please chime in.
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rsavage · 09-21-2012, 11:57 AM

My tach could be off although the one in the Burb was a column mounted Sun. Non lock up torque converter slippage? In any event, you will slow the rpms down significantly over your 3.73's and it would have to improve your mileage. I don't daily drive either of my trucks so don't even check mileage. Neither one gets good mileage - probably 8 -10. Let us know how you make out once you get it installed.

09-20-2012, 12:50 PM	#1
Already Gone 70+ ( Old Skool Club ) Join Date: Nov 2003 Location: Saskatoon, Saskatchewan ,Canada Posts: 9,109	New Diff for my Big Block Maybe this will help somewhat with my gas mileage. With my 3:73's and TH400 the 396 turns 3000 RPM at 70 MPH and with this 3:08 (and bonus,its a posi) my RPMs should go down to about 2400 which I hope will get me 2-3 MPG more which will bring me to the 15/16mpg instead of 12 mpg. Attached Images __________________ 1972 C10 Custom/Deluxe 613 Highlander 406/700R4 1999 White Tahoe LS 4x2 with Z56 Police Package 1992 K1500 GMC Suburban Members I have personally met: MusicMan70 - HeavyD - ChewyChevy67 - StingRay -71SWB4x4 - 67 Burb - DeadheadNM - too much stuff - bc65 - das601 " Circumsatances Do Not Change Responsibility " " The Sky is not the Limit, Your Mind is." Marilyn Monroe .. RIP Charlie Watts 1941 - 2021

09-20-2012, 03:28 PM	#2
northerngmc Registered User Join Date: Jan 2004 Location: Whitehorse YK Canada Posts: 2,651	Re: New Diff for my Big Block Greg, paint it gold and it might even get better! I measure fuel mileage of my 396 by tankfulls/year! __________________ Blaine 59 GMC 9314 a.k.a. Whoopee 68 C20 50th Anniversary a.k.a. The Rat http://www.flickr.com/photos/northerngmc/ http://www.members.tripod.com/P-wee/index_032.htm 1.61803 is one H of a lot better than 3.14159, no really it is!

09-20-2012, 10:50 PM	#3
AusTx68 Registered User Join Date: Nov 2008 Location: Italy Posts: 1,278	Re: New Diff for my Big Block Sell the 3:08 posi and get you an overdrive tranny so you can keep the 3:73s. Just a thought. __________________ '68 C-10 SWB 383 Stroker (Sold 3/2/2013) '87 R2500 LWB 454 TBI converted to Carb

09-21-2012, 06:45 AM	#4
special-K Special Order Join Date: Nov 2004 Location: Mt Airy, MD Posts: 85,861	Re: New Diff for my Big Block Greg,good move. Most BBCs came through with 3.08s anyway. You know it will perform right. That said,I love me some growlin' gears. I like to feel the pull of the motor. If I had my drothers,I'd have 4.10s with overdrive...or at least keep the 3.73s. I know I need to change with today's fuel costs. I like to drive them and it's getting expensive to be hard headed. I've been scoffing at those downsizing and going for mileage for decades and just forked out the few extra bucks to have what I want. Times is a changin',though (or did they already and it's finally sinking in?) __________________ "BUILDING A BETTER WAY TO SERVE THE USA"......67/72......"The New Breed" GMC '67 C1500 Wideside Super Custom SWB: 327/M22/3.42 posi.........."The '67" (project) GMC '72 K2500 Wideside Sierra Custom Camper: 350/TH350/4.10 Power-Lok..."The '72" (rolling) Tim "Don't call me a redneck. I'm a rough cut country gentleman" R.I.P. ~ East Side Low Life ~ El Jay ~ 72BLUZ ~ Fasteddie69 ~ Ron586 ~ 67ChevyRedneck ~ Grumpy Old Man ~

09-21-2012, 08:02 AM	#5
rsavage Registered User Join Date: Aug 2009 Location: Alden NY Posts: 2,705	Re: New Diff for my Big Block Don't think you are going to be at 2400 at 70 mph. My Blazer has 31" tires and 3.07's and at 24-2500 rpms I am at 55-56 mph. I have a TH350 but it is one to one in drive just like your TH400. __________________ 1961 C1 Corvette 1959 El Camino 350 TPI, 9" 4 w disc 69 Blazer K5 - sold July '20 2021 Durango RT 5.7

09-21-2012, 11:14 AM	#7
rsavage Registered User Join Date: Aug 2009 Location: Alden NY Posts: 2,705	Re: New Diff for my Big Block Yes, I have had both covers off . Have a tach dash and have checked speedo with my Garmin as I had to buy a new adapter for the taller tires, so was interested in checking accuracy. __________________ 1961 C1 Corvette 1959 El Camino 350 TPI, 9" 4 w disc 69 Blazer K5 - sold July '20 2021 Durango RT 5.7

09-21-2012, 11:25 AM	#8
rsavage Registered User Join Date: Aug 2009 Location: Alden NY Posts: 2,705	Re: New Diff for my Big Block I see what you mean with the calculators - just not what I am seeing in the real world use? My Burb also had 3.07's in it and with a TH350 was also right around 24-2500 at 55mph. __________________ 1961 C1 Corvette 1959 El Camino 350 TPI, 9" 4 w disc 69 Blazer K5 - sold July '20 2021 Durango RT 5.7

09-21-2012, 11:37 AM	#9
Already Gone 70+ ( Old Skool Club ) Join Date: Nov 2003 Location: Saskatoon, Saskatchewan ,Canada Posts: 9,109	Re: New Diff for my Big Block I too have a dash tach and have checked my speed with my Garmin and with my speedo at 95 MPH the Garmin was reading 93 MPH which matches using my trans, rear end and tire factors. If somone has any ideas why there might be discrepencies here please chime in. Posted via Mobile Device __________________ 1972 C10 Custom/Deluxe 613 Highlander 406/700R4 1999 White Tahoe LS 4x2 with Z56 Police Package 1992 K1500 GMC Suburban Members I have personally met: MusicMan70 - HeavyD - ChewyChevy67 - StingRay -71SWB4x4 - 67 Burb - DeadheadNM - too much stuff - bc65 - das601 " Circumsatances Do Not Change Responsibility " " The Sky is not the Limit, Your Mind is." Marilyn Monroe .. RIP Charlie Watts 1941 - 2021

09-21-2012, 11:57 AM	#10
rsavage Registered User Join Date: Aug 2009 Location: Alden NY Posts: 2,705	Re: New Diff for my Big Block My tach could be off although the one in the Burb was a column mounted Sun. Non lock up torque converter slippage? In any event, you will slow the rpms down significantly over your 3.73's and it would have to improve your mileage. I don't daily drive either of my trucks so don't even check mileage. Neither one gets good mileage - probably 8 -10. Let us know how you make out once you get it installed. __________________ 1961 C1 Corvette 1959 El Camino 350 TPI, 9" 4 w disc 69 Blazer K5 - sold July '20 2021 Durango RT 5.7



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