6066 (1960-1966) GMC Truck Club
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6066 (1960-1966) GMC Truck Club (https://6066gmcclub.com/index.php)
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-   -   Better gas Mileage (https://6066gmcclub.com/showthread.php?t=47904)

BlockHead February 2nd, 2014 07:04 AM
Re: Better gas Mileage
 
Quote:
Originally Posted by BlockHead (Post 52083)
I put a 3:54 on 3 days ago on my 64 with a 305 and it is a substantial difference but still need an overdrive if you ask me since the torque will handle the high gears on the Nevada flats. Wouldn't that be cool to be lugging or chugging along at 80 with no problem? My friend is a tranny/driveline guy and says that Volvo had an overdrive that might work. If not it needs a Brownie or Gear vendor. Going to put a Holley 500 on this week since the old carb I rebuilt was good for a few days and won't idle now. Not messing with it. R and R with Holley sounds good to me.
Finished the Holly 500 and need to modify new air cleaner for the big vent hose. That dude runs like a champ with that carb. WOW man! Will jet for 5500 foot altitude. If anybody has an overdrive or brownie let me know.

66birdman February 26th, 2014 03:27 AM
Re: Better gas Mileage
 
I guess I must be lucky or something. I have a 66 C15 with a completely stock 351E, SM420 and Dana 60 with either 4.11s or 4.56s (havent dug that far into it yet) and I get around 14mpg cruising at 55ish. Not sure what my RPMs are since I dont have a tach but I did put slightly oversized tires (235/85/R16) on it to make it capible of highway speeds. Side note the vintage salvage yard here where I live has a 4 speed with an aftermarket overdrive still attached to a 65 GMC if anybody wants me to go ask about it. I dont remember if the tranny was a SM420 or a SM465 though.

tommyduncan February 26th, 2014 04:17 AM
Re: Better gas Mileage
 
Quote:
Originally Posted by 66birdman (Post 52318)
I guess I must be lucky or something. I have a 66 C15 with a completely stock 351E, SM420 and Dana 60 with either 4.11s or 4.56s (havent dug that far into it yet) and I get around 14mpg cruising at 55ish. Not sure what my RPMs are since I dont have a tach but I did put slightly oversized tires (235/85/R16) on it to make it capible of highway speeds. Side note the vintage salvage yard here where I live has a 4 speed with an aftermarket overdrive still attached to a 65 GMC if anybody wants me to go ask about it. I dont remember if the tranny was a SM420 or a SM465 though.
At 55mph with that tire size 4.56s=2658rpms
4.10s= 2390
3.54s=2063

To get that good of mileage I'm guessing someone might have changed your years already.
My best friend owns a gear shop and he told me that 3:31s are available for the DANA 60. He looked it up right in front of me but he might have made a mistake...

66birdman February 26th, 2014 04:21 AM
Re: Better gas Mileage
 
anything is possible and I am no expert on gears but when I bought the truck I got it from the original owners son who pulled it out of the barn after it sat since 1995. He told me it was all stock. Guess it could have been some sort of special order or something.

tommyduncan February 26th, 2014 04:56 AM
Re: Better gas Mileage
 
Quote:
Originally Posted by 66birdman (Post 52320)
anything is possible and I am no expert on gears but when I bought the truck I got it from the original owners son who pulled it out of the barn after it sat since 1995. He told me it was all stock. Guess it could have been some sort of special order or something.
Mine has 4:10s and I get 8mpg. Usually when I look at the tach I am around 3100rpm, which is about 70mph. Since I posted earlier I realized that I never go 55... in this town you will get run down by a cement truck at that speed. I brought my GPS and the speedo runs a little slow also.

66birdman February 26th, 2014 05:05 AM
Re: Better gas Mileage
 
Actually now that you mention it I may have been going a bit slower than 55. The truck has been parked and torn apart for a while now and I cant remember what the GPS speedo on my phone said. 55 is what my truck was saying but im sure that is off by at least 5mph

ab positive bulldogs February 26th, 2014 03:18 PM
Re: Better gas Mileage
 
Quote:
Originally Posted by 66birdman (Post 52318)
I guess I must be lucky or something. I have a 66 C15 with a completely stock 351E, SM420 and Dana 60 with either 4.11s or 4.56s (havent dug that far into it yet) and I get around 14mpg cruising at 55ish. Not sure what my RPMs are since I dont have a tach but I did put slightly oversized tires (235/85/R16) on it to make it capible of highway speeds. Side note the vintage salvage yard here where I live has a 4 speed with an aftermarket overdrive still attached to a 65 GMC if anybody wants me to go ask about it. I dont remember if the tranny was a SM420 or a SM465 though.
Be careful I have a stock 351E with the Dana 60 and 4.10 gears I installed a tach to make sure I am not hitting redline on the freeway and I am at 3100RPM at about 45mph with 33" tires

ab positive bulldogs February 26th, 2014 03:22 PM
Re: Better gas Mileage
 
Quote:
Originally Posted by tommyduncan (Post 52319)
At 55mph with that tire size 4.56s=2658rpms
4.10s= 2390
3.54s=2063

To get that good of mileage I'm guessing someone might have changed your years already.
My best friend owns a gear shop and he told me that 3:31s are available for the DANA 60. He looked it up right in front of me but he might have made a mistake...
I don't know how to do that math for that BUT like I said in my previous post at 45MPH I am at about 3100RPM with the Dana and 4.10 gears

adamfgarner February 26th, 2014 03:31 PM
Re: Better gas Mileage
 
I have been following this thread and would like to usurp some advice while on this topic...I have a Dana60 rear end as well with the 4.10 gears...I recently bought a 72 Chevy 3/4 ton Dana rear with 3.54 gears I want to use...the brakes are larger as are the lugs...does anyone have any experience or advice for or against this swap (other than the "all original" argument )??....my brother-in-law tells me I made need to add a proportioning valve...anyone else have input before I take my truck apart???Bueller? Bueller?

66birdman February 27th, 2014 12:10 AM
Re: Better gas Mileage
 
Quote:
Originally Posted by ab positive bulldogs (Post 52326)
I don't know how to do that math for that BUT like I said in my previous post at 45MPH I am at about 3100RPM with the Dana and 4.10 gears
I am not saying this for sure but your tach may be off. According to every calculator and formula i have found with that set up you should be running about 1850-1900 rpm with that set up


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