Better gas Mileage

[adamfgarner]

for what it's worth....I have the sm420, Dana 4.10 gears and 265/75/16 tires on my 1500 series...my tach reads just at 2000 for 40 right under 2500 at 50 and close to 3000 at 60...there are several calculators available on-line though there will be discrepancies as your tires get smaller with use and are slightly different from maker to maker...I am doing my first mileage test with this drivetrain...will post the changes when I do the 3.54 swap....as for my research I have found that 3.54 is as low as the Dana 60 carrier will accommodate...the 3.31 gearing might be a Dana 44 or other model???

[tommyduncan]

Originally Posted by ab positive bulldogs

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

Originally Posted by 66birdman

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

I agree! maybe his tach and his speedo? Most 33's are closer to 32" but that is still off.

Originally Posted by adamfgarner

for what it's worth....I have the sm420, Dana 4.10 gears and 265/75/16 tires on my 1500 series...my tach reads just at 2000 for 40 right under 2500 at 50 and close to 3000 at 60...there are several calculators available on-line though there will be discrepancies as your tires get smaller with use and are slightly different from maker to maker...I am doing my first mileage test with this drivetrain...will post the changes when I do the 3.54 swap....as for my research I have found that 3.54 is as low as the Dana 60 carrier will accommodate...the 3.31 gearing might be a Dana 44 or other model???

Going by the listed diameter for your tire you are fairly close. I did the math and got 1778@40mph.
With 265/65/17 my speedo/rpm read the same as yours but my speedo reads about 10% slower than actual speed.

My buddy that told me about the DANA 60 3:31 owns a diff shop and he was surprised to see them listed also. I am trying to get them ordered but he is a busy guy...

[tommyduncan]

My friend must be busy so I downloaded Motive gears catalog(where he saw them)and they don't list 3:31 gears. They do list them for DANA 80 so I suspect he saw it wrong...

[bigblockv6]

Originally Posted by ab positive bulldogs

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

There definitely something wrong with your figures. Though it's been a quite sometime I do remember my 68 K2500 with 4.56 and stock 33 inch tire would run somewhere between 2200 to 2500 rpms at 55mph.

[kieth]

Here is a link that will help you guys figure out which gears to run with what tire diameter and transmission. Everyone is talking speed which is only one component of this problem, making sure you have enough torque available is important as well as speed.
Take a look at the spreadsheets which are available in this thread. The file names include a number which is the diameter of the rear tire in inches from the ground to the top of the tire. If you are trying to build a truck that cruises down the freeway I normally shoot for 2000rpm at 65mph or something close to it.
My 66 GMC has a TBI 350 200hp with a 700r4 transmission, 460ft/lbs of torque to the ground, 30" tire diameter, 3:73 rear axle ratio, .70 Od ratio.
Click on the file with the proper tire diameter and the excel spreadsheet shows different engines, transmission ratios, for a given diameter. My truck is a good highway cruiser @460ft/lbs of torque but if I was pulling a trailer I would gear it differently.....or just shift down to 3rd gear 1:1 ratio and have plenty of power but have to turn higher rpms so my fuel mileage would go down.
See what you think.......hope it helps you....figure out the gearing, speed issue.......go down to posts 6 & 7 for the links...

ps see how much torque your truck currently has then try to keep the torque close to or higher than the 460ft/lbs depending on what you want the truck to do, i.e. highway cruiser or stump puller...

http://6066gmcclub.com/forum/showthread.php?t=48090


Here is a link to the 33" tire diameter spreadsheet:

http://6066gmcclub.com/forum/attachm...7&d=1395158086

[kieth]

Originally Posted by bigblockv6

There definitely something wrong with your figures. Though it's been a quite sometime I do remember my 68 K2500 with 4.56 and stock 33 inch tire would run somewhere between 2200 to 2500 rpms at 55mph.

here is a spreadsheet with 32" tire diameter:

http://6066gmcclub.com/forum/attachm...6&d=1394467957


I will run one for 33's and post it later today.....Kieth

[BlockHead]

Originally Posted by 66birdman

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.

Did you ever find out if that guy has that trans?

[BlockHead]

Did you ever find out if that guy has that overdrive and trans? I am close enough to come and get it. I called the vintage place in MT Home and they only have 3 speeds.

[66birdman]

Sorry I disappeared for a while. I went out and talked to the guy at Vintage and I actually laid eyes on that OD that he says he doesnt have.....when I asked about it I got the typical hoarder answer of "ooooh I better hold onto that for one of my projects". Sorry to get your hopes up about it.

[kieth]

You want to be careful when changing too many things at once, you might end up with a truck that falls on its face........but looks good on paper.

here is a link to some information that we published on this forum, you will need the transmission, rear tire diameter, and then know what rear axle ratio you currently have.
With these spreadsheets you will be able to see how much torque you currently have to the ground with your current set up, then compare the speed lines and available torque before you change the rear axle ratios or transmissions.......post me here if you have any questions.....Kieth
here is the link http://6066gmcclub.com/forum/showthread.php?t=48090