Better gas Mileage

[ab positive bulldogs]

OK GURUS...I have a 67 GMC with a 351E in it...because of my financial situation she will be my daily driver for a while. So far I have added power steering, Holley 500 carb, rebuilt my heads, and rebuilt the tranny. Now I have heard i can change to 3:54 gears I believe for better gas mileage or get a new tranny....what do you recommend is cheaper and or better? Which tranny do you guys recommend i swap to for better gas mileage and better freeway cruising speeds? Thanks in advanced!

[BarryGMC]

Gear swap is the cheapest, and has the fastest payback.And Its the first thing that I do. As money permits then maybe a gear vendors OD on top of the gear swap. This is cheaper to do and more flex able than a 480le swap. Some will say the 460le or its earlier kin. Well thats fine until you hook up a heavy trailer.

[Rick]

A rear end gear swap is a possibility, however this will cost around/about $500. This will work if you are sure that you will be getting what you are looking for. You might also consider some sort of overdrive transmission. You will have to investigate all possibilities with this. My 62, has 3:07 rear axel plus a .78% overdrive. This gives me 1750 RPM at 65 mph. Do the math before spending money.

[tommyduncan]

You just got two conflicting answers. Welcome to the forum!

Since you just rebuilt your transmission I doubt you would want to replace it?

The easiest overdrive swap is a t5 manual... if you can find the correct version and it isn't already trashed. It is also a fairly light duty trans so if you still want to use your truck for a truck it might not last. Some people have great luck with them on here but no one I know has. I destroyed a brand new one in a 4 cylinder S-10 when I was young and dumb(now I'm just dumb).

A swap to an o/d auto would require finding an adapter(not cheap or easy to find), probably shortening/modifying your driveshaft, building or buying a cross member, cutting a hole in the floor for a shifter, buying a shifter, trans cooler, cooler lines, possible trans tunnel clearance issues... and a TV cable depending on trans.

Do you know what your rear ratio is now? Most of these trucks are geared pretty low. There are gear ratio calculators online that help you figure what is best for your needs.

[raycow]

Ok, here is a third conflicting answer.

You didn't say what kind of transmission you have in there now.

If you have a 3 on the tree, a 3 speed + overdrive will be almost a bolt-in. These were a factory option on Chevy cars and trucks from 1955 until about 1969 and are still fairly easy to find. They will bolt to your stock bellhousing.

If you have a 4 speed, the easiest swap will be a T-5 from an S-10. That's what I installed on my truck about a dozen years ago and I have been happy with it ever since. This will also bolt to your stock bellhousing. If you use the truck seriously hard, you might want a stronger 5 speed like an NV4500. This one will NOT bolt to your bellhousing without an adapter like this:
http://www.advanceadapters.com/products/712550--dodge-nv4500-5-speed-transmission-to-a-gm-v8-adapter-plate-kit/

If you have an automatic, you probably have an adapter on the engine now that will let you use a Chevy pattern transmission. The most durable of these (in stock form) is the 4L80E, but that one requires an electronic controller. Non-electronic transmissions which will work are 700-R4 and 200-4R. These are not as strong in stock form as the 4L80, but can be modified to survive severe usage.

Ray

[tommyduncan]

I had a 3 on the tree with od in a 61 Brookwood wagon with a power pack 283. Loved that car!

[GMCv6inND]

oh boy, using as a daily driver! hey, I love driving my 67 around and i put a 351c in last year and installed the holley 2-barrell. My pickup came with the 3:08 rear-end and 3 on the tree. it starts, runs and cruises nicely - turns around 2000 at 65-70 mph. I only checked my mileage once and it was just a 90 mile trip in decent wx. I got 11 mpg. but i still love it. anyway, I think your best bet may be the highway gears AND an overdrive trans if thats possible. I would love to find an original 3-speed overdrive to install but would HAVE to be careful with the torque. good luck and have some fun!

[ab positive bulldogs]

OK so I passed by Hoopers Rear Ends today and i now now that I have a Dayna 60 rear end so the gears are 4:10's...guy said he can go as high as 3:54's BUT now I am wondering if that would be too much...and loose too much low end? I really need to do something since i am red lining at 50 mph right now...

[BlockHead]

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.

[tommyduncan]

OK so I passed by Hoopers Rear Ends today and i now now that I have a Dayna 60 rear end so the gears are 4:10's...guy said he can go as high as 3:54's BUT now I am wondering if that would be too much...and loose too much low end? I really need to do something since i am red lining at 50 mph right now...

Most of the recommendations I see are 3:54s. The formula to figure out what rpm at what mph isn't hard to figure out.

[BlockHead]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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

[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]

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

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

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]

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/attachment.php?attachmentid=1307&d=1395158086

[kieth]

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/attachment.php?attachmentid=1296&d=1394467957


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

[BlockHead]

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