Rear differential gearing?
 

hey just curious if anyone knows the original rear gearing? I lifted my back end and it my driveshaft only turns 2.5 revolutions to the the tires one. That's seems low? My driveshaft isny hooked to anything it's just wired loosely to the bottom of the truck. Trying to decide if I wanted to try to run the original rear but unsure of its geared that way. I attached a photo of it. I guess i dont know if it is original. Thanks!

Re: Rear differential gearing?
 

I'm going to say 4 something and maybe even into the 5's.

Go to the GM heritage center archives and find your year model and open the pfd and scroll down and it will tell you what gear ratios were available for that model of truck
https://www.gmheritagecenter.com/gm-...tion-kits.html

Re: Rear differential gearing?
 

What year/model is the truck?

Re: Rear differential gearing?
 

stock for 3100 is 4:11 or there abouts, low mpg, lots of power, very little speed. 2100 rpm at 50 mph
stock for 3800 is 5:10
i run a 3:27 in truk, very easy to maintain hwy speed or get a ticket, 21 mpg and 1200 rpm at 50 mph

Re: Rear differential gearing?
 

55-59 3100 stock rear end ration is 3.90 : 1, so Ogre is correct. The 3600 and 3800 were 5.27-ish :1

Re: Rear differential gearing?
 

Sorry it's a 57 gmc 100

Re: Rear differential gearing?
 

Thanks ! And yes I want mpg so need to locate something else. What is your 3.27 out of?

Re: Rear differential gearing?
 

Easiest way to know ratio is take cover off and count teeth...or look for ratio marks on ring gear...
Or this from silversport transmission...
Mark the wheel and the drive shaft with chalk or masking tape. Turn the wheel TWO revolutions and count the number of driveshaft turns. The number of driveshaft rotations will help you determine your rear axle ratio. For example, if it turned 3 and a half times, it's a 3.50:1 ratio.

Re: Rear differential gearing?
 

Thanks so mine did almost 5 wi th the 2.tire turns so a 4.9 then? I got confused on the turn once for posi or 2x for not. When I spin the tire it turns the other side also so I thought that meant I was only supposed to do the 1 rotation test?

Re: Rear differential gearing?
 

I never had much faith in the turn the driveshaft or axle and count the turns of the other one thing. From the looks of things you need a new gasket on the rear axle cover anyhow and it is probably well past due for a gear oil change so I'd just put a pan under it, pull the cover and count.

Gear ratio after all is done? That would be what you and the truck are comfortable with the way you drive it and how far. 80% around town with maybe one 100 mile trip to an event or nifty spot a year. Stock gears will probably be fine even though gas mileage might suffer a tad. If every other weekend means a road trip to an event within 200 miles or so one way or you are driving it on the freeway a lot, you want gears that allow for more comfortable engine speed at he speed you plan to drive. 55 on a 55 mph road is comfortable if that is where the truck wants to run but 55 on a 70/75 or now 80 speed limit freeway turns you into a road hazard.

Checking the Map says Fruita is a few clicks out of Grand Junction on 70 (I almost burned my clutch up pulling Uhaul up a grade in that area in 1977) where you probably jump on the highway in one direction or another quite often and head out. With that I'd say 3.3 to 3.7 gears are probably what you want for a stick without overdrive and if you have a turbo 350 or 400 3.0 gears might be better.

Still it comes down to
Engine=
Trans =
Rear tire diameter=

How you use it. Just cruising and going places mostly local? towing a camp trailer? Crawling around the back roads? Long road trips to events or nifty places?

All of those have a bearing on gear selection.

Re: Rear differential gearing?
 

Thank you for the info. Oh burning a clutch no fun! We do have some mountains around here haha.
I hope and intend to be mostly a daily driver and yes some small trips so I was hoping for better gearing and mileage. I dont plan to interstate much but you never know haha. Mostly 55 but some 65. So yeah maybe need to search for something else for a different gearing option. Intending a 700r4 tranny and automatic. AND havent bought tires yet so that's another adventure

Re: Rear differential gearing?
 

truk has an 86 trans am rearend w/ posi and disc brakes, don't look for a 3:27, not very common
3rd gen camaro/firebird/trans am will fit under your 57 gmc, gear ratios available from 2.72 to 3.73
most are 10 bolt, my 86 is an oddball 9 bolt borg warner
find one with the ratio you want, disk brakes and get the torque arm with it
torque arm is great for traction control, anti wheel hop and spring wrap (truk is on stock springs)

Re: Rear differential gearing?
 

It is not an axle from the 50s. Is it 5 or 6 lug? Looks like 70s 12 bolt GM 1/2 ton truck diff. Gearing should be fine for your purposes.