What's new
CK5
Register an account today to become a member! Once signed in, you'll be able to participate on this site by adding your own topics and posts, as well as connect with other members.

Identifying what gears are in a differential?

M

muzzfactor

Registered Member
Joined
Feb 25, 2008
Posts
29
Reaction score
0
Location
San Antonio, TX
Hey guys, I'm headed out to the junkyard this weekend looking for a new front and rearend for my truck ('84 GMC shortbed) and I'd like to know how I can tell what gears are in the rearends. I currently have 4.56's with a 700r and 35's and it feels like it's running to many rpms at 70 mph, so I'm looking for a rearend with either 3.73's or 4.10's in it. I've read that everybody recommends the 14ff, but I don't have the money to change the wheels so I need to stick with a six lug. Thanks.
 
Last edited:
The absolute best method is to pull the diff cover and read the numbers stamped on the ring gear to get your ratio.

You absolutely need to do this before purchase anyways, it's the only way to know what you are getting for your money in regards to condition.

As an example, had to get an axle for a friends S10 Blazer. Needed 3.42, and wanted to replace the gov-locked axle with another gov-locked axle. Went to a wrecking yard, EVERY axle in the yard was marked as "open". Popped them open, at least 50% were gov-locked. *Because* the yard guys didn't know what they were doing, and tried to figure out what it was solely by spinning one axle. Don't trust the yard.

The 'yards around here, IF they mark/annotate the ratio, they do it by looking at the RPO code. While that probably works 90% of the time, it doesn't when it's not stock.
 
^ What Dorian said. I just want to add that 4.56 is the perfect gear for 35s with a 700r4. 3.73 or 4.10 will increase the need for downshifts, possibly lead to "hunting" back and forth between 3rd and 4th on the highway, and increase the stress on your trans. Are you really sure you want to do that? If you do go to a different ratio, at least go with the 4.10.:crazy:
 
4.10's will be fine with a 700 and 35's.

def pull the covers and check the teeth out and count them if you cant find any numbers on them.
 
BTW, the numbers on the ring gear won't say the ratio, it'll have the tooth count for both gears, i.e. 41-10 for a 4.10 gearset.
 
Thanks for all the replies guys. I had decided to go with 4.56's after I did alot of research on this forum and like you said throttleissues, they are supposed to be the perfect ratio for the 700r with 35's, but i do alot of hwy driving and when i'm running around 75mph it just feels like the rpms are running high. I don't know exactly what the rpms are since I don't have a tach.
 
At 75 mph while in OD your RPM's will be 2298 give or take a few depending on actual tire size (just becasue it's a 35" tire doesn't mean it actually measures 35". Most tires are slightly smaller than stated size.) Given the assumption that your tire measures 34.5" your RPM's will be 2331 @ 75 MPH
 
I run 31's, 4,56's and a th400 and i dont think it revs too much (only do 65mph though).
 
I use them for towing my car to the track otherwise the drop hitch is just too much!

I normally run 36's, have some 38's to try now. I need a lower hitch setup that is not so scary (i run a 10'' drop hitch with the 31's).
 
Well, it sounds like big lift and little tires for towing. That's gotta look hilarious. Wouldn't it work better to raise the trailer (flip the axles or something) so it was a closer match to the truck? I have seen one huge drop hitch that had turnbuckles back to brackets on the frame rails to stabilize it. I think it was an 16" drop, so it's still a little spooky. Just a coupla thoughts.
Oops, looks like we hijacked his thread.:o Sorry Muzz, but to be honest, we already answered your questions anyway.:D
 
Yup, it does look rediculous. I like having the CG of the car/trailer low so i don't want to raise it up. I will start towing with the 36's on next summer i think, doing a motor swap first.

03-07-071151.jpg

03-07-071150.jpg
 
Niiice! Well, at least your running really sweet rims with the 31's.:D The truck doesn't seem very high though, so I guess you have a really low racecar hauler. It all works, and I'm very much in favor of the "whatever gets the job done" way of doing things.:waytogo:
 
kevin400ex said:
4.10's will be fine with a 700 and 35's.

def pull the covers and check the teeth out and count them if you cant find any numbers on them.
Click to expand...

Yup, if you never plan on using overdrive again... :crazy:

Rene
 
You must log in or register to reply here.

Latest Posts

Top Bottom