What gear ratio did 85 lwb 305w700r4 come with

[true74enduro]

What axle gear ratio did 85 k10 lwb 305w700r4s come with? My truck is stock my grandpa never changed anything on it. I checked the glove box I didn't see where it should say axle gear ratio. I'm not looking to have a street machine nor am I wanting a tow rig. I just want a good dd with good performance. My truck has 17x8 wheels with 265 70 17 rubber. I might want to change it to 20" incubus recoil someday. Any help on what a good gear ratio would be awesome.

[piecesparts]

Any idea as to what your RPMs are when cruising down the highway at 60 or 70 MPH? You could have one of several rear end combinations, since the 1985 trucks was in the transition from old school to the overdrive world. I would guess at a 3.42 gear ratio as it was the most common, but you could have a 3.73 or 4.10 gearing. If you are serious about towing, then a 3.73 would be a good choice with the right size of tires, however if your plans are to go BIG on tires, then a 4.10 or lower would be the choice. Your present tire diameter is 31.6" in diameter, so from that point, I would hope that you are running a 3.73 or 4.10.

[true74enduro]

I don't have a tach on my truck in confident in saying about 3,500 to 4,000 rpm though. I would like to perform well I don't plan on towing or fourwheeling.

[piecesparts]

The calculations for the RPMs that you just gave, while using 70 MPH as a speed, puts you into a very low geared rear end.

Example: if you have 265/70/17 tire at 31.6" diameter with a RPM of 3000 at 70 MPH, a transmission with a .7 OD, then you would have a5.75 rear end ratio. I am not sure that is what you have. The RPMs would be important to this calculation.

[true74enduro]

Makes sense I'll get a tach and repost my rpms for better figuring out my rear gear ratio. At least this way I wont have to pull the diff cover off. Thanks for your replies

[Keith Seymore]

Assuming your truck does not have California emissions: the base axle for a '85 K10 with a 305 and overdrive trans = 3.08:1

Base rear axle ratio is not shown on the SPID label, or is shown as "GQ1" (base axle ratio).

If he had ordered an optional ratio then the specific RPO would show on the SPID (GU6 = 3.42; GT4 = 3.73, etc).

K

[true74enduro]

That's awesome thanks for the help Ill double check my glove box to see if I can find axle ratio. I also have a lot of other documents from when my grandpa originally owned it. What would yall say would be a good gear ratio to run? I don't want to have a truck with no low end torq but I want good get up and to be able to keep up on the free way.

[beamn7]

My 85 C10 (not K10) came with 305/700R4 and 3:08's from the factory. I swapped out the rears to 3:73's and run 31" tires. Great combo.

[true74enduro]

I found out I have 3.73 rear gears in my truck. So what would be a better ratio for a little more git up for going on the freeway as ares here in salt lake is 70mph so I don't want to be full on the throttle trying to keep up.