Why the gear ratio has to match
A rear axle and differential are built around a specific gear ratio, commonly written as something like 3.42, 3.73 or 4.10, and that ratio has to match the front axle on four-wheel-drive trucks and the transmission's programming on every vehicle. Install a differential with the wrong ratio and the speedometer reads incorrectly, fuel economy and towing performance suffer, and on 4WD trucks a ratio mismatch between front and rear axles can damage the transfer case under sustained driving. This is the single most important thing we confirm before quoting a used axle or differential.
How we confirm the ratio
The axle tag, a metal plaque riveted to the differential cover or axle tube, usually states the ratio directly. Where the tag is missing, GM's glovebox RPO label lists a code like GU6, GT4 or GT5 that decodes to a specific ratio, and other manufacturers list similar codes on a build sheet or door-jamb label tied to the VIN. Counting ring-and-pinion teeth is the fallback when no label survives. Tell us what you have, including a photo of the tag if you can get one, and we take it from there.
Locking, limited-slip, or open
A locking differential, like GM's G80 Eaton or Ford's electronic locker, engages both rear wheels together for traction and is a different unit entirely from an open differential that lets the wheels spin independently. Limited-slip units sit in between. Installing the wrong type changes how the truck handles, and on trucks ordered with a locker, a mismatched open differential is a noticeable downgrade in traction. We confirm which type your truck left the factory with, or which one you want to upgrade to, before quoting.
Rear axle & differential sourcing FAQ
Check the metal tag on the differential cover or axle tube first; it often states the ratio directly. If it is missing, your VIN and, on GM trucks, the RPO code on the glovebox label usually decode to the ratio. Send us a photo of whatever you find and we confirm it.
That depends on how your truck was originally equipped and what you want from it. A locking differential engages both wheels for traction; an open differential does not. We check your build first, then talk through whether to match it or upgrade.
Yes, within reason, and some owners do it for towing or fuel economy. Changing the ratio usually means the front axle needs to match on 4WD trucks and can affect speedometer calibration. Tell us what you are trying to achieve and we advise on what is realistic.
Locating and confirming a match typically takes a business day. Freight then adds five to ten business days depending on distance. Call and we give you a specific timeline for your truck.