Differential Sourcing FAQ: Open vs Limited Slip vs Locking, Gear Ratios, and What Actually Goes Wrong

I'm not the one under the truck. That's Ray, who's been setting up rearends for 28 years and can hear a bad pinion bearing from the parking lot. I'm the office administrator at a 45-person parts and install operation. I handle differential and drivetrain ordering — roughly $410,000 a year across 11 vendors — and I report to both operations and finance.

Which means I'm the one who eats it when the wrong carrier shows up on a Friday afternoon, or when a supplier hands over a handwritten receipt and finance kicks the expense report back. (That one cost me $2,400 out of my own department budget in 2021. I check invoicing before I check price now.)

These are the questions I actually get — from our counter guys, from other shops who call us, and from my own past self.

• What's the difference between open, limited slip, and locking?
• How do I figure out what differential gear ratio I need?
• Why won't a 4.10 gear set fit the carrier that held my 3.55s?
• Why does the same truck differential quote at $520 and $1,480?
• Do I need new side bearing spacers with a swap?
• Is a locking differential worth it for a fleet?
• How do I vet a differential supplier?

What's the real difference between an open, a limited slip, and a locking differential?

An open differential splits torque evenly between the two wheels — which means the wheel with the least grip decides how much torque actually reaches the ground. One tire on ice, and that tire gets the power. Cheap, simple, fine for a commuter car.

A limited slip differential (LSD for short) puts either a clutch pack or a set of helical gears between the two sides so it can bias torque toward the wheel that's still gripping. You don't do anything. It just works — until the clutch packs wear and it starts behaving like an open diff again.

A locking differential ties both axles together, either automatically (a ratcheting locker) or on command (air, electric, or cable). Locked on dry pavement, both wheels turn the same speed whether they want to or not.

Here's the part people get backwards: the assumption is a locker is "stronger" than an LSD and an LSD is stronger than an open. That's not the relationship. The question is where you want the stress to show up. A locked axle on dry pavement pushes the bind into your U-joints and axle shafts. An LSD slips instead — which is exactly why the clutch packs eventually wear out. It's not weaker. It's absorbing the load somewhere else.

How do I figure out what differential gear ratio I need?

The math is not complicated: ring gear teeth divided by pinion teeth. A 41-tooth ring and an 11-tooth pinion is a 3.73. A 41 and a 10 is a 4.10. That's the whole calculation.

Finding what you already have is the harder half. Check the tag bolted to the housing or diff cover, the build sticker on the door jamb, or run the VIN. If a previous owner regeared and didn't retag it, the tag lies — we've been burned by that more than once, and the only fix is counting teeth with the cover off.

Choosing a ratio comes down to tire size and duty cycle. Taller tires effectively lower your ratio (fewer engine RPM per mile), so a truck on 33s with a 3.55 gear set turns into a dog and cooks the transmission on long grades. Going numerically higher — 4.10, 4.56, meaning more RPM per mile — restores what the bigger tires took away. Towing heavy or running mountains, same direction. Highway-only fleet, the opposite. And if it's a 4WD, front and rear have to match, or you're buying a transfer case next.

Why won't a 4.10 gear set fit the carrier that held my 3.55s?

This is the question nobody asks until the gears are on the bench. On a lot of axles, there's a carrier break — the differential case itself is a different part depending on the ratio range. The break lands somewhere between 3.55 and 3.73 on most common axles, but it's not universal, and it's not the same on every housing.

Practically: a 4.10 ring gear won't bolt to the carrier that held your 3.55s. You either buy the correct carrier for the new ratio, or you buy a "thick" gear set made specifically to sit on the low-ratio carrier. Both work. Ordering the wrong one costs you a week.

If you've ever had a pallet arrive and realized halfway through the install that it's the wrong case, you know that specific sinking feeling. Ask the ratio and the carrier break before the PO goes out. A supplier who can't answer that question in one reply is telling you something.

Why does the same truck differential quote at $520 and $1,480?

Last spring we quoted the same application, same ratio, three suppliers: $520, $860, $1,480. Same truck. Same fitment.

The spread comes from what's actually in the box. New versus remanufactured versus used. "Rebuilt" meaning the housing was cleaned, painted, and resealed with the original bearings still in it. New clutch packs versus reused ones. Carrier only, versus a complete dropout with yoke, cover, and flanges. A one-year warranty versus ninety days.

FTC advertising guidelines require that claims be truthful, not misleading, and substantiated with evidence. But that doesn't mean every supplier uses "remanufactured" to mean the same thing. Ask for the parts list.

People assume the high quote is markup. Usually it's the reverse — the seller who can tell you exactly which bearings and which clutch packs went in charges more because there's more in there. The causation runs the other way. That's not a reason to always buy the top quote, but it is a reason to stop treating the lowest number as the honest one.

Do I need new side bearing spacers when the differential gets swapped?

Yes, plan on it. Side bearing spacers (sometimes shims, depending on the axle) do two jobs at once: they set the carrier's side-to-side position, which sets backlash, and they set bearing preload. Both of those are what keep the gear pattern where it belongs.

Your old shim stack is a starting point, nothing more. A different carrier case can be a few thousandths off from the one that came out, and a few thousandths is the difference between a quiet rearend and a howl at 55 mph. Same thickness does not mean same preload.

Two other things I've learned the hard way, secondhand from Ray: never reuse a crush sleeve on a rebuild, and if the axle is coming apart more than once, a solid spacer pays for itself. Every setup spec — backlash, pinion depth, preload — comes from the axle manufacturer's service manual. Those numbers are not universal, and anyone quoting you a single "correct" backlash for all axles is guessing.

Is a locking differential worth the money for a fleet?

I have mixed feelings about this one. On one hand, a locked rear gets a loaded truck out of a muddy jobsite without a tow, and for a contractor running unimproved access roads, that's not a luxury. On the other hand, drivers complain about tire chirp and accelerated tire wear, and two years ago one of our accounts tore up a set of axle shafts because a driver left it locked on the highway and didn't notice.

We ran the numbers on a 12-truck account: the locker build ran about $700 more per truck than an LSD build. Best case, it saves a stuck truck — roughly $1,900 in lost hours plus a tow, on their numbers, not mine. Worst case, it's axles and a differential. The expected value said yes. The downside felt expensive.

Where we landed: LSD for on-road fleets and anything a driver isn't trained on, selectable lockers for off-road work where the operator knows when to engage it, and automatic ratcheting lockers only when the customer genuinely wants one and understands the noise. There's no single right answer here, and anyone selling you one is selling you something.

How do I vet a differential supplier?

Two years ago I had four hours to source a semi truck differential before a customer's rig came off a lift we needed. Normally I get three quotes, verify the axle tag, and confirm ratio and spline count before I commit. There was no time, so I went with our regular supplier on trust alone. It worked out. It could just as easily not have.

What I check now, in order:

• Application verification. Will they confirm fitment from a VIN and an axle tag, in writing, or do they just say "should fit"?
• Build detail. Can they name the bearings, the clutch packs, the gear set source?
• Invoicing and terms. Clean invoices, net terms, a real core return policy. This is the one that bit me.
• Warranty specifics. Who pays labor when it fails? (Almost nobody. Know that going in.)
• Stock versus drop-ship. Drop-ship isn't wrong, but it changes your lead time and who owns the problem.

The reason we've kept buying from Diffpoint Sourcing is narrow and specific: they're differential-focused rather than general parts, they cover the whole spread from car differentials to truck and semi truck differentials, and when I send a VIN and an axle tag I get a specific answer about fitment and the right side bearing spacers instead of a maybe. They aren't the cheapest quote in the stack. After the $2,400 receipt, I stopped optimizing for that.

If you're sourcing an automotive differential and the supplier can't tell you the carrier break, the ratio, and what's inside the case, keep calling.