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November 09, 2026 9 min read
By Troy Kane | V-Twins to V-8s
Ever watch somebody do a burnout and wonder why one car leaves a single black stripe while another leaves two?
You've probably seen it happen. One car lights up a single rear tire, while another leaves two long black stripes on the pavement.
What's the difference?
A lot of it comes down to what's inside the rear axle: an open differential or a limited-slip differential, commonly called Positraction or simply Posi by Chevrolet enthusiasts.
I've got two Chevy C10 chassis in the shop, one with an open differential and another with a Positraction rear end. In the video above, I'm using them to demonstrate how each operates.
And there's something about a one-wheel burnout that might surprise you. That spinning tire could be rotating twice as fast as you think.
An open differential is one of the most common differential designs found in cars and trucks.
Under normal driving conditions, when both drive tires have similar traction, an open differential does its job without much attention from the driver.
The limitation becomes apparent when one tire has considerably less traction than the other.
An open differential delivers essentially equal torque to both axle shafts. That means the usable driving torque is limited by the tire with the least available traction.
If one tire is on ice, loose gravel, or another slippery surface, that tire can spin while the other barely moves.
It's also why you sometimes see an old muscle car doing a burnout with only one rear tire spinning.
On my older Chevy C10 chassis, I can demonstrate this without taking the rear axle apart.
With the pinion held stationary, I rotate one rear wheel. The opposite wheel turns in the other direction.
When I release the pinion, the wheels and pinion can rotate together, depending on the resistance within the assembly.
Now, if I hold one rear tire stationary and rotate the pinion, the free tire turns while the other stays put.
That's the behavior I'm demonstrating in the video.
And it brings up something important that a lot of people don't realize.
Here's something worth remembering the next time you see somebody spinning only one rear tire.
When one tire is completely stationary, the other tire rotates at twice the speed it would if both wheels were turning equally at the same differential carrier speed.
Think about that.
If you're doing a one-wheel burnout and a correctly calibrated, drivetrain-based speedometer indicates 60 MPH, the spinning tire could be rotating at the equivalent of 120 MPH.
60 MPH indicated. 120 MPH tire speed.
The differential carrier rotates at the average speed of the two axle shafts. With one wheel stationary, the other wheel rotates at twice the carrier speed.
That can become a serious issue when you're spinning tires at high RPM.
Tires have speed ratings and operating limitations for a reason. Excessive rotational speed, heat, tire age, and existing damage can all increase the risk of failure.
A tire spinning at the equivalent of 120 MPH while the vehicle is barely moving isn't experiencing the same conditions as a tire rolling down the highway.
Technical note: This example assumes one stationary wheel, a correctly calibrated drivetrain-based speedometer, and appropriate tire sizing. Actual road speed and readings from wheel-speed-sensor-based systems may differ.
Now let's look at the other C10 chassis.
This one has a Positraction rear end.
Positraction is General Motors' familiar name for limited-slip differential technology. You'll also hear it called Posi or LSD.
The purpose of a limited-slip differential is to improve usable traction when the two drive wheels have different amounts of grip.
Instead of allowing one tire to spin freely with very little resistance, a limited-slip differential can resist excessive wheel-speed differences or bias more torque toward the tire with better traction, depending on its design.
There are several types of limited-slip differentials, and they don't all operate the same way.
Some use clutch packs, while others use internal gearing to provide their traction advantage.
On my newer C10 chassis, I rotate the pinion and show both rear wheels turning together.
Turn the pinion forward, and both wheels rotate forward. Turn it backward, and both wheels rotate backward.
That's the behavior I'm demonstrating with this Positraction rear axle.
But here's an important point.
Seeing both wheels turn together with the axle off the ground doesn't automatically prove you have Positraction.
An open differential can also rotate both wheels together under certain conditions.
The real advantage of limited slip becomes apparent when the drive wheels have different amounts of available traction.
| Feature | Open Differential | Limited Slip / Posi |
|---|---|---|
| Normal street driving | Good | Good |
| Allows different wheel speeds | Yes | Yes |
| Traction when one tire slips | Limited | Generally better |
| Can produce a one-wheel burnout | Yes | Yes, under some conditions |
| Can spin both tires | Yes | Yes |
| Complexity | Generally simpler | Varies by design |
| Maintenance | Usually straightforward | Depends on design |
| Performance applications | Limited traction advantage | Often preferred |
A limited-slip differential doesn't guarantee both tires will always spin together.
Its performance depends on the design, available traction, condition of the unit, and how much torque is being applied.
But when you're trying to put power to the pavement, a properly functioning limited-slip differential can offer a substantial advantage.
That depends on what you're building.
For a stock restoration or a classic that's primarily used for cruising, an open differential may be perfectly adequate.
It's simple, predictable, and generally inexpensive to maintain.
If you're restoring a vehicle to factory-original specifications, retaining the original open differential may also make sense.
For a street-performance car, muscle car, or modified pickup, a quality limited-slip differential is often worth considering.
It can improve traction during acceleration, especially when one tire has less grip than the other.
That's particularly useful when you're working with increased horsepower, wider tires, or a performance-oriented drivetrain.
But don't choose a differential simply because somebody tells you that you need a Posi.
Build the rear axle for the way you actually intend to drive the vehicle.
If you've got an open rear end and you're tired of spinning one tire, there are several ways to improve traction.
And depending on what rear axle you're working with, you may not have to replace everything inside the housing.
Here are three options worth considering.
One of the most familiar upgrades for a classic muscle car is a traditional clutch-type limited-slip differential.
These units use clutch packs and spring pressure to resist excessive differences in wheel speed and improve traction.
The Eaton Posi is a good example.
It's a proven design that's been popular in street-performance vehicles for decades. Many clutch-type units can also be rebuilt as their friction components wear.
For somebody restoring a classic Chevrolet or building a traditional street machine, this is certainly an option worth considering.
Best suited for: Classic restorations, street-performance vehicles, and traditional muscle-car builds.
Explore: Eaton Posi Differentials at Summit Racing
Another option is the Eaton Detroit Truetrac.
Unlike a traditional clutch-type Positraction unit, the Truetrac uses internal helical gears to bias torque toward the wheel with better traction.
There are no conventional limited-slip clutch packs to wear out.
These units are known for smooth operation and are available for a wide variety of rear axle applications.
For a street-driven classic or performance pickup, a gear-driven limited-slip differential can be an excellent choice.
Best suited for: Street-performance vehicles, modified classics, and drivers looking for smooth, low-maintenance traction improvement.
Explore: Eaton Detroit Truetrac Differentials at Summit Racing
Here's another option that's been around for years, and one I'm familiar with from selling these products.
Powertrax offers traction systems that can be installed inside certain existing open differential carriers.
The Powertrax Lock-Right replaces the original spider and side gears with an automatic locking mechanism.
On compatible applications, you can retain the original differential carrier and ring-and-pinion setup.
That can make the conversion less involved than replacing the entire carrier and resetting the gear setup.
Powertrax also offers the No-Slip Traction System, which is designed to provide smoother and quieter operation than a traditional ratcheting locker.
There's an important distinction here.
Powertrax Lock-Right and No-Slip units are automatic lockers, not conventional limited-slip differentials.
They improve traction differently, and their engagement can be more noticeable during street driving.
But for somebody looking to improve traction while retaining a compatible open carrier, they're definitely worth investigating.
Best suited for: Owners who want improved traction without replacing a compatible existing carrier, particularly in performance and off-road applications.
Explore: Powertrax Differential Upgrades at Summit Racing
Make sure you know exactly what rear axle you're working with.
You'll need to verify the axle model, spline count, carrier compatibility, gear ratio requirements, and intended application.
Don't assume that every Chevrolet 12-bolt rear end uses the same parts. Truck and passenger-car applications can differ.
And remember, installing a replacement differential carrier generally requires proper bearing setup, backlash adjustment, and verification of the gear contact pattern.
A drop-in locker may avoid disturbing the ring-and-pinion setup on compatible applications, but that doesn't mean every installation is identical.
Buy the correct components for your specific axle, not just the vehicle's year and model.
Affiliate disclosure: Some product links in this article may earn V-Twins to V-8s a commission at no additional cost to you. I recommend researching compatibility and choosing parts based on your actual application.
This is a common question, especially when buying a classic car or working with an axle of unknown history.
Factory axle identification codes, build documentation, and manufacturer specifications can help identify what differential was originally installed.
But on a vehicle that's 40, 50, or 60 years old, there's no guarantee the original components are still inside.
A previous owner may have replaced the entire rear axle or changed the differential carrier.
You may have heard that you can jack up the rear axle, rotate one tire, and watch which direction the other tire turns.
That can provide clues, but it's not a definitive test.
Clutch preload, brake drag, internal friction, and differential design can all affect what happens when you rotate the wheels by hand.
If you're buying a used rear axle or planning an upgrade, verify what's actually inside before spending money.
With an open differential, both axle shafts receive essentially equal torque. When one tire has less available traction, it may spin while the other remains stationary or turns more slowly.
Positraction is General Motors' familiar name for limited-slip differential technology. Other manufacturers use different names for similar traction-enhancing systems.
Yes. An open differential can spin both tires under certain traction conditions. Seeing two black stripes doesn't automatically prove a vehicle has limited slip.
Yes. Limited-slip differentials have operating limits, and some designs may allow significant wheelspin when traction is extremely uneven or the unit is worn.
In many cases, yes. Depending on the axle, you may be able to install a replacement limited-slip carrier or a compatible traction-enhancing conversion system.
Some Powertrax systems also allow you to convert an existing open carrier into an automatic locker without replacing the entire carrier.
No. A conventional limited-slip differential isn't the same as a permanently locked axle. Its operation depends on the particular differential design.
Now that you understand the difference between an open differential and Positraction, there's another important piece of information worth knowing about your rear axle:the gear ratio.
Whether you're building a classic car, swapping rear ends, or just trying to figure out what's already under your truck, knowing your gear ratio can tell you a lot about how your vehicle will perform.
And here's the good news.You don't have to remove the differential cover or take anything apart to find out.
In another shop demonstration using my Chevy C10, I'll show you how to use a couple of reference marks and count pinion rotations to estimate your rear-end gear ratio.
Read More: How to Check Your Rear-End Gear Ratio Without Taking It Apart
Whether you're restoring an original classic, building a street machine, or upgrading an older pickup, understanding the difference between an open differential and limited slip can help you make better decisions.
An open differential works perfectly well for many applications. But when improved traction becomes a priority, there are several proven upgrade options worth considering.
And the next time you see somebody leave one black stripe instead of two, you'll have a better understanding of what's happening inside that rear axle.
But now I've got another question for you.
Why do we need a differential in the first place? Why not just connect both rear wheels together and make them turn at the same speed all the time?
There's a good mechanical reason for it, and that's what we're going to cover in the next installment of College of Classic Car Knowledge.
Until then, tell me what you think in the comments.
Troy Kane
V-Twins to V-8s — Real-World Automotive Knowledge
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This site documents real builds, tools, and shop work from my own projects. Some pages are showcases. Some are how-tos and tool reviews. If you’re working on a project and want experienced guidance, I offer one-on-one coaching.