How do I find out what gear ratio my Ford rear end has?
Most Ford rear ends display the gear ratio on a tag or stamping on the differential, or in the vehicle’s build data; if those aren’t readable, you can determine it by turning the driveshaft or by counting ring- and pinion-teeth. This guide explains reliable methods and what to do if you don’t have a readable tag.
Where to look for an official axle ratio
The quickest and most reliable way to know your gear ratio is to read an identification tag or stamped code on the axle, or to check the vehicle’s original documentation. Use the following options in order.
- Axle identification tag: Many Ford axles, including 8.8" and 9" units, carry a metal tag on the differential cover or nearby that shows the ratio (for example, 3.55, 3.73, 4.10). Read the numbers clearly and note the format used by Ford for that model year.
- A stamping on the differential housing or carrier: Some axles print the ratio directly onto the housing or on the carrier with a stamped number. Look along the edge of the differential cover or on the side of the third member for a small numeric marking.
- Vehicle build data or door/window sticker: On many modern Ford trucks and cars, the axle ratio is listed on the vehicle’s data plate (often on the door jamb). Look for an entry labeled AXLE, which may show a numeric ratio.
- VIN decode and factory documentation: If you have access to the VIN, Ford’s build sheet or reputable online decoder can reveal the original axle ratio installed at the factory for that vehicle.
If you find a readable tag or data plate, that is the fastest way to know the exact gear ratio without disassembly or measurements.
Verifying the ratio by measurement
If there’s no tag or the tag is unreadable, you can verify the ratio by a careful physical measurement of rotation. This method works without removing the cover, but it requires patience and safe lifting procedures.
- Ensure safety: Park on a level surface, set the parking brake, chock the wheels, and if possible, elevate the rear with jack stands so the wheels can rotate freely.
- Prepare the driveshaft: With the vehicle in neutral and the transmission in Park (automatic) or a neutral gear (manual), you will rotate the driveshaft by hand or with a helper.
- Count wheel revolutions per driveshaft revolution: Slowly rotate the driveshaft exactly one full turn and count how many complete turns the wheels (or a single wheel) make. The final drive ratio equals the number of wheel revolutions per one driveshaft revolution (for example, about 3.55 wheel revolutions per driveshaft turn indicates a 3.55:1 ratio).
- Take precautions against slip: If you’re rotating both wheels or the axle assembly, ensure the parking brake is not applied and that nothing binds. Use wheel chocks and a helper to avoid accidental movement.
This method gives you a practical read on the final drive ratio as installed in your vehicle, independent of any incomplete or unreadable tags.
Alternative method: count gear teeth (requires cover removal)
For a precise, tag-free determination, you can count the actual gear teeth. This approach requires removing the differential cover and documenting gear counts, so proceed only if you’re comfortable with basic drivetrain work and sealing the cover afterward.
- Drain the differential fluid and remove the differential cover according to your model’s service procedure.
- Count the teeth on the ring gear (the large gear attached to the carrier) and on the pinion gear (the smaller gear driven by the driveshaft).
- Calculate ratio: Divide the ring gear teeth by the pinion gear teeth. For example, a 42-tooth ring gear with a 12-tooth pinion yields a 3.50:1 ratio.
- Reassemble with new gasket or sealant, refill differential fluid to the proper level, and test drive to confirm smooth operation.
Counting teeth is the definitive approach when there are no readable identification marks, but it involves disassembly and reassembly, so proceed with care or consult a professional.
Troubleshooting and caveats
Some Ford rear ends use unconventional gearing codes or have different labeling conventions by year and model. If you encounter a code that isn’t obviously a simple number, cross-check with Ford’s official service literature, lineage-specific parts catalogs, or trusted online axle-code databases. Always confirm via multiple sources when possible to avoid misidentification, especially if you’re planning gear swaps or other drivetrain work.
When to seek professional help
If you’re unsure about locating tags, decoding codes, or performing a safe measurement, a professional mechanic or a Ford dealership can identify the axle ratio quickly using factory documentation or diagnostic tools. This is particularly important if you’re planning gear changes or performance upgrades to avoid incorrect gearing and potential driveline damage.
Summary
To determine your Ford rear end gear ratio, start with any visible axle tag, housing stamping, or the vehicle’s data plate or VIN build data. If those aren’t readable, you can verify by turning the driveshaft and counting wheel revolutions per driveshaft revolution, or by removing the differential cover to count ring and pinion teeth. Each method has its own risk and level of effort, so choose the approach that matches your comfort with disassembly and safety. In doubt, consult a professional to ensure accuracy and proper reassembly.
