2016 Kenworth T680 — Transmission & Axle Gear Ratios

Transmission gear ratios

Below are the most common transmission options spec’d on 2016 Kenworth T680 highway tractors. Exact ratios depend on the specific model code stamped on the transmission tag; always verify against the unit’s data plate or build sheet.

Eaton Fuller FR/FRO 10‑speed Overdrive (manual)

Notes: This representative FR/FRO overdrive gearset is widely used in FR‑/FRO‑15/16/18 series 10‑speeds paired with PACCAR MX‑13 and Cummins ISX15 in 2016 T680s.

Eaton UltraShift Plus 10‑speed Overdrive (automated manual)

Notes: UltraShift Plus uses the same underlying Eaton Fuller gearset as the manual FR/FRO 10‑speed, with automated clutch and shift actuation. Common on linehaul T680s focused on fuel economy.

Allison 4500 RDS (6‑speed automatic; less common on T680)

Notes: Occasionally spec’d for specialized applications; most T680 highway tractors will be Eaton manual or automated manual.

Other transmissions (availability varies by build): Eaton Fuller 13‑speed and 18‑speed overdrive models were also available on some 2016 T680s for heavier GCWs. Gear ratios vary by exact model (e.g., RTLO‑16913A, RTLO‑18918B); verify against the transmission tag if equipped.

Available axle ratios

Kenworth T680 tandems in 2016 commonly used Meritor or Dana/Spicer differentials. Ratio availability varied by axle model, engine, and intended duty cycle.

Notes: Actual axle model and code are stamped on the axle tag (see below). Some ratios may be limited to specific carriers and tire sizes. Always match overall cruise RPM with your transmission’s top‑gear ratio and tire revs/mile.

How to identify your axle ratio

• Axle tag on the differential: The most reliable method on a T680 is the metal tag riveted to the differential carrier (both forward and rear axles on tandems). It lists axle model (e.g., Meritor RT40‑145 or MT40‑14X; Dana/Spicer DSP40/DSP41), the serial number, and the numeric ratio (e.g., 3.36). Clean the tag and record all characters.
• Transmission/axle data plates: Eaton Fuller transmissions have an aluminum ID tag on the case with the model number; this, combined with the build sheet, confirms the gearset. Axle carriers also have stamped numbers on the housing if the tag is missing.
• VIN/build sheet: Any Kenworth dealer can pull the original spec sheet from the VIN, which will list axle models and ratios as built. Many fleets keep a copy of the chassis spec in the truck’s document binder.
• Spin test (approximate): Chock one wheel on a tandem, raise the other, mark the driveshaft, and count driveshaft turns while the wheel makes two full rotations. Divide driveshaft turns by two to estimate the axle ratio. Do this safely with proper supports.

Towing vs MPG impact

• Numerically lower ratios (e.g., 2.26–2.79) reduce engine RPM at cruise and generally deliver the best fuel economy on flat terrain when paired with an overdrive or super‑overdrive top gear. Startability and gradeability decline, so spec these only when GCW and routes allow.
• Mid ratios (3.08–3.36) strike a balance for regional haul or mixed terrain, preserving reasonable startability while keeping cruise RPM controlled.
• Higher ratios (3.55–4.30) multiply torque for heavy loads, frequent stops, and steep grades. Expect higher cruise RPM and reduced MPG unless paired with deep overdrive gearing and tall tires.

Always consider the full driveline: transmission top‑gear ratio, tire revs/mile, typical cruise speed, and target engine’s “sweet spot” RPM band.

Common issues

• Ring and pinion whine: Often grows with load and road speed; can indicate improper setup or wear. Check fluid level/condition and backlash.
• Carrier bearing (center support) wear: Causes vibration or rumble on throttle changes. Inspect U‑joints and the center bearing on the inter‑axle driveline.
• Limited‑slip/inter‑axle differential clutch wear: Diminished traction transfer or chatter in tight turns; service clutches and verify correct friction modifier where specified by the axle maker.

Find a driveline shop on Repairius.