2020 Isuzu NQR Transmission & Axle Gear Ratios

Transmission gear ratios

The 2020 Isuzu NQR typically comes with the Aisin A465 6-speed automatic transmission. This transmission is known for its durability and smooth shifts, making it well-suited for the demands of a medium-duty truck.

Available axle ratios

Isuzu offers a range of axle ratios for the NQR model, allowing buyers to optimize their trucks for specific applications, whether that's maximizing fuel economy or enhancing towing and hauling capabilities. While specific RPO codes can vary, the following are common ratios found in the 2020 NQR.

How to identify your axle ratio

Identifying your axle ratio is crucial for proper maintenance, parts ordering, and understanding your truck's performance characteristics. For Isuzu NQR trucks, several methods can be used:

• Door-Jamb RPO Sticker (Limited Use): While GM vehicles commonly use a door-jamb RPO sticker to list axle codes (often starting with "GU" for gear ratios), Isuzu's labeling can be less consistent in this regard. It's always worth checking, but don't rely solely on this method.
• Axle Tag: The most reliable method is to locate the metal tag attached to one of the differential cover bolts on the rear axle housing. This tag will typically have the axle ratio stamped on it. You might need to clean off dirt or grease to read it clearly.
• VIN Decoder: Some online VIN decoders, especially those tailored for commercial vehicles, may be able to provide details on the original build specifications, including the axle ratio. Be aware that aftermarket changes to the axle are not reflected here.
• Counting Ring Gear Teeth: As a last resort, if no tag is present or readable, the axle ratio can be determined by removing the differential cover and counting the teeth on the ring gear and the pinion gear. Divide the number of ring gear teeth by the number of pinion gear teeth to get the ratio.

Towing vs MPG impact

The choice of axle ratio significantly impacts the balance between towing capability and fuel economy:

• Numerically Lower Ratio (e.g., 3.73): A numerically lower axle ratio means the engine turns fewer revolutions per minute (RPM) at a given road speed. This generally translates to better fuel economy, especially at highway speeds, as the engine is operating less strenuously. However, it also reduces torque multiplication at the wheels, making the truck less effective for heavy towing or hauling.
• Numerically Higher Ratio (e.g., 4.33): Conversely, a numerically higher axle ratio causes the engine to spin at higher RPMs for the same road speed. This increases torque at the wheels, providing more pulling power for towing and hauling heavy loads, or navigating steep grades. The trade-off is typically reduced fuel economy, as the engine is working harder.

For the 2020 Isuzu NQR, selecting a 4.10 axle ratio provides a good balance for general-purpose use, offering decent towing capacity without severely impacting fuel efficiency. Trucks intended for consistent heavy hauling or operating in hilly terrain would benefit from the 4.33 ratio, while those used for lighter express delivery where fuel economy is paramount might opt for the 3.73 if available.

Common issues

While Isuzu NQR drivelines are generally robust, like any mechanical component, the rear axle can experience issues over time, especially under heavy usage:

• Ring & Pinion Whine: A persistent high-pitched whine from the rear of the truck, particularly during acceleration or deceleration, often indicates wear or improper setup of the ring and pinion gears. This can be due to insufficient lubrication, improper gear mesh, or worn bearings.
• Carrier Bearing Wear: The carrier bearings support the differential carrier within the axle housing. Worn carrier bearings can cause a humming or rumbling noise that changes with vehicle speed and can lead to excessive play in the driveline.
• Limited-Slip Clutch Wear (if equipped): If your NQR is equipped with a limited-slip differential, the internal clutch packs can wear out over time. Symptoms might include a chattering or clunking noise when turning, or a noticeable lack of traction when one wheel loses grip.

Regular differential fluid changes with the correct type and viscosity of gear oil are crucial for preventing these issues and extending the life of your NQR's axle.

Find a driveline shop on Repairius.