2018 GMC Sierra 1500 Transmission & Axle Gear Ratios
A comprehensive guide to understanding the powertrain specifications of your truck.
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Transmission gear ratios
The 2018 GMC Sierra 1500 offered a few transmission options, each designed to optimize performance and fuel economy for various applications. Below are the internal gear ratios for each available transmission.
6-speed automatic (Hydra-Matic 6L80)
| Gear | Ratio |
|---|---|
| 1st | 4.03:1 |
| 2nd | 2.36:1 |
| 3rd | 1.53:1 |
| 4th | 1.15:1 |
| 5th | 0.85:1 |
| 6th | 0.67:1 |
| Reverse | 3.06:1 |
8-speed automatic (Hydra-Matic 8L90)
| Gear | Ratio |
|---|---|
| 1st | 4.56:1 |
| 2nd | 2.97:1 |
| 3rd | 2.08:1 |
| 4th | 1.69:1 |
| 5th | 1.27:1 |
| 6th | 1.00:1 |
| 7th | 0.85:1 |
| 8th | 0.65:1 |
| Reverse | 3.82:1 |
10-speed automatic (Hydra-Matic 10L80)
| Gear | Ratio |
|---|---|
| 1st | 4.70:1 |
| 2nd | 2.98:1 |
| 3rd | 2.06:1 |
| 4th | 1.77:1 |
| 5th | 1.52:1 |
| 6th | 1.28:1 |
| 7th | 1.00:1 |
| 8th | 0.85:1 |
| 9th | 0.69:1 |
| 10th | 0.64:1 |
| Reverse | 4.87:1 |
Available axle ratios
The 2018 GMC Sierra 1500 offered a range of axle ratios to cater to diverse needs, from maximizing fuel efficiency to heavy-duty towing.
| Ratio | Available With (engine/trim/package) | RPO/Axle Code | Best For |
|---|---|---|---|
| 3.08:1 | 5.3L V8 (some configurations) | GU4 | Fuel Economy |
| 3.23:1 | 4.3L V6, 5.3L V8 | GU5 | Balanced |
| 3.42:1 | 5.3L V8, 6.2L V8 (standard with Max Trailering Package) | GU6 | Towing/Balanced |
| 3.73:1 | Not typically available for 2018 1500 models (more common in HD) | N/A | Heavy Towing (if available) |
How to identify your axle ratio
For GMC vehicles, the easiest way to identify your axle ratio is by checking the RPO (Regular Production Option) sticker. This sticker is typically found on the inside of the glove box door or the driver's side door jamb. Look for codes starting with "GU" followed by a number, such as GU4 (3.08:1), GU5 (3.23:1), or GU6 (3.42:1).
If the RPO sticker is missing or unreadable, you can sometimes find an axle tag on the differential housing itself. This metal tag will usually have the gear ratio stamped on it. Another option is to use a VIN decoder, many of which can provide a detailed breakdown of your vehicle's factory options, including the axle ratio.
Towing vs MPG impact
The choice of axle ratio significantly impacts both towing capability and fuel economy. A numerically lower axle ratio (e.g., 3.08:1) means the engine operates at lower RPMs at a given road speed. This generally translates to better fuel economy on the highway but reduces torque multiplication, making it less ideal for heavy towing.
Conversely, a numerically higher axle ratio (e.g., 3.42:1) allows the engine to spin at higher RPMs for the same road speed. This provides increased torque to the wheels, enhancing towing capacity and acceleration, but typically at the expense of fuel efficiency. For the 2018 GMC Sierra 1500, the 3.42:1 ratio, especially when paired with the Max Trailering Package, is the preferred choice for those who frequently tow heavy loads.
Common issues
While the differentials in the 2018 GMC Sierra 1500 are generally robust, a few common issues can arise:
- Ring & Pinion Whine: A high-pitched whine that increases with vehicle speed can indicate wear or improper setup of the ring and pinion gears. This often becomes noticeable during deceleration or at specific speed ranges.
- Carrier Bearing Noise: A humming or growling noise, especially when turning or under load, can point to worn carrier bearings within the differential. Early detection and replacement can prevent more extensive damage.
- Limited-Slip Clutch Wear: If your Sierra is equipped with a limited-slip differential, over time, the clutch packs can wear out. Symptoms include a chattering or clunking noise from the rear end, particularly during tight turns, or a decrease in traction in slippery conditions. Regular differential fluid changes with the correct friction modifiers can extend the life of these clutches.
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More specs for the 2018 GMC Sierra 1500
Oil Capacity & Filter
Capacity, weight, filter PN, drain plug torque, change interval.
Fluid Type Specs
Engine oil, coolant, ATF, diff, brake, PS, DEF — with OEM spec numbers.
Coolant Capacity & Type
Total system capacity, OEM coolant spec, flush interval, bleed tips.
Transmission Fluid & Service
Trans model, fluid spec, capacity, filter PN, service interval.
Torque Specifications
Lug nuts, drain plug, caliper, axle nut, plug, key fasteners.
Brake Pad & Rotor Specs
Pad PNs, rotor size, min thickness, caliper torque.
Tire Size & Pressure
OEM size, cold psi, bolt pattern, wheel size, TPMS.
Battery Size & Group
Group size, CCA, AGM/flooded, dimensions, reset note.
A/C Refrigerant & Oil
R-134a or R-1234yf, charge oz, PAG oil spec.
Bulb Sizes (Headlight, Tail, Fog)
Headlight, fog, tail, turn, reverse, interior.
Factory Maintenance Schedule
Service intervals by mileage, severe-duty notes, known issues.
Towing Capacity & Payload
Max tow, payload, GVWR/GCWR, hitch class by trim.
Fuse Box Diagram & Amperage
Fuse box locations, layout, amps, and what each fuse controls.
Serpentine Belt Routing & Length
Belt routing, length, rib count, OEM + aftermarket PNs.
Wheel Alignment Specifications
Camber, caster, toe ranges (front & rear), adjustability notes.
Paint Color Codes
All factory colors with paint codes, sticker location, touch-up tips.
Wiper Blade Sizes
Driver/passenger/rear blade size, attachment, OEM + cross-refs.
Spark Plug Type & Gap
Plug PN, gap, torque, interval by engine.
Key Fob Programming
DIY programming steps, FCC ID, fob PN, battery, transponder.
Same spec — other model years
Maintenance & repair guides
2018 GMC Sierra 1500 Maintenance Schedule →
Available maintenance guidance; confirm the manufacturer schedule.
Repair planning references →
Plan the scope of work; detailed procedures need configuration-matched evidence.
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